Type of Gland


Cable Glands

  • A device designed to permit the entry of cable in to electrical equipment which provide sealing ,retention and earthing, bonding, grounding, insulation, strain relief or combination of all these.
  • Gland should maintain overall integrity of enclosure in to which it is to be fitted.

Gland Selection

  • Gland should be selected on following Points
  1. Type of Cable
  2. Gland Size
  3. Entry Type/Thread Specification of application
  4. Ingress Protection required.
  5. Material
  • Type of Cable:
  • Unarmored: Unarmored Cable will require outer seal within Gland to not only Provide ingress protection but also degree of retention.
  • Armored: Gland that required clamping mechanism to terminate the armored both mechanically and electrically.
  • The Gland will usually be required to provide ingress protection by sealing outer sheath and retention by clamping amour.

Type of Glands:

  1. Brass Indoor Type Gland
  2. Brass Outdoor Type Gland
  3. Brass Straitening Unarmored Cable Gland
  4. Brass Weather Proof Gland
  5. PG Threaded Gland:
  6. Industrial Type Gland

1)    Brass Indoor Type Gland

  • This Gland is quite handy in use with various types of cable whether plastic, rubberized, metal or any other.
  • Application: Dry indoor, for use with all type of SWA cables, plastic or rubber sheathed cable.
  • Brass indoor gland suitable for single wire armored, plastic or rubber sheathed cable. Recommended to use with shroud for additional ingress protection.

  • Cable Type: Steel Wire Amour.
  • Amour Clamping: Two Part Amour Lock.

2)      Brass Outdoor Type Gland

  • This come in stunning high quality material for use in outdoor or indoor application with various types of cables sheathed or unsheathed.
  • Brass indoor and outdoor gland popularly used with single wire armored.
  • Plastic or rubber sheathed cable. Terminates and secure cable armoring and outer seal grips sheath of cable thus ensuring mechanical strength and earth continuity.
  • CW brass glands are also supplied with integral earth facilities.
  • Recommended to use PVC shroud for additional ingress protection

  • Application: 
  • a)     Outdoor or indoor, for use with all type of SWA cables, plastic or rubber sheathed cable.
  • b)    Most suitable for SWA, plastic of rubber (Elastomeric) sheathed cables.
  • c)     Used in dry indoor conditions.
  • d)    No loose parts and easy to install.
  • e)     Save times & money.
  • Gland size: 20 mm to 75 mm (S & L)
  • Accessories :Earth Tag, PVC Shroud, Neo prime Rubber & LSF Rubber, PVC Washer, Brass Lock Nut.
  • Cable Type: Wire Braid Armor.
  • Armor Clamping: Three Parts (With Lock Nut).

(3) Brass Straitening Unarmored Cable Gland

  • Nickel plated or natural brass A2 type cable glands are used with variety of unarmored or rubber sheathed cables.
  • Brass indoor and outdoor cable gland suitable for all types of unarmored cables, plastic or rubber sheathed cables.

 

  • Application:
  1. For use with unarmored elastomeric and plastic insulated cables. 
  2. Indoor & Outdoor whenever it is required to provide sealing on cable outer sheath.
  • Size  : Metric – 20mm to 75mm (S/L)
  • Accessories: Earth Tag, PVC Shroud, Neo prime Rubber & LSF Rubber, PVC Washer, Brass Lock Nut.
  • Cable Type : Unarmored

4)    Brass Weather Proof Gland

  • Unlike other types of cable glands, This type cable gland is used precisely with single armored various types of swa cables whether plastic or rubber sheathed ones. this type cable gland is known for its uninterrupted services once the gland is fixed to the desired wires and wire components.
  • Suitable for SWA or rubber sheathed cables.
  • Outer seal grips bedding layer of cable for use in most climatic conditions.
  • Weather proof and water proof.
  • Design has separate armor lock rings. Can be supplied with integral earth facility.
  • Gland size: 20 mm to 75 mm (S & L)

         

  • Application :
  1. Outdoor or indoor, for use with single armored, all type of SWA cable, plastic or rubber sheathed cable.
  2. E1W Gland is Weatherproof & Waterproof Cable Gland
  • Cable Type :  Steel Wire Armour
  • Armour Clamping: Three Part Armour Lock
  • Sealing Technique: Compression & Displacement Type
  • Sealing Area(s): Inner & Outer Sheath

5)    PG Threaded Gland:

  • Nickel chrome plated PG threaded cable gland is a custom made threaded gland to meet the needs from the meet industries. Apart from the round headed PG threaded cable gland, we also offer hexagonal gland or any other like spherical rectangular or any other dimensional PG threaded cable gland as per the specification of the customer.

 

6)      Industrial Cable Gland:

  • Brass gland suitable for wire braid armored, plastic or rubber sheathed cable. Terminates and secure cable armoring and outer seal grips sheath of cable thus ensuring mechanical strength and earth continuity.
  •  Recommended to use PVC shroud for additional ingress protection

 

 

  • Cable Type: Wire Braid Armour
  • Armour Clamping : Three Part (With Lock Nut)
  • Sealing Technique: Compression Type.
  • Brass gland suitable for steel tape armored, plastic or rubber sheathed cables. Terminates and secure cable armoring and outer seal grips sheath of cable thus ensuring mechanical strength and earth continuity.
  • Recommended to use PVC shroud for additional ingress protection
  • Cable Type : Steel Tape Armour
  • Armour Clamping : Three Part (With Lock Nut)
  • Sealing Technique: Compression Type.

 What is difference between Single Compression and Double Compression?

  • Double compression glands provide extra support to the heavy armored cables entering or exiting the panel while single compression glands are used for light armored cables.
  • Normal Cable Gland is also called Single Compression Cable Gland. As the name suggests, while you tighten the gland, the grip or compression is effected only at one p [lace (i.e.) at the cable armour only. There is scope for moisture and corrosive vapour to enter the gland and thus into the cable.
  • Whereas in Double-Compression Gland, the compression happens both at the cable armour as well as at the inner sheath. This is sort of two sealing. Hence, chances of moisture or vapour entry are minimised. Hence these glands are also known as Weather-proof cable glands or Flame-proof cable glands.
  • The basic difference between single and double compression

1)    Parts of Double comp

  • Gland body
  • Gland body Nut
  •  Cone
  • Cone Ring
  •  Neopen Rubber seal.
  •  Rubber Washer
  •  Check Nut.

2)    Single Comp Parts

  •  Gland body
  • Gland body Nut
  • Neopen Rubber seal.
  • Rubber Washer
  • Check Nut
  • Flat washer
  • The Basic difference between Single and Double Comp is in Single comp there no cone and cone ring.
  • The mechanical support for the cable is only Neopen rubber seal, When u tighting the cable.
  • In double camp gland the mechanical support to the cable only cone and cone ring. When doing glanding the cable armor sits on the cone and cone ring act as a lock for armor.
  • Single compression and double compression glands are used on the basis of area classification. Those who are affiliated with oil and gas sector they will easily understand about area classification.
  • In zone 0 where the presence of hydrocarbon is obvious (IIC) double compression gland is used because the flame path in case of double compression gland is much more than in case of single compression gland.
  • The logic behind this is that if there is any explosion inside the terminal box of the motor no flame should be able to come out through the cable gland in order to prevent fire hazards but where there is no presence of hydrocarbons i.e. no danger of fire hazards (IIA/ IIB) single compression glands are used.
  • It has nothing to do with mechanical strength. Even in case of lighting fixtures used in IIC zone double compression glands are used.

Gland Size Selection


Gland Selection Table:

600 / 1000v stranded copper conductors pvc insulated with steel wire amour and PVC sheathed overall. (BS 6346 : 1997)

Cable Size Conductor

Numbers of Cores

Nom. Area (mm2)

Neutral

1

2

3

31/2

4

5

7

10

12

19

27

37

48

1.5

16

16

20S

20S

20S

20

20

25

25

32

32

2.5

20S

20S

20S

20S

20

25

25

25

32

32

40

4

20S

20

20

20

25

25

25

32

40

6

20

20

20

10

25

25

25

16

25

25

25

25

16

25

32

32

32

35

16

32

32

32

40

50

25

25

32

32

32

40

70

35

25

32

40

40

40

95

50

25

40

40

50S

50S

120

70

32

40

50S

50

50

150

70

32

50S

50

50

63S

185

95

32

50

50

63S

63

240

120

40

50

63S

63

75S

300

150

40

63S

63

75S

75

300

185

40

63

63

75

75

400

185

50S

63

75S

75

75

500

50

630

50

800

63S

1000

63

                             
                             
                             

600 / 1000v stranded copper conductors xlpe/swa/pvc cable and PVC sheathed overall.  (BS 5467 : 1989)

Cable Size Conductor

Numbers of Cores

Nom. Area (mm2)

Neutral

1

2

3

31/2

4

5

7

10

12

19

27

37

48

1.5

20S

20S

20S

20S

20S

20

25

25

32

32

32

2.5

20S

20S

20S

20S

20

25

25

32

32

40

40

4

20S

20S

20

20

25

25

25

32

40

40

50S

6

20

20

20

10

20

25

25

16

25

25

25

25

16

25

32

32

32

35

16

32

32

32

32

50

25

25

25

32

32

32

70

35

25

32

32

40

40

95

50

25

32

40

50S

50S

120

70

32

40

40

50

50

150

70

32

40

50S

50

50

185

95

32

50S

50

63S

63S

240

120

40

50

63S

63

63

300

150

40

63S

63

75S

75S

300

185

40

63S

63

75S

75

400

185

50S

63S

75S

75

75

500

50

630

50

800

63S

1000

63

 

Single Compression Heavy Duty as per IS12943
Cable Overload Diameter (mm)  
From

To

Nipple Thread in (mm)
9.0 12.0 16 x 1.5
12.0 15.0 16 x 1.5
12.0 15.0 20 x 1.5
15.0 18.0 20 x 1.5
16.0 20.0 25 x 1.5
20.0 24.0 25 x 1.5
24.0 28.0 32 x 1.5
28.0 32.0 40 x 1.5
33.0 37.0 40 x 1.5
37.5 42.0 50 x 1.5
43.0 48.0 50 x 1.5
49.0 54.0 63 x 1.5
55.0 60.0 63 x 1.5
61.0 66.0 75 x 1.5
67.0 72.0 75 x 1.5
73.0 78.0 82 x 1.5
79.0 84.0 90 x 1.5
84.5 92.0 100 x 1.5
93.0 100.0 110 x 1.5
101.0 115.0 125 x 1.5

 

ARMOUR CABLE (Single Compression Medium Duty)
Cable Overload Diameter (mm)  
From

To

Nipple Thread in (mm)
6.0 9.0 3/8″
9.0 12.0 1/2″
12.0 14.0 5/8″
14.0 16.0 3/4″
16.0 18.0 7/8″
18.0 21.5 1″
21.5 26.0 1 1/8″
26.0 31.0 1 1/4″
31.0 34.0 1 3/8″
34.0 38.0 1 1/2″
38.0 42.0 1 3/4″
42.0 48.0 2″
48.0 54.0 2 1/4″
54.0 60.0 2 1/2″
60.0 66.0 2 3/4″
66.0 72.0 3″
72.0 78.0 3 1/4″
78.0 84.0 3 1/2″
84.0 92.0 4″

 

UN-ARMOUR CABLE (Double Compression Medium Duty)
Cable Overload Diameter (mm)  
From

To

Nipple Thread Size
5.0 11.0 3/4″
11.0 14.0 3/4″
14.1 18.0 1″
18.1 20.0 1″
20.1 23.0 28 mm
23.1 26.0 1 1/4″
26.1 30.0 1 1/2″
30.1 33.0 1 1/2″
33.1 36.0 42 mm
36.1 41.0 2″
41.1 44.0 2″
44.1 52.0 2 1/2″
52.1 55.0 2 1/2″
55.1 60.0 70 mm
60.1 66.0 3″
66.1 75.0 3 1/4″
75.0 80.0 3 1/2″
81.0 90.0 4″
90.0 100.0 4 1/2″

 

ARMOUR CABLE (Double Compression Medium Duty)
Cable Overload Diameter (mm)  
From

To

Nipple Thread Size
6.0 12.0 3/4″
12.0 16.5 3/4″
16.5 18.5 3/4″
16.5 18.5 1″
18.5 20.0 1″
18.5 20.0 3/4″
20.0 23.0 1″
23.0 26.0 1″
23.0 26.0 1 1/4″
26.0 30.0 1 1/4″
26.0 30.0 1 1/2″
30.0 33.0 1 1/2″
30.0 33.0 1 1/4″
33.0 37.0 1 1/2″
37.0 41.0 2″
41.0 46.0 2″
46.0 52.0 2″
46.0 52.0 2 1/2″
52.0 54.0 2 1/2″
54.0 61.0 2 1/2″
61.0 66.0 3″
66.0 72.0 3″
72.0 78.0 3 1/4″
78.0 84.0 3 1/2″
84.0 94.0 4″
94.0 104.0 4 1/2″

————————————————————————————————————————-

EHV XLPE – Current Rating


EHV XLPE Cable:

3.8 / 6.6 KV(6.6 KV Earthed) Single Core AL/COPPER COND, XLPE INSULATED  CABLES As per IS:7098 (Part-II)

Cross-sectional area        (Sq MM)  

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps 

Short Circuit Current Rating for 1Sec.duration in K. Amps 

Aluminum Conductor

Copper Conductor

Aluminum

Copper

Ground Duct Air Ground Duct Air
1cX25    23    100    90    120    130    115    155    2.35    3.58  
1cX 35    24    120    105    145    155    140    185    3.29    5.00  
1cX50    25    140    125    170    185    160    220    4.70    7.15  
1cX70    27    175    155    215    225    195    275    6.58    10.01  
1cX 95    28    205    180    260    265    235    340    8.93    13.59  
1cX120    30    235    205    305    300    265    390    11.28    17.16  
1cX150    32    260    230    345    335    295    440    14.10    21.45  
1cX185    34    295    260    395    380    330    510    17.39    26.46  
1cX240    37    340    300    470    435    380    600    22.56    34.32  
1cX300    39    385    335    540    490    425    680    28.20    42.90  
1cX400    44    0.57    440    380    630    550    480    790    37.60  
1cX 500    47    0.60    495    430    730    610    530    910    47.00  
1cX 630    51    0.67    560    480    840    680    580    1030    59.22  
1cX800    57    0.76    620    530    960    740    630    1140    75.20  
1cX1000    61    0.82    680    580    1070    790    670    1250    94.00  
                   

3.8 / 6.6 KV(6.6 KV Earthed) Three Core AL/COPPER COND, XLPE INSULATED  CABLES As per IS:7098 (Part-II)

Cross-sectional area        (Sq MM)  

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps 

Short Circuit Current Rating for 1Sec.duration in K. Amps 

Aluminum Conductor

Copper Conductor

Aluminum

Copper

Ground Duct Air Ground Duct Air
3cX 25    40    95    82    105    120    105    135    2.35    3.58  
3cX 35    42    115    97    125    145    125    165    3.29    5.01  
3cX 50    45    130    115    150    170    150    195    4.70    7.15  
3cX 70    49    160    140    190    210    180    240    6.58    10.01  
3cX 95    54    190    165    230    250    215    295    8.93    13.59  
3cX 120    58    220    190    260    280    240    335    11.28    17.16  
3cX 150    61    245    210    295    310    270    380    14.10    21.45  
3cX 185    65    275    240    335    350    305    430    17.39    26.46  
3cX 240    72    315    275    395    400    350    500    22.56    34.32  
3cX 300    77    355    310    450    445    390    570    28.20    42.90  
3cX 400    88    400    350    520    500    440    650    37.60    57.20  
                   

6.6 / 11 KV (6.6KV Un-Earthed/ 11 KV Earthed) Single Core AL/COPPER COND, XLPE INSULATED, CABLES As per IS:7098 (Part-II)

Cross-sectional area        (Sq MM)  

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps 

Short Circuit Current Rating for 1Sec.duration in K. Amps 

Aluminum Conductor

Copper Conductor

Aluminum

Copper

Ground Duct Air Ground Duct Air
1cX25    24    100    90    120    130    115    155    2.35    3.58  
1cX 35    25    120    105    145    155    140    185    3.29    5.00  
1cX50    26    140    125    170    185    160    220    4.70    7.15  
1cX70    28    175    155    215    225    195    275    6.58    10.01  
1cX 95    30    205    180    260    265    235    340    8.93    13.59  
1cX120    32    235    205    305    300    265    390    11.28    17.16  
1cX150    33    260    230    345    335    295    440    14.10    21.45  
1cX185    36    295    260    395    380    330    510    17.39    26.46  
1cX240    39    340    300    470    435    380    600    22.56    34.32  
1cX300    41    385    335    540    490    425    680    28.20    42.90  
1cX400    44    440    380    630    550    480    790    37.60    57.20  
1cX 500    47    495    430    730    610    530    910    47.00    71.50  
1cX 630    51    560    480    840    680    580    1030    59.22    90.10  
1cX800    57    620    530    960    740    630    1140    75.20    114.40  
1cX1000    61    680    580    1070    790    670    1250    94.00    143.00  
                   

6.6 / 11 KV (6.6KV Un-Earthed/ 11 KV Earthed) Three Core AL/COPPER COND, XLPE INSULATED, CABLES As per IS:7098 (Part-II)

Cross-sectional area        (Sq MM)  

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps 

Short Circuit Current Rating for 1Sec.duration in K. Amps 

Aluminum Conductor

Copper Conductor

Aluminum

Copper

Ground Duct Air Ground Duct Air
3cX 25    43    95    82    105    120    105    135    2.35    3.58  
3cX 35    46    115    97    125    145    125    165    3.29    5.01  
3cX 50    50    130    115    150    170    150    195    4.70    7.15  
3cX 70    54    160    140    190    210    180    240    6.58    10.01  
3cX 95    58    190    165    230    250    215    295    8.93    13.59  
3cX 120    62    220    190    260    280    240    335    11.28    17.16  
3cX 150    65    245    210    295    310    270    380    14.10    21.45  
3cX 185    70    275    240    335    350    305    430    17.39    26.46  
3cX 240    76    315    275    395    400    350    500    22.56    34.32  
3cX 300    80    355    310    450    445    390    570    28.20    42.90  
3cX 400    90    400    350    520    500    440    650    37.60    57.20  
                   

11 KV(11 KV Un-Earthed) Single Core AL/COPPER COND., XLPE INSULATED  CABLES As per IS:7098 (Part-II)

Cross-sectional area        (Sq MM)  

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps 

Short Circuit Current Rating for 1Sec.duration in K. Amps 

Aluminum Conductor

Copper Conductor

Aluminum

Copper

Ground Duct Air Ground Duct Air
1cX25    28    100    90    120    130    115    155    2.35    3.58  
1cX 35    29    120    105    145    155    140    185    3.29    5.00  
1cX50    31    140    125    170    185    160    220    4.70    7.15  
1cX70    33    175    155    215    225    195    275    6.58    10.01  
1cX 95    34    205    180    260    265    235    340    8.93    13.59  
1cX120    37    235    205    305    300    265    390    11.28    17.16  
1cX150    38    260    230    345    335    295    440    14.10    21.45  
1cX185    40    295    260    395    380    330    510    17.39    26.46  
1cX240    43    340    300    470    435    380    600    22.56    34.32  
1cX300    44    385    335    540    490    425    680    28.20    42.90  
1cX400    48    440    380    630    550    480    790    37.60    57.20  
1cX 500    53    495    430    730    610    530    910    47.00    71.50  
1cX 630    56    560    480    840    680    580    1030    59.22    90.10  
1cX800    61    620    530    960    740    630    1140    75.20    114.40  
1cX1000    65    680    580    1070    790    670    1250    94.00    143.00  
                   

11 KV(11 KV Un-Earthed) Three Core AL/COPPER COND., XLPE INSULATED  CABLES As per IS:7098 (Part-II)

Cross-sectional area        (Sq MM)  

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps 

Short Circuit Current Rating for 1Sec.duration in K. Amps 

Aluminum Conductor

Copper Conductor

Aluminum

Copper

Ground Duct Air Ground Duct Air
3cX 25    54    95    82    105    120    105    135    2.35    3.58  
3cX 35    57    115    97    125    145    125    165    3.29    5.01  
3cX 50    60    130    115    150    170    150    195    4.70    7.15  
3cX 70    64    160    140    190    210    180    240    6.58    10.01  
3cX 95    69    190    165    230    250    215    295    8.93    13.59  
3cX 120    73    220    190    260    280    240    335    11.28    17.16  
3cX 150    76    245    210    295    310    270    380    14.10    21.45  
3cX 185    80    275    240    335    350    305    430    17.39    26.46  
3cX 240    85    315    275    395    400    350    500    22.56    34.32  
3cX 300    91    355    310    450    445    390    570    28.20    42.90  
3cX 400    98    400    350    520    500    440    650    37.60    57.20  
                   
                   

38 / 66KV (66 KV Un-Earthed) single Core AL COND, XLPE INSULATED  CABLES

Cross-sectional area        (Sq MM

ARMOURED CABLE

Overall Diameter (mm)

Weight

Current Rating in Amps

Ground

Duct

1cX 95  

51

2700

195

240

1cX120  

53

2900

220

280

1cX150  

55

3300

240

335

1cX185  

58

3600

275

380

1cX240  

60

3900

320

450

1cX300  

62

4300

360

510

1cX400  

66

4900

410

595

1cX 500  

69

5300

460

690

1cX 630  

75

6400

525

800

1cX800  

79

7300

590

910

1cX1000  

84

8400

650

1010

         
         

XLPE Cable-Current Rating


XLPE Insulated Armored & Unarmored Cables:

1.1 KV SINGLE CORE AL/COPPER COND,XLPE INSULATED CABLES As per IS:7098 (Part-I)

Cross-sectional area        (Sq MM)

UN-ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
1cX4

8

 —  —  —  48  47  45  0.376  0.572
1cX 6  9  48  45  45  60  59  57  0.564  0.858
1cX10  10  62  62  61  80  78  77  0.940  1.430
1cX16  11  81  80  83  104  102  106  1.504  2.288
1cX25  12  99  90  115  130  115  145  2.350  3.575
1cX 35  13  117  110  135  155  140  175  3.290  5.005
1cX50  15  138  125  170  185  165  215  4.700  7.150
1cX70  16  168  155  210  225  200  270  6.580  10.01
1cX 95  18  204  185  255  265  235  330  8.930  13.59
1cX120  20  230  210  300  300  265  380  11.28  17.16
1cX150  22  265  230  342  335  300  430  14.10  21.45
1cX185  24  295  260  385  380  335  495  17.39  26.46
1cX240  27  340  300  450  435  385  590  22.56  34.32
1cX300  30  390  335  519  490  430  670  28.20  42.90
1cX400  33  450  380  605  550  480  780  37.60  57.20
1cX 500  36  500  430  700  610  530  900  47.00  71.50
1cX 630  40  555  485  809  680  590  1020  59.22  90.09
1cX800  47  625  530  935  740  630  1140  75.20  114.40
1cX1000  51  690  570  1065  780  660  1250  94.00  143.00
                   

1.1 KV SINGLE CORE AL/COPPER COND,XLPE INSULATED CABLES As per IS:7098 (Part-I)

Cross-sectional area        (Sq MM)

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
1cX4  10  —  —  —  48  47  45  0.376  0.572
1cX 6  11  45  45  40  60  59  57  0.56  0.858
1cX10  12  59  62  53  80  78  77  0.94  1.43
1cX16  13  76  80  73  104  102  106  1.50  2.29
1cX25  14  99  90  115  130  115  145  2.35  3.58
1cX 35  15  117  110  140  155  140  175  3.29  5.01
1cX50  17  138  125  170  185  165  215  4.70  7.15
1cX70  19  168  155  210  225  200  270  6.58  10.01
1cX 95  22  204  185  255  265  235  330  8.93  13.59
1cX120  24  230  210  300  300  265  380  11.28  17.16
1cX150  25  265  230  342  335  300  430  14.10  21.45
1cX185  28  295  260  385  380  335  495  17.39  26.46
1cX240  30  340  300  450  435  385  590  22.56  34.32
1cX300  33  390  335  519  490  430  670  28.20  42.90
1cX400  38  450  380  605  550  480  780  37.60  57.20
1cX 500  41  500  430  700  610  530  900  47.00  71.50
1cX 630  46  555  485  809  680  590  1020  59.22  90.09
1cX800  51  625  530  935  740  630  1140  75.20  114.40
1cX1000  56  690  570  1065  780  660  1250  94.00  143.00
                   

1.1 KV Two CORE AL/COPPER COND,XLPE INSULATED As per IS:7098(Part-I)

Cross-sectional area        (Sq MM)

UN-ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
2cX4  13  34  28  30  44  37  39  0.376  0.572
 2cX 6  14  43  37  40  55  47  50  0.564  0.858
2cX10  17  57  48  53  74  61  67  0.940  1.430
2cX16  17  78  61  70  94  78  85  1.50  2.29
2cX25  19  95  80  99  120  100  125  2.35  3.58
2cX35  20  116  94  117  145  120  155  3.29  5.01
2cX50  22  140  110  140  170  145  190  4.70  7.15
2cX70  25  170  140  176  210  175  235  6.58  10.01
2cX95  28  200  165  221  250  210  290  8.93  13.59
2cX120  31  225  185  258  285  240  330  11.28  17.16
2cX150  33  255  210  294  315  270  375  14.10  21.45
2cX185  37  285  235  339  355  300  435  17.39  26.46
2cX240  41  325  270  402  410  350  510  22.56  34.32
2cX300  44  370  305  461  460  390  590  28.20  42.90
2cX400  48  435  350  542  520  440  670  37.60  57.20
2cX500  54  481  405  624  580  480  750  47.00  71.50
2cX630  62  537  470  723  680  575  875  59.22  90.09
                   
                   

1.1 KV Two CORE AL/COPPER COND,XLPE INSULATED As per IS:7098(Part-I)

Cross-sectional area        (Sq MM)

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
2cX4  15  34  28  30  44  37  39  0.376  0.572
 2cX 6  16  43  37  40  55  47  50  0.564  0.858
2cX10  18  57  48  53  74  61  67  0.940  1.430
2cX16  19  78  61  70  94  78  85  1.50  2.29
2cX25  21  95  80  99  120  100  125  2.35  3.58
2cX35  23  116  94  117  145  120  155  3.29  5.01
2cX50  25  140  110  140  170  145  190  4.70  7.15
2cX70  28  170  140  176  210  175  235  6.58  10.1
2cX95  31  200  165  221  250  210  290  8.93  13.59
2cX120  34  225  185  258  285  240  330  11.28  17.16
2cX150  37  255  210  294  315  270  375  14.10  21.45
2cX185  40  285  235  339  355  300  435  17.39  26.46
2cX240  45  325  270  402  410  350  510  22.56  34.32
2cX300  49  370  305  461  460  390  590  28.20  42.90
2cX400  52  0.33  435  350  542  520  440  670  37.60
2cX500  60  0.34  481  405  624  580  480  750  47.00
2cX630  66  0.36  537  470  723  680  575  875  59.22
                   

1.1 KV Three CORE AL/COPPER COND,XLPE INSULATED As per IS:7098(Part-I)

Cross-sectional area        (Sq MM)

UN-ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
3cX 4  14  34  28  30  44  37  39  0.376  0.572
3cX 6  16  43  37  40  55  47  50  0.564  0.858
3cX 10  18  57  48  53  74  61  67  0.940  1.430
3cX 16  18  78  61  70  94  78  85  1.50  2.29
3cX 25  21  95  80  99  120  100  125  2.35  3.58
3cX 35  22  116  94  117  145  120  155  3.29  5.01
3cX 50  25  140  110  140  170  145  190  4.70  7.15
3cX 70  30  170  140  176  210  175  235  6.58  10.01
3cX 95  32  200  165  221  250  210  290  8.93  13.59
3cX 120  35  225  185  258  285  240  330  11.28  17.16
3cX 150  39  255  210  294  315  270  375  14.10  21.45
3cX 185  43  285  235  339  355  300  435  17.39  26.46
3cX 240  49  325  270  402  410  350  510  22.56  34.32
3cX 300  53  370  305  461  460  390  590  28.20  42.90
3cX 400  59  435  350  542  520  440  670  37.60  57.20
3cX 500  66  481  405  624  580  480  750  47.00  71.50
3cX 630  73  537  470  723  680  575  875  59.22  90.09
                   

1.1 KV Three CORE AL/COPPER COND., PVC INSULATED CABLES As per IS:1554 (Part-I)

Cross-sectional area        (Sq MM)

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
3cX 4  18  34  28  30  44  37  39  0.376  0.572
3cX 6  19  43  37  40  55  47  50  0.564  0.858
3cX 10  20  57  48  53  74  61  67  0.940  1.430
3cX 16  20  78  61  70  94  78  85  1.50  2.29
3cX 25  23  95  80  99  120  100  125  2.35  3.58
3cX 35  25  116  94  117  145  120  155  3.29  5.01
3cX 50  29  140  110  140  170  145  190  4.70  7.15
3cX 70  32  170  140  176  210  175  235  6.58  10.01
3cX 95  35  200  165  221  250  210  290  8.93  13.59
3cX 120  39  225  185  258  285  240  330  11.28  17.16
3cX 150  43  255  210  294  315  270  375  14.10  21.45
3cX 185  48  285  235  339  355  300  435  17.39  26.46
3cX 240  53  325  270  402  410  350  510  22.56  34.32
3cX 300  58  370  305  460  460  390  590  28.20  42.90
3cX 400  65  435  350  542  520  440  670  37.60  57.20
3cX 500  72  481  405  624  580  480  750  47.00  71.50
3cX 630  81  537  470  723  680  575  875  59.22  90.09
                   

1.1 KV Three CORE AL/COPPER COND,XLPE INSULATED As per IS:7098(Part-I)

Cross-sectional area (Sq MM)

UN-ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
 3.5X25  22  95  80  99  120  100  125  2.35  3.58
 3.5X35  24  116  94  117  145  120  155  3.29  5.01
 3.5X50  27  140  110  140  170  145  190  4.70  7.15
 3.5X70  31  170  140  176  210  175  235  6.58  10.01
 3.5X95  34  200  165  221  250  210  290  8.93  13.59
 3.5X120  38  225  185  258  285  240  330  11.28  17.16
 3.5X150  43  255  210  294  315  270  375  14.10  21.45
 3.5X185  46  285  235  339  355  300  435  17.39  26.46
 3.5X240  52  325  270  402  410  350  510  22.56  34.32
 3.5X300  57  370  305  461  460  390  590  28.20  42.90
 3.5X400  65  435  350  542  520  440  670  37.60  57.20
 3.5X500  73  481  405  624  580  480  750  47.00  71.50
 3.5X630  82  537  470  723  680  575  875  59.22  90.09
                   
                   

1.1 KV Three & Half CORE AL/COPPER COND,XLPE INSULATED As per IS:7098(Part-I)

Cross-sectional area (Sq MM)

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
 3.5X25  25  95  80  99  120  100  125  2.35  3.58
 3.5X35  27  116  94  117  145  120  155  3.29  5.01
 3.5X50  30  140  110  140  170  145  190  4.70  7.15
 3.5X70  35  170  140  176  210  175  235  6.58  10.01
 3.5X95  38  200  165  221  250  210  290  8.93  13.59
 3.5X120  42  225  185  258  285  240  330  11.28  17.16
 3.5X150  46  255  210  294  315  270  375  14.10  21.45
 3.5X185  51  285  235  339  355  300  435  17.39  26.46
 3.5X240  56  325  270  402  410  350  510  22.56  34.32
 3.5X300  60  370  305  461  460  390  590  28.20  42.90
 3.5X400  71  435  350  542  520  440  670  37.60  57.20
 3.5X500  79  481  405  624  580  480  750  47.00  71.50
 3.5X630  88  537  470  723  680  575  875  59.22  90.09
 

1.1 KV Four CORE AL/COPPER COND,XLPE INSULATED  CABLES As per IS:7098 (Part-I)

Cross-sectional area (Sq MM)

UN-ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
4cX4  17  34  28  30  44  37  39  0.376  0.572
4cX 6  18  43  37  40  55  47  50  0.564  0.858
4cX 10  20  57  48  53  74  61  67  0.940  1.430
4cX16  20  78  61  70  94  78  85  1.50  2.29
4cX 25  24  95  80  99  120  100  125  2.35  3.58
4cX35  26  116  94  117  145  120  155  3.29  5.01
4cX 50  29  140  110  140  170  145  190  4.70  7.15
4cX70  34  170  140  176  210  175  235  6.58  10.01
4cX 95  37  200  165  221  250  210  290  8.93  13.59
4cX 120  41  225  185  258  285  240  330  11.28  17.16
4cX 150  45  255  210  294  315  270  375  14.10  21.45
4cX 185  50  285  235  339  355  300  435  17.39  26.46
4cX 240  56  325  270  402  410  350  510  22.56  34.32
4cX 300  63  370  305  461  460  390  590  28.20  42.90
4cX 400  70  435  350  542  520  440  670  37.60  57.20
4cX 500  79  481  405  624  580  480  750  47.00  71.50
4cX 630  88  537  470  723  680  575  875  59.22  90.09
                   

1.1 KV Four CORE AL/COPPER COND,XLPE INSULATED  CABLES As per IS:7098 (Part-I)

Cross-sectional area (Sq MM)

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
4cX4  18  34  28  30  44  37  39  0.376  0.572
4cX 6  19  43  37  40  55  47  50  0.564  0.858
4cX 10  21  57  48  53  74  61  67  0.940  1.430
4cX16  22  78  61  70  94  78  85  1.50  2.29
4cX 25  26  95  80  99  120  100  125  2.35  3.58
4cX35  28  116  94  117  145  120  155  3.29  5.01
4cX 50  32  140  110  140  170  145  190  4.70  7.15
4cX70  37  170  140  176  210  175  235  6.58  10.01
4cX 95  40  200  165  221  250  210  290  8.93  13.59
4cX 120  44  225  185  258  285  240  330  11.28  17.16
4cX 150  49  255  210  294  315  270  375  14.10  21.45
4cX 185  54  285  235  339  355  300  435  17.39  26.46
4cX 240  65  325  270  402  410  350  510  22.56  34.32
4cX 300  68  370  305  460  460  390  590  28.20  42.90
4cX 400  76  435  350  542  520  440  670  37.60  57.20
4cX 500  86  481  405  624  580  480  750  47.00  71.50
4cX 630  94  537  470  723  680  575  875  59.22  90.09

PCV Cable-Current Rating


Flexible PVC Insulated Cable

PVC- Insulated 1 core unsheathed / Sheathed flexible cord with copper conductor 1.1KV                                         ( As per IS:694 – 1990 )

Nominal Conductor Area (Sq.mm)

UN SHEATHED

SHEATHED

Maximum Conductor Resistance at 20°C (ohm/km)

Current Carrying Capacity (Amp)

Nominal Thickness of Insulation (mm)

Maximum overall Diameter (mm)

Approx. Weight (kg/100m)

Nominal Thickness of Insulation (mm)

Nominal Thickness of Sheath (mm)

Maximum Overall Diameter (mm)

Approx. Weight (kg/100m)

1cX0.5

0.6

2.3

0.9

0.6

0.9

4.5

2.6

39

4

1cX0.75

0.6

2.5

1.2

0.6

0.9

4.7

3

26

7

1cX1

0.6

2.7

1.5

0.6

0.9

4.9

3.4

19.5

11

1cX1.5

0.6

3

2

0.6

0.9

5.4

4.3

13.3

15

1cX2.5

0.7

3.7

3.2

0.7

1

6.2

6

7.98

19

1cX4

0.8

4.4

4.9

0.8

1

7

8.2

4.95

26

1cX6

0.8

5

7

0.8

1

7.4

10.3

3.3

35

1cX10

1

7

11.9

1

1

7.7

13

1.91

46

1cX16

1

9

20.1

1

1

9.8

21.8

1.21

62

1cX25

1.2

10

27.4

1.2

1.1

12

32.1

0.78

80

1cX35

1.2

11.4

36.7

1.2

1.1

13

42.7

0.554

102

1cX50

1.4

13.5

52.5

1.4

1.2

15

59.7

0.386

138

1cX70

1.4

16

72.3

1.4

1.2

17.8

80.6

0.272

214

1cX95

1.6

18

96.1

1.6

1.4

20.7

108.1

0.206

254

1cX120

1.6

20.5

120.3

1.6

1.4

22.2

132.5

0.161

300

                   

PVC- Insulated and PVC – Sheathed 2 core flexible cord with copper conductor 1.1KV                                     ( As per IS:694 – 1990 )

Nominal Conductor Area (Sq.mm)

Nominal Thickness of Insulation (mm)

Nominal Thickness of Sheath (mm)

2 CORE CIRCULAR

2 CORE FLAT

Maximum Conductor Resistance at 20°C (ohm/km)

Current Carrying Capacity (Amp)

Maximum overall Diameter (mm)

Approx. Weight (kg/100m)

Maximum overall Diameter (mm)

Approx. Weight (kg/100m)

2cX0.5

0.6

0.9

7.2

5.5

4.9×7.2

4.7

39

4

2cX0.75

0.6

0.9

7.8

6.5

5.2×7.8

5.5

26

7

2cX1

0.6

0.9

8

7.5

5.4×8.0

6.3

19.5

11

2cX1.5

0.6

0.9

8.6

9.2

5.6×8.6

8

13.3

15

2cX2.5

0.7

1

10.5

13.5

6.6×10.5

11.2

7.98

19

2cX4

0.8

1

12

19

7.2×12.0

15.8

4.95

26

                   

PVC- Insulated and PVC – Sheathed 3 core flexible cord with copper conductor 1.1KV                                           ( As per IS:694 – 1990 )

Nominal Conductor Area (Sq.mm)

Nominal Thickness of Insulation (mm)

Nominal Thickness of Sheath (mm)

3 CORE CIRCULAR

3 CORE FLAT

Maximum Conductor Resistance at 20°C (ohm/km)

Current Carrying Capacity (Amp)

Maximum overall Diameter (mm)

Approx. Weight (kg/100m)

Maximum overall Diameter (mm)

Approx. Weight (kg/100m)

3cX0.5

0.6

0.9

7.6

6.4

0.9

5.1

39

4

3cX0.75

0.6

0.9

8.2

7.6

0.9

6.1

26

7

3cX1

0.6

0.9

9.2

10.9

0.9

7.1

19.5

11

3cX1.5

0.6

0.9

9.2

10.9

0.9

8.7

13.3

15

3cX2.5

0.7

1

11

16.2

1

13

7.98

19

3cX4

0.8

1

12.5

23.7

1

18.6

4.95

26

 PVC Insulated Armored & Unarmored Cables:

1.1 KV SINGLE CORE AL/COPPER COND., PVC INSULATED CABLES As per IS:1554 (Part-I)

Cross-sectional area        (Sq MM)

UN-ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminum

Copper

Ground Duct Air Ground Duct Air
1cX4  8  —  —  —  39  38  35  0.304  0.460
1cX 6  9  39  37  35  49  48  44  0.456  0.690
1cX10  10  51  51  47  65  64  60  0.760  1.150
1cX16  11  66  65  64  85  83  82  1.220  1.84
1cX25  13  86  84  84  110  110  110  1.900  2.88
1cX 35  14  100  100  105  130  125  130  2.660  4.03
1cX50  16  120  115  130  155  150  165  3.800  5.75
1cX70  17  140  135  155  190  175  205  5.320  8.05
1cX 95  19  175  155  190  220  200  245  7.220  10.90
1cX120  21  195  170  220  250  220  280  9.120  13.80
1cX150  23  220  190  250  280  245  320  11.40  17.30
1cX185  25  240  210  290  305  260  370  14.10  21.30
1cX240  28  270  225  335  345  285  425  18.20  27.30
1cX300  30  295  245  380  375  310  475  22.80  34.50
1cX400  35  325  275  435  400  335  550  30.40  46.00
1cX 500  38  345  295  480  425  355  590  38.00  57.50
1cX 630  43  390  320  550  470  375  660  47.90  72.50
1cX800  48  450  380  610  530  425  725  60.80  92.00
1cX1000  52  500  415  680  590  740  870  76.00  115.00
                   

1.1 KV SINGLE CORE AL/COPPER COND., PVC INSULATED CABLES As per IS:1554 (Part-I)

Cross-sectional area        (Sq MM)

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
1cX4  11  31  30  27  39  38  35  0.304  0.460
1cX 6  12  39  37  35  49  48  44  0.456  0.690
1cX10  13  51  51  47  65  64  60  0.760  1.150
1cX16  14  66  65  64  85  83  82  1.220  1.840
1cX25  15  86  84  84  110  110  110  1.900  2.880
1cX 35  16  100  100  105  130  125  130  2.660  4.030
1cX50  18  120  115  130  155  150  165  3.800  5.750
1cX70  20  140  135  155  190  175  205  5.320  8.050
1cX 95  22  175  155  190  220  200  245  7.220  10.90
1cX120  24  195  170  220  250  220  280  9.120  13.80
1cX150  26  220  190  250  280  245  320  11.400  17.30
1cX185  29  240  210  290  305  260  370  14.100  21.30
1cX240  32  270  225  335  345  285  425  18.200  27.60
1cX300  33  295  245  380  375  310  475  22.800  34.50
1cX400  39  325  275  435  400  335  550  30.400  46.00
1cX 500  42  345  295  480  425  355  590  38.000  57.50
1cX 630  48  390  320  550  470  375  660  47.880  72.50
1cX800  52  450  380  610  530  423  725  60.800  92.00
1cX1000  59  500  414  680  590  471  870  76.000  115.00
                   

1.1 KV Two CORE AL/COPPER COND., PVC INSULATED, UN-ARMOURED CABLES As per IS:1554 (Part-I)

Cross-sectional area        (Sq MM)

UN-ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
2cX4  14  32  27  27  41  35  35  0.304  0.460
 2cX 6  17  40  34  35  50  44  45  0.456  0.690
2cX10  18  55  45  47  70  58  60  0.760  1.150
2cX16  17  70  58  59  90  75  78  1.220  1.840
2cX25  19  90  76  78  115  97  105  1.900  2.880
2cX35  21  110  92  99  140  120  125  2.660  4.030
2cX50  24  135  115  125  165  145  155  3.800  5.750
2cX70  26  160  140  150  205  180  195  5.320  8.050
2cX95  30  190  170  185  240  215  230  7.220  10.90
2cX120  32  210  190  210  275  235  265  9.120  13.80
2cX150  34  240  210  240  310  270  305  11.40  17.300
2cX185  38  275  240  275  350  300  350  14.10  21.280
2cX240  42  320  275  325  405  345  410  18.20  27.600
2cX300  46  355  305  365  450  385  465  22.80  34.500
2cX400  52  385  345  420  490  485  530  30.40  46.000
2cX500  54  425  380  475  540  460  605  38.00  57.500
2cX630  65  465  415  540  640  550  785  47.90  72.550
                   
                   
                   

1.1 KV Two CORE AL/COPPER COND., PVC INSULATEDCABLES As per IS:1554 (Part-I)

Cross-sectional area        (Sq MM)

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
2cX4  18  32  27  27  41  35  35  0.304  0.460
 2cX 6  19  40  34  35  50  44  45  0.456  0.690
2cX10  20  55  45  47  70  58  60  0.760  1.150
2cX16  20  70  58  59  90  75  78  1.220  1.840
2cX25  22  90  76  78  115  97  105  1.90  2.880
2cX35  23  110  92  99  140  120  125  2.66  4.030
2cX50  26  135  115  125  165  145  155  3.80  5.750
2cX70  29  160  140  150  205  180  195  5.32  8.050
2cX95  33  190  170  185  240  215  230  7.22  10.90
2cX120  35  210  190  210  275  235  265  9.12  13.80
2cX150  37  240  210  240  310  270  305  11.40  17.30
2cX185  41  275  240  275  350  300  350  14.10  21.30
2cX240  47  320  275  325  405  345  410  18.20  27.60
2cX300  50  355  305  365  450  385  465  22.80  34.50
2cX400  58  385  345  420  490  485  530  30.40  46.00
2cX500  64  425  380  475  540  460  605  38.00  57.50
2cX630  72  465  415  540  640  550  785  47.90  72.50
                   

1.1 KV Three CORE AL/COPPER COND., PVC INSULATED CABLES As per IS:1554 (Part-I)

Cross-sectional area        (Sq MM)

UN-ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
3cX 4  16  28  23  23  36  30  30  0.304  0.460
3cX 6  18  35  30  30  45  38  39  0.456  0.690
3cX 10  19  46  39  40  60  50  52  0.760  1.150
3cX 16  19  60  50  51  77  64  66  1.220  1.840
3cX 25  22  76  63  70  99  81  90  1.900  2.880
3cX 35  24  92  77  86  120  99  110  2.660  4.030
3cX 50  27  110  95  105  145  125  135  3.800  5.750
3cX 70  30  135  115  130  175  150  165  5.320  8.050
3cX 95  34  165  140  155  210  175  200  7.220  10.900
3cX 120  37  185  155  180  240  195  230  9.120  13.800
3cX 150  40  210  175  205  270  225  265  11.40  17.300
3cX 185  44  235  200  240  300  255  305  14.10  21.300
3cX 240  50  275  235  280  345  295  355  18.20  27.600
3cX 300  55  305  260  315  385  335  400  22.80  34.500
3cX 400  62  335  290  375  425  360  435  30.40  46.000
3cX 500  69  370  320  425  470  390  520  38.00  57.500
3cX 630  77  405  350  480  555  470  675  47.90  72.500
                   

1.1 KV Three CORE AL/COPPER COND., PVC INSULATED CABLES As per IS:1554 (Part-I)

Cross-sectional area        (Sq MM)

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
3cX 4  18  28  23  23  36  30  30  0.304  0.460
3cX 6  19  35  30  30  45  38  39  0.456  0.690
3cX 10  21  46  39  40  60  50  52  0.760  1.150
3cX 16  21  60  50  51  77  64  66  1.220  1.840
3cX 25  23  76  63  70  99  81  90  1.900  2.880
3cX 35  26  92  77  86  120  99  110  2.660  4.030
3cX 50  29  110  95  105  145  125  135  3.800  5.750
3cX 70  33  135  115  130  175  150  165  5.320  8.050
3cX 95  37  165  140  155  210  175  200  7.220  10.900
3cX 120  39  185  155  180  240  195  230  9.120  13.800
3cX 150  43  210  175  205  270  225  265  11.400  17.300
3cX 185  49  235  200  240  300  255  305  14.100  21.300
3cX 240  54  275  235  280  345  295  355  18.200  27.600
3cX 300  59  305  260  315  385  335  400  22.800  34.500
3cX 400  68  335  290  375  425  360  435  30.400  46.000
3cX 500  75  370  320  425  470  390  520  38.000  57.500
3cX 630  84  405  350  480  555  470  675  47.900  72.500
                   

1.1 KV Three & Half CORE AL/COPPER COND., PVC INSULATED CABLES As per IS:1554 (Part-I)

Cross-sectional area (Sq MM)

UN-ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
 3.5X25  24  76  63  70  99  81  90  1.90  2.88
 3.5X35  26  92  77  86  120  99  110  2.66  4.03
 3.5X50  29  110  95  105  145  125  135  3.80  5.75
 3.5X70  32  135  115  130  175  150  165  5.32  8.05
 3.5X95  36  165  140  155  210  175  200  7.22  10.90
 3.5X120  40  185  155  180  240  195  230  9.12  13.80
 3.5X150  44  210  175  205  270  225  265  11.40  17.30
 3.5X185  48  235  200  240  300  255  305  14.10  21.30
 3.5X240  54  275  235  280  345  295  355  18.20  27.60
 3.5X300  62  305  260  315  385  335  400  22.80  34.50
 3.5X400  68  335  290  375  425  360  435  30.40  46.00
 3.5X500  77  370  320  425  470  390  520  38.00  57.50
 3.5X630  87  405  350  480  555  470  675  47.90  72.50
                   
                   

1.1 KV Three & Half CORE AL/COPPER COND., PVC INSULATED CABLES As per IS:1554 (Part-I)

Cross-sectional area (Sq MM)

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
 3.5X25  26  76  63  70  99  81  90  1.90  2.88
 3.5X35  28  92  77  86  120  99  110  2.66  4.03
 3.5X50  31  110  95  105  145  125  135  3.80  5.75
 3.5X70  36  135  115  130  175  150  165  5.32  8.05
 3.5X95  39  165  140  155  210  175  200  7.22  10.90
 3.5X120  43  185  155  180  240  195  230  9.12  13.80
 3.5X150  47  210  175  205  270  225  265  11.40  17.30
 3.5X185  53  235  200  240  300  255  305  14.10  21.30
 3.5X240  58  275  235  280  345  295  355  18.20  27.60
 3.5X300  65  305  260  315  385  335  400  22.80  34.50
 3.5X400  75  335  290  375  425  360  435  30.40  46.00
 3.5X500  84  370  320  425  470  390  520  38.00  57.50
 3.5X630  92  405  350  480  555  470  675  47.90  72.50
                   

1.1 KV Four CORE AL/COPPER COND., PVC INSULATED  CABLES As per IS:1554 (Part-I)

Cross-sectional area (Sq MM)

UN-ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
4cX4  16  28  23  23  36  30  30  0.304  0.460
4cX 6  18  35  30  30  45  38  39  0.456  0.690
4cX 10  20  46  39  40  60  50  52  0.760  1.150
4cX16  23  60  50  51  77  64  66  1.220  1.840
4cX 25  26  76  63  70  99  81  90  1.900  2.880
4cX35  30  92  77  86  120  99  110  2.660  4.030
4cX 50  34  110  95  105  145  125  135  3.800  5.750
4cX70  38  135  115  130  175  150  165  5.320  8.050
4cX 95  43  165  140  155  210  175  200  7.220  10.900
4cX 120  46  185  155  180  240  195  230  9.120  13.800
4cX 150  51  210  175  205  270  225  265  11.400  17.300
4cX 185  55  235  200  240  300  255  305  14.100  21.300
4cX 240  60  275  235  280  345  295  355  18.200  27.600
4cX 300  66  305  260  315  385  335  400  22.800  34.500
4cX 400  73  335  290  375  425  360  435  30.400  46.000
4cX 500  82  370  320  425  470  390  520  38.000  57.500
4cX 630  92  405  350  480  555  470  675  47.900  72.500
                   

1.1 KV Four CORE AL/COPPER COND., PVC INSULATED  CABLES As per IS:1554 (Part-I)

Cross-sectional area (Sq MM)

ARMOURED CABLE

Overall Diameter (mm)

Normal Current Rating in Amps

Short Circuit Current Rating for 1Sec.duration in K. Amps

Aluminums Conductor

Copper Conductor

Aluminums

Copper

Ground Duct Air Ground Duct Air
4cX4  18  28  23  23  36  30  30  0.304  0.460
4cX 6  21  35  30  30  45  38  39  0.456  0.690
4cX 10  22  46  39  40  60  50  52  0.760  1.150
4cX16  23  60  50  51  77  64  66  1.220  1.840
4cX 25  27  76  63  70  99  81  90  1.900  2.880
4cX35  30  92  77  86  120  99  110  2.660  4.030
4cX 50  34  110  95  105  145  125  135  3.800  5.750
4cX70  37  135  115  130  175  150  165  5.320  8.050
4cX 95  42  165  140  155  210  175  200  7.220  10.900
4cX 120  47  185  155  180  240  195  230  9.120  13.800
4cX 150  51  210  175  205  270  225  265  11.40  17.300
4cX 185  56  235  200  240  300  255  305  14.10  21.300
4cX 240  62  275  235  280  345  295  355  18.20  27.600
4cX 300  70  305  260  315  385  335  400  22.80  34.500
4cX 400  76  0.70  335  290  375  425  360  435  30.40
4cX 500  86  0.70  370  320  425  470  390  520  38.00
4cX 630  96  0.70  405  350  480  555  470  675  47.90

Minimum Electrical Clearance.


Minimum Electrical Clearance As Per BS:162.

INDOOR
Voltage in KV Phase to earth in mm Phase to phase in mm
0.415 15.8 19.05
0.600 19.05 19.05
3.3 50.8 50.8
6.6 63.5 88.9
11 76.2 127.0
15 101.6 165.1
22 139.7 241.3
33 222.25 355.6

Minimum Electrical Clearance As Per BS:162.

OUTDOOR
Voltage in KV Phase to earth in mm Phase to phase in mm
6.6 139.7 177.8
11 177.8 228.6
22 279.4 330.2
33 381 431.8
66 685.8 787.4
110 863.6 990.6
132 1066.8 1219.2
220 1778 2057.4

 Minimum Working Clearance:

OUTDOOR SWITCHYARD
Voltage in KV To ground in mm Between section(mm)
11 2750 2500
33 3700 2800
66 4000 3000
132 4600 3500
220 5500 4500

 Minimum Ground Clearance As Per IE-1956(Rule 77)

Voltage in KV To ground in mm
132 6.10
220 7.00
400 8.84
800 12.40
   

Minimum Clearance between Lines Crossing Each Other (IE-1957)

System Voltage 132KV 220KV 400KV 800KV
Low & Medium 3.05 4.58 5.49 7.94
11-66KV 3.05 4.58 5.49 7.94
132KV 3.05 4.58 5.49 7.94
220KV 4.58 4.58 5.49 7.94
400KV 5.49 5.49 5.49 7.94
800KV 7.94 7.94 7.94 7.94

Minimum Height above Railway As Per IE-1957

Voltage Broad Meter & Narrow Gauges
Above 66KV up to 132KV 14.60 Meter
Above 132KV up to 220KV 15.40 Meter
Above 220KV up to 400KV 17.90 Meter
Above 400KV up to 500KV 19.30 Meter
Above 500KV up to 800KV 23.40 Meter

Various Air clearances to be provided as per IE rule 64

Voltage KV 33KV 66KV 110KV 220KV 400KV
BIL (Kvp) 170 325 550 1050 1425
P-E (cm) 30 63 115 240 350
P-P(cm) 40 75 135 210 410
P-G (Meter) 3.7 4.0 4.6 5.5 8.0
Section Clearance(Mt) 2.8 3.0 3.5 4.3 6.5

Clearances from Buildings of HT and EHT voltage lines IE Rule 80

Vertical Distance
High voltage lines up to 33KV 3.7 Meter
Extra High Voltage 3.7 Meter + Add 0.3 meter for every additional 33KV
Horizontal clearance between the conductor and Building
High Voltage Up to 11 KV 1.2 Meter
11KV To 33KV 2.0 Meter
Extra High Voltage 2.0 Meter + Add 0.3 meter for every additional 33KV

Clearance above ground of the lowest conductor As per IE Rule 77

Over head Line Across Street
Low and Medium Voltage 5.8 Meter
High Voltage 6.1 Meter
Over head Line Along  Street (Parallel To Street)
Low and Medium Voltage 5.5 Meter
High Voltage 5.8  Meter
Over head Line Without Across or Along  Street
Low/Medium /HT line up to 11KV If Bare Conductor 4.6 Meter
Low/Medium /HT line up to 11KV If Insulated Conductor 4.0 Meter
Above 11  KV Line 5.2 Meter
Above 33KV Line 5.8 Meter + Add 0.3 meter for every additional 33KV

Clearance between conductors and Trolley / Tram wires (IE Rule 78)

Low and Medium Voltage 1.2 Meter
High Voltage Line Up to 11KV 1.8 Meter
High Voltage Line Above to 11KV 2.5 Meter
Extra High Voltage Line 3.0 Meter
   

 Clearances from Buildings of low & medium voltage lines(IE Rule 79 )

For  Flat roof, Open Balcony, Verandah Roof and lean to Roof
Line Passes Over Building Vertical Clearance 2.5 Meter
Line Passes Adjustment of Building Horizontal Clearance 1.2 Meter
For pitched Roof
Line Passes Over Building Vertical Clearance 2.5 Meter
Line Passes Adjustment of Building Horizontal Clearance 1.2 Meter

Fuse


What is Fuses

  • A fuse is a device that protects a circuit from an over current condition only.  It has a fusible link directly heated and destroyed by the current passing through it.    A  fuse  contains  a  current- carrying element sized so that the heat generated by the flow of normal current through it does not  cause  it  to  melt  the  element;  however,  when  an  over current  or  short-circuit  current  flows through the fuse, the fusible link will melt and open the circuit.
  • A device that protects a circuit by fusing opens its current-responsive element when an over-current passes through it. An over-current is either due to an overload or a short circuit condition.
  • The Underwriter Laboratories (UL) classifies fuses by letters e.g. class CC, T, K, G, J, L, R, and so forth. The class letter may designate interrupting rating, physical dimensions, and degree of current limitation.
  • As per NEC and ANSI/IEEE standard 242 [2] – A current limiting fuse is a fuse that will interrupt all available currents above its threshold current and below its maximum interrupting rating, limit the clearing time at rated voltage to an interval equal to or less than the first major or symmetrical loop duration, and limit peak let-through current to a value less than the peak that would be possible with the fuse replaced by a solid conductor of the same impedance.

Fuse Construction:

  • The typical fuse consists of an element which is surrounded by filler and enclosed by the fuse body. The element is welded or soldered to the fuse contacts (blades or ferrules).
  • The element is a calibrated conductor. Its configuration, mass and the materials employed are selected to achieve the desired electrical and thermal characteristics.
  • The element provides the current path through the fuse. It generates heat at a rate dependent on its resistance and the load current.
  • The heat generated by the element is absorbed by the filler and passed through the fuse body to the surrounding air. The filler material, such as quartz sand, provides effective heat transfer and allows for the small element cross-section typical in modern fuses.
  • The effective heat transfer allows the fuse to carry harmless overloads .The small element cross section melts quickly under short-circuit conditions. The filler also aids fuse performance by absorbing arc energy when the fuse clears an overload or short circuit.
  • When a sustained overload occurs, the element will generate heat at a faster rate than the heat can be passed to the filler. If the overload persists, the element will reach its melting point and open. Increasing the applied current will heat the element faster and cause the fuse to open sooner. Thus, fuses have an inverse time current characteristic: that is, the greater the over current, the less time required for the fuse to open the circuit.
  • This characteristic is desirable because it parallels the characteristics of conductors, motors, transformers, and other electrical apparatus. These components can carry low-level overloads for relatively long periods without damage. However, under high-current conditions, damage can occur quickly. Because of its inverse time current characteristic, a properly applied fuse can provide effective protection over a broad current range, from low-level overloads to high-level short circuits.

Commonly used terms for Fuse

  • I2t (Ampere Square second): A measure of the thermal energy associated with current flow.I2t is equal to (I RMS) 2 X t, where is the duration of current flow in seconds.A measure of thermal energy associated with current flow. It can be expressed as melting I2t, arcing I2t or the sum of them as Clearing I2t. Clearing I2t is the total I2t passed by a fuse as the fuse clears a fault, with t being equal to the time elapsed from the initiation of the fault to the instant the fault has been cleared. Melting I2t is the minimum I2t required to melt the fuse element
  • Interrupting Rating (Abbreviated I.R.)Same as breaking capacity or short circuit rating. The maximum current a fuse can safely interrupt at rated voltage. Some special purpose fuses may also have a “Minimum Interrupting Rating”. This defines the minimum current that a fuse can safely interrupt. Safe operation requires that the fuse remain intact. Interrupting ratings may vary with fuse design and range from 35 amperes AC for some 250V metric size (5 x 20mm) fuses up to 200,000 amperes AC for the 600V industrial fuses (for example, ATDR series).
  • Clearing I2t: The total I2t passed by a fuse as the fuse clears a fault, with being equal to the time elapsed from the initiation of the fault to the instant the fault has been cleared.
  • Melting I2t: The minimum I2t required melting the fuse element.
  • Ampere Rating: The continuous current carrying capability of a fuse under defined laboratory conditions. The ampere rating is marked on each fuse.
  • Available Fault Current: The maximum short-circuit current that can flow in an unprotected circuit.
  • Coordination: The use of over current protective devices that will isolate only that portion of an electrical system that has been overloaded or faulted.
  • Current limiting Range: currents a fuse will clear in less than ½ cycles, thus limiting the actual magnitude of current flow.
  • Element: A calibrated conductor inside a fuse that melts when subjected to excessive current. The element is enclosed by the fuse body and may be surrounded by an arc quenching medium such as silica sand. The element is sometimes referred to as a link.
  • Fast acting Fuse: This is a fuse with no intentional time-delay designed into the overload range. It is sometimes referred to as a “single-element fuse” or “non-delay fuse.”
  • Fault Current: Short-circuit current that flows partially or entirely outside the intended normal load current path of a circuit component. Values may be from hundreds to many thousands of amperes.
  1. Ferrule: copper mounting terminals of fuses with amp ratings up to 60 amperes. The cylindrical terminals at each end of a fuse fit into fuse clips.
  • Current limiting Fuse: A fuse that meets the following three conditions:
  1. 1. interrupts all available over currents within its interrupt rating.
  1. 2. Within its current limiting range, limits the clearing time at rated voltage to an interval equal to, or less than, the first major or symmetrical current loop duration.
  1. 3. Limits peak let-through current to a value less than the available peak current. The maximum level of fault current that the fuse has been tested to safely interrupt.
  • Arcing timeThe amount of time from the instant the fuse link has melted until the over current is interrupted, or cleared.
  • Clearing time The total time between the beginning of the over current and the final opening of the circuit at rated voltage by an over current protective device. Clearing time is the total of the melting time and the arcing time.
  • Fast acting fuse A fuse which opens on overload and short circuits very quickly. This type of fuse is not designed to withstand temporary overload currents associated with some electrical loads. UL listed or recognized fast acting fuses would typically open within 5 seconds maximum when subjected to 200% to 250% of its rated current.IEC has two categories of fast acting fuses:
  1. F= quick acting, opens 10x rated current within 0.001 seconds to 0.01 seconds
  1. FF = very quick acting, opens 10x rated current in less than 0.001 seconds
  • Overload Can be classified as an over current which exceeds the normal full load current of a circuit by 2 to 5 times its magnitude and stays within the normal current path.
  • Resistive load An electrical load which is characterized by not drawing any significant inrush current. When a resistive load is energized, the current rises instantly to its steady state value, without first rising to a higher value.
  •  RMS Current The R.M.S. (root mean square) value of any periodic current is equal to the value of the direct current which,flowing through a resistance, produces the same heating effect in the resistance as the periodic current does.
  • Short circuit An over current that leaves the normal current path and greatly exceeds the normal full load current of the circuit by a factor of tens, hundreds, or thousands times.
  • Time delay fuse A fuse with a built-in time delay that allows temporary and harmless inrush currents to pass without operating, but is so designed to open on sustained overloads and short circuits. UL listed or recognized time delay fuses typically open in 2 minutes maximum when subjected to 200% to 250% of rated current. IEC has two categories of time delay fuses:
  1. T= time lag, opens 10x rated current within 0.01 seconds to 0.1 seconds
  1. TT = long time lag, opens 10x rated current within 0.1 seconds to 1 second
  •  Voltage rating A maximum open circuit voltage in which a fuse can be used, yet safely interrupt an over current. Exceeding the Voltage rating of a fuse impairs its ability to clear an overload or short circuit safely.
  • Over current A condition which exists in an electrical circuit when the normal load current is exceeded. Over currents take on two separate characteristics-overloads and short circuits.
  • Threshold Current: The magnitude of symmetrical RMS available current at the threshold of the current-limiting range, where the fuse becomes current-limiting when tested to the industry standard.
  •  Threshold ratio: A threshold ratio is a relationship of threshold current to a fuse’s continuous current rating.

             Threshold Ratio = Fuse Threshold Current / Fuse Continuous Current.

Maximum threshold ratio for various types of fuses:

Fuse Class Ratio
CLASS RK5 65
CLASS RK1 30
CLASS J 30
CLASS CC 30
CLASS L 30 (601-1200 Amps)
CLASS L 35(1201-2000 Amps)
CLASS L 40 (2001-4000 Amps)
  • A current limiting fuse may be current limiting or may not be current limiting. The current limiting characteristic depends on the threshold ratio and available fault current.
  • Let’s consider an example of 1500 kVA radial service feeding a fusible switchboard with 2000 amps class L fuses. As per ANSI C 57 [3] standard, a typical impedance value for this size of a transformer is 5.75%; this value is a key factor in calculating the short circuit current.
  • All utility’s network provides a specific fault current at a specific location which depends on various factors, e.g.; cable lengths, cable size, X/R ratio and etc. If we ignore this limitation and assume that there is an unlimited fault current available from a utility, then let’s calculate short circuit current from a 1500 kVA transformer at 480 volts
  • The formula to calculate short circuit current (Isc)
  • ISC = (KVA X 10,000) / (1.732 X VOLT X %Z).
  • ISC = 1500 X 10,000 / 1.732 X 480 X 5.75
  • ISC = 31378.65 Amp.

Type of Fuse:

  •  A fuse unit essentially consists of a metal fuse element or link, a set of contacts between which it is fixed and a body to support and isolate them. Many types of fuses also have some means for extinguishing the arc which appears when the fuse element melts. In general, there are two categories of fuses.
  1. Low voltage fuses.
  2.  High voltage fuses.
  • Usually isolating switches are provided in series with fuses where it is necessary to permit fuses to be replaced or rewired with safety.
  •  In absence of such isolation means, the fuses must be so shielded as to protect the user against accidental contact with the live metal when the fuse is being inserted or removed.

LOW VOLTAGE FUSES

  • Low voltage fuses can be further divided into two classes namely
  1. Semi-enclosed or Rewire able type.
  2. Totally enclosed or Cartridge type.

(1) Re Wire able Fuse:

  • The most commonly used fuse in ‘house wiring’ and small current circuit is the semi-enclosed or rewire able fuse. (also sometime known as KIT-KAT type fuse). It consist of a porcelain base carrying the fixed contacts to which the incoming and outgoing live or phase wires are connected and a porcelain fuse carrier holding the fuse element, consisting of one or more strands of fuse wire, stretched between its terminals.

  •  The fuse carrier is a separate part and can be taken out or inserted in the base without risk, even without opening the main switch. If fuse holder or carrier gets damaged during use, it may be replaced without replacing the complete unit.
  • The fuse wire may be of lead, tinned copper, aluminum or an alloy of tin lead.
  • The actual fusing current will be about twice the rated current. When two or more fuse wire are used, the wires should be kept apart and a de rating factor of 0.7 to 0.8 should be employed to arrive at the total fuse rating.
  • The specification for re wire able fuses are covered by IS: 2086-1963. Standard ratings are 6, 16, 32, 63, and 100A.
  • A fuse wire of any rating not exceeding the rating of the fuse may be used in it that is a 80 A fuse wire can be used in a 100 A fuse, but not in the 63 A fuse. On occurrence of a fault, the fuse element blows off and the circuit is interrupted. The fuse carrier is pulled out, the blown out fuse element is replaced by new one and the supply can is resorted by re-inserting the fuse carrier in the base.
  • Though such fuses have the advantage of easy removal or replacement without any danger of coming into the contact with a lie part and negligible replacement cost but suffers from following disadvantages:
  1. Unreliable Operations.
  2. Lack of Discrimination.
  3. Small time lag.
  4. Low rupturing capacity.
  5. No current limiting feature.
  6. Slow speed of operations.

(2) Totally Enclosed Or Cartridges Type Fuse:

  •  The fuse element is enclosed in a totally enclosed container and is provided with metal contacts on both sides. These fuses are further classified as
  1. D-type.
  2. Link type.
  • Link type cartridges are again of two type’s viz. Knife blade or bolted type.

A) D- Type Cartridges Fuses

  • It is a non interchangeable fuse comprising s fuse base, adapter ring, cartridge and a fuse cap. The cartridge is pushed in the fuse cap and the cap is screwed on the fuse base. On complete screwing the cartridge tip touches the conductor and circuit between the two terminals is completed through the fuse link. The standard ratings are 6, 16, 32, and 63 amperes.

  • The breaking or rupturing capacity is of the order of 4k A for 2 and 4 ampere fuses the 16k A for 63 A fuses.
  • D-type cartridge fuse have none of the drawbacks of the re wire able fuses. Their operation is reliable. Coordination and discrimination to a reasonable extent and achieved with them.

B) Link type Cartridge or High Rupturing Capacity (HRC)

  • Where large numbers of concentrations of powers are concerned, as in the modern distribution system, it is essential that fuses should have a definite known breaking capacity and also this breaking capacity should have a high value. High rupturing capacity cartridge fuse, commonly called HRC cartridge fuses, have been designed and developed after intensive research by manufactures and supply engineers in his direction.

  • The usual fusing factor for the link fuses is 1.45. the fuses for special applications may have as low as a fusing factor as 1.2.
  • The specification for medium voltage HRC link fuses are covered under IS: 2202-1962.

        (A) Knife Blade Type HRC Fuse:


  • It can be replaced on a live circuit at no load with the help of a special insulated fuse puller.

       (B) Bolted Type HRC Link Fuse:

                                            

   

  • it has two conducting plates on either ends. These are bolted on the plates of the fuse base. Such a fuse needs an additional switch so that the fuse can be taken out without getting a shock.
  • Preferred ratings of HRC fuses are 2, 4, 6, 10, 16, 25, 30, 50, 63, 80, 100, 125, 160, 200, 250, 320, 400, 500, 630,800, 1000 and 1,250 amperes.

 Fuse Selection Guide

  • The fuse must carry the normal load current of the circuit without nuisance openings. However, when an over current occurs the fuse must interrupt the over current, limit the energy let-through, and withstand the voltage across the fuse during arcing. To properly select a fuse the followings must be considered:
  • Normal operating current (The current rating of a fuse is typically de rated 25% for operation at 25C to avoid nuisance blowing. For example, a fuse with a current rating of 10A is not usually recommended for operation at more than 7.5A in a 25C ambient.)
  • Overload current and time interval in which the fuse must open.
  • Application voltage (AC or DC Voltage).
  • Inrush currents, surge currents, pulses, start-up currents characteristics.
  • Ambient temperature.
  • Applicable standards agency required, such as UL, CSA, and VDE.
  • Considerations: Reduce installation cost, ease of removal, mounting type/form factor, etc

Recommended UL Current Limiting Fuse Classes:

TIME DELAY FUSE TYPE

Class Voltage Current
Class-L (LCL) 600V AC 601 – 6000A
Class RK1 (LENRK) 250V AC 0.6 -600A
Class RK1 (LESRK) 600V AC 0.5 -600A
Class RK5 (ECNR) 250V AC 0.1 -600A
Class RK5 (ECSR) 600V AC 0.1 -600A
Class J (JDL) 600V AC 1 -600A
Class CC (HCTR) 600V AC 0.25 -10A
     

FAST ACTING TYPE FUSE(Non/time-delay)

Class Voltage Current
Class-T (TJN) 300V AC 1 – 800A
Class-T (TJS) 600V AC 1 – 800A
Class-L (LCU) 600V AC 601– 6000A
Class-RK1(NCLR) 250V AC 1 – 600A
Class-RK1(SCLR) 600V AC 1 – 600A
Class J (JFL) 600V AC 1 -600A
Class CC (HCLR) 600V AC 0.1 -30A
     

 Fuse Class:

(1) Class L, fuses 

  • They provide a minimum time delay of 4 seconds at 500% of their rated current to handle harmless inrush currents, plus they are 20% more current limiting than any other Class L fuse.
  • That means optimal over current protection for service entrances, large motors, feeders and other circuits.
  • Range from 601 to 6000 amperes, 600V AC, 300kA
  • I.R., and an exclusive 500V DC, 100kA I.R., through 3000A.

Features

  • Fastest operation under short circuit conditions
  • Most current limiting for lowest peak let-thru current
  • Replaces all older Class L fuses
  • Pure silver links for long fuse life
  • AC and DC ratings
  • High-grade silica filler for fast arc quenching

Applications

  • Mains and feeders
  • Large motors
  • Lighting, heating and general loads
  • Power circuit breaker backup
  • UPS DC links, battery disconnects and other DC applications

Application notes

  • Mains and feeders — Can size at 100% of expected full load, unless equipment manufacturers specify
  • Motor starters — Consult your motor control manufacturer’s recommendations.
  • Lighting, heating and general loads — Can size at 100% to 125%, depending on load make-up.
  • Transformers — Due to the high inrush currents that can be experienced with transformers, size fuse to carry 12 times transformer full load for 0.1 second and 25 times full load for 0.01 second.

 (2) Class J, fuses 

  • The most current-limiting UL-class fuse, provide optimal performance, prevent interchangeability with old fuses, and save valuable panel space. So you can use smaller, more economical fuse blocks and IEC contactors to provide superior protection for dedicated or combined motor, lighting, heating and transformer loads.
  • Plus their time delay characteristic allows for use in a wide range of applications.
  •  Rated from 1 to 600 amperes, 600V AC, 300kA I.R., and 500V DC, 100kA I.R., listed to UL 248-8, they’re the right fuses for any new installation.

Features

  • Most current-limiting UL-class fuses
  • Timesaving Smart Spot™ indicator
  • Unique dimensions prevent misapplications
  • Optional mechanical indicator available on 70A to 600A AJT fuses

Applications

  • Motor circuits
  • Mains and feeders
  • Branch circuits
  • Lighting, heating and general loads
  • Transformers and control panels
  • Circuit breaker backup
  • Bus duct
  • Load centers

Application notes

  • Mains and feeders: Can size at 125% of load for NEC and CEC code compliance.
  • Motor starters: For typical starting duty and optimal coordination, fuse rating should not exceed 150% of motor FLA. Where “no-damage” tests have been conducted, follow the control gear manufacturer’s fuse ampere rating recommendations.
  • Lighting, heating and general loads: Can size at 125% of combined load for NEC and CEC code compliance.
  • Transformers: Due to the high inrush currents that can be experienced with transformers, size fuse to carry 12 times transformer full load for 0.1 second and 25 times full load for 0.01 second.

 (3) Class RK1 fuses:

  • Significantly more current limiting than Class RK5, K and H fuses, upgrading your existing feeder and branch circuits to arc flash category “0”. They also offer plenty of application flexibility, with ratings from 1/10A to 600A (250V or 600V), 300kA I.R.

Features

  • Highly current limiting to achieve HRC “0”
  • Timesaving Smart Spot™ indicator
  • Brass end caps (blade style) for cooler operation and superior performance
  • Rejection-style design

Applications

  • Motors
  • Safety switches
  • Transformers
  • Branch circuit protection
  • Disconnects
  • Control panels
  • General-purpose circuits

Application notes

  • Mains and feeders: Can size at 125% of load for NEC and CEC code compliance.
  • Motor starters: For typical starting duty and optimal coordination, fuse rating should not exceed 150% of motor FLA. Where “no damage” tests have been conducted, follows the control gear manufacturer’s fuse ampere rating recommendations.
  • Lighting, heating and general loads: Can size at 125% of combined load for NEC and CEC code compliance.
  • Transformers :Due to the high inrush currents that can be experienced with transformers, size fuse to carry 12 times transformer full load for 0.1 second and 25 times full load for 0.01 second.

(4) Class CC, fuses 

  • Choose our highly current-limiting fuses when you need maximum fault protection for sensitive branch circuit components and small motors. They deliver the best time delay characteristics and exceptional cycling ability for frequent motor starts and stops without nuisance opening. They’re available in 1/4A to 30A, 600VAC, 200kA I.R.

Features

  • Highly current limiting
  • Best time-delay characteristics in a Class CC fuse
  • Exceptional cycling ability for frequent motor stops and starts
  • Rejection-style design

Applications

  • Small motors
  • Contactors
  • Branch circuit protection

Application notes

  • Motor starters: for typical starting duty. Where “no damage” tests have been conducted, follows the control gear manufacturer’s fuse ampere rating recommendations.
  • Lighting, heating and general loads: Can size at 125% of combined load for NEC and CEC code compliance.

(5) Class CC, fuses 

  • Class CC fuses provides the time delay needed to handle the high inrush currents of control transformers, solenoids, and similar inductive loads.
  • They’re available in 1/10A to 30A, 600V AC, 200kA I.R.

Features

  • Highly current limiting
  • Rejection-style design
  • Special time-delay characteristics for transformer loads

Applications

  • Control transformers
  • Solenoids
  • Inductive loads
  • Branch circuit protection

Application notes

  • Control transformers, solenoids and similar inductive loads: For control transformers 600V AC or less with ratings up to 2000VA.fuses are designed to handle 40 times the transformer’s primary full load amperes for 0.01 second.
  • Lighting, heating and general loads: Can size at 125% of combined load for NEC and CEC code compliance.

 (6) Class RK5, fuses:

  •  Voltage / Ampere: 250V (1A to 200Amp), 600V (3A to 200A)

Description:

  • The time delay characteristics of these fuses typically allows them to be sized closer to the running ampacity of inductive loads to reduce cost and improve over current protection

Application:

  • Use in AC power distribution system mains, feeders, and branch circuits.
  • Recommended for high inrush inductive loads, like motors and transformers, and non inductive loads like lighting, and heating loads.

 (7) Class Midget fuses (600V, 0.5To 50A):

 Description:

  • Provides supplemental protection to end-use equipment with a 100KA interruption rating, 600VAC. Fast acting design responds quickly to both overloads and short-circuit protection.

Application:

  • Recommended for control circuits, street lighting, HID lighting, and electronic equipment protection

 (8) Class Midget fuses (250V, 0.5To 50A)

 Description:

  • Provides supplemental protection to end-use equipment with a 10,000A interruption rating, economical laminated paper tube

Application:

  • Recommended to use as supplemental protection for non inductive control loads and lighting circuits

 (9) Class Midget fuses (500V, 0.25To 30A)

 Description:

  • Provides supplemental protection to high inrush loads. has a 10,000A interruption rating, 500VAC. Fiber tube construction.

Application:

  • Recommended to use as supplemental protection for inductive control loads such as transformers and solenoids.

(10) Class Midget fuses (250V, 0.5To 30A)

 Description:

  • Provides supplemental protection to high inrush loads. has a 10,000A interruption rating, fiber tube construction. Dual element allows harmless inductive surges to pass without opening

Application:

  • Recommended to use as supplemental protection for inductive control loads such as transformers and solenoids

(11) Class 1 1/4″ x 1/4″ Ceramic (250,125V, 0.5To 30A)

 Description:

  • Fast acting 1/4″ x 1-1/4″ ceramic tube construction.

Application:

  • Recommended to use as supplemental protection for inductive control loads such as transformers and solenoids.

(12) Class 1 1/4″ x 1/4″ Glass (250,32V, 0.5To 30A)

 Description:

  • Fast acting 1/4″ x 1-1/4″ glass tube construction.

Application:

  • Recommended as supplemental protection for electronic applications.

 (13) Class 5mmx20mm Glass (250,125V, 0.063To 15A)

 Description:

  • Fast acting 5mmx20mm glass tube construction.

Application:

  • Recommended as supplemental protection for electronic applications.

(14)  Class 5mmx20mm Glass (250,125V, 0.5To 10A)

 Description:

  • Medium Time Delay 5mm x 20mm glass tube construction.

Application:

  • Recommended as supplemental protection for electronic applications.

 (15) Class 1 1/4″ x 1/4″ Ceramic (250, 0.5To 20A)

 Description:

  • Time Delay 1/4″ x 1-1/4″ ceramic tube construction.

Application:

  • Recommended as supplemental protection for electronic applications.

 (16) Class 1 1/4″ x 1/4″ Glass (250,32V, 0.0625To 20A)

 Description:

  • Time Delay 1/4″ x 1-1/4″ glass tube construction.

Application:

  • Recommended as supplemental protection for electronic applications

 (17) Class 5mmx20mm Glass (250, 0.5To 10A)

 Description:

  • Fast acting 5mm x 20mm glass tube construction

Application:

  • Recommended as supplemental protection for electronic applications.

 (18) Class 5mmx20mm Glass (250, 0.25To 6.3A)

 Description:

  • Time Delay 5mm x 20mm glass tube construction.

Application:

  • Recommended as supplemental protection for electronic applications.

 Selection of Fuse for Main and Branch Circuits:

 1 Main Service Conductor Cable Limiters (NEC 240,230.82):

  • Select by cable size and mounting terminal configurations required.

2 Main Service Circuit Fuses–Mixed Loads:

  • Size fuses same as item 6.

3 Transformer Circuit Fuses (NEC 450.3b, 240.3, 240.21, 430.72 (c) as required):*

  • (a) PRIMARY FUSES: Size fuses not over 125%. As exceptions exist, refer to the appropriate NEC® paragraphs.
  • Recommended fusesL Time Delay- Class RK1, Class RK5, Class L, Class J)
  • (b) SECONDARY FUSES (Sum of following): 125% of the continuous load + 100% of non-continuous load. Fuse size not to exceed 125% of transformer secondary rated amps.
  • RECOMMENDED FUSES: Class RK1, Class RK5, ClassJ LENRK, ECNR, NCLR, JDL or LCU.

4 Branch Circuit Fuse Size, No Motor Load (NEC 240.3, 210.20):

  • 100% of non-continuous load, +125% of continuous load.
  • *Do not exceed conductor ampacity. Recommended fuses: LENRK, ECNR, NCLR, JDL, LCU, or LCL.

5 Branch Circuit Fuse Size, No Motor Load (NEC240.3, 210.20):

  • 100% of non-continuous load, + 125% of continuous load. Fuse may be sized 100% when used with a continuous rated switch. Recommended fuses same as 4.

6 Feeder Circuit Fuse Size, Mixed Load (NEC 240.3, 430.63, 430.24):

  • (a) 100% of non-continuous, non-motor load + 125% of continuous, non-motor load.
  • (b) Determine non-continuous motor load (NEC430.22 (e).1.) Add to “a” above.
  • (c) Determine A/C or refrigeration load. (NEC 440.6). Add to “a” above.
  • (d) Feeder protective device shall have a rating or setting not greater than the rating of the largest branch device and sum of the FLCs of the other motors.(NEC 430.62)
  • (e) Recommended fuses: LENRK/LESRK, JDL, ECNR/ECSR, LCU, LCL.

7 Feeder Circuit Fuse Size, 100% Motor Load (NEC 240.3, 430.62 (a).

  • (a)Determine non-continuous motor load (NEC430.22 (e).
  • (b)Determine load of A/C or refrigeration equipment (NEC 440.6). Add to “a” above.
  • (c) Feeder protective device shall have a rating or setting not greater than the rating of the largest branch device and sum of the FLCs of the other motors.(NEC 430.62)
  • (d) Recommended fuses: LENRK/LESRK, JDL, ECNR/ECSR or LCL.

8 Branch Circuit Fuse Size, Individual Motor Load, With Fuse Overload Protection (No Starter Overload Relays): (NEC 430.32, 430.36):

  • (a) Motors with 1.15 Service Factor or temperature rise not over 40 Degrees C., size fuses at not more than 125% of the motor nameplate current rating.
  • (b) For all other A-C motors, size fuses at not more than 115 %.
  • (c) Best protection is obtained by measuring motor running current and sizing fuses at 125% of measured current for normal motor operation. Reference to “Average Time/Current Curves” is recommended.
  • (d) Recommended Fuses: LENRK/LESRK, JDL, or ECNR/ECSR

.9 Branch Circuit Fuse Size, Individual Motor Load, With Starter Overload Relays: (NEC 430.32, 430.52):

  • (a) For “back-up” NEC® overload, ground fault and short circuit protection size the fuses the same as (8 a, b) above, or the next standard size larger.
  • (b) The fuse sizes in a) above may be increased as allowed by NEC® references. Generally, dual element fuses should not exceed 175% of motor nameplate F.L.A. and non-UL defined time-delay fuses not more than 300 %.
  • (c) Recommended fuses: LENRK/LESRK, JDL, ECNR/ ECSR or LCL.

10 Fuse Sizing for Individual Large Motors With F.L.A. Above 480 Amps or Otherwise Require Class L Fuses – (NEC 430.52):

  • Application Tips:
  • Size fuses as closely as practical to the ampacity of the protected circuit components without the probability of unnecessary fuse opening from harmless, transient current surges. This usually requires a choice between time-delay and non-time-delay fuses.
  • Use Class R fuse clips with Class R fuses to prevent installation of fuses with less interrupting rating or current limitation. Class H fuse reducers cannot be used with Class R fuse clips.
  •  When a conductor is oversized to prevent excess voltage drop, size the fuses for the ampacity of protected circuit components instead of over sizing fuses for the larger conductor.

  Selection of Fuse for Motor Protection:

  •  Group installation is an approach to building multi-motor control systems in accordance with Section 430-53 of the National Electrical Code. The selection of components used in group installations is a simple process which consists of several steps.
  • First is the selection of the appropriate fuse as Branch Circuit Protective Device (BCPD).
  • Second is the selection of the appropriate motor starter and protector.
  • Third, the selected MMP must be checked for UL listing with the selected BCPD and the available short circuit current at the application location.

1. Fused disconnect

  • Calculate maximum fuse size according to NEC 430-53 (c).
  • Imax (fuse size) =175% x FLC (full load current for largest motor) + the sum of FLC (full load current for largest motor) + the sum of FLC values for other motors on that branch using NEC Table 430-150 on the right.
  • Select fuse from NEC Table 240-6 below. Where Imax falls between two fuse ampere ratings NEC 430-53 (c) permits going to the next high ampere rating.

2. Motor protector selection

  • Select the proper MMP catalog number for each motor load from the based on the actual motor full load current (FLA) using the “Thermal setting range” column for reference.

3. MMP Interruption ratings

  • Using the interruption ratings table on the next page, identify the system application voltage and interrupting capacity for the type of fuse selected in step1 above.
  • NEC 240-6 Standard fuse amperes 15, 20, 25, 30, 40, 45, 50, 60, 70, 80, 90, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600
  • Examples: Select components for protecting the following 3-phase, 460VAC, squirrel cage induction motors. The nameplate data are: 1/2 HP, 1.0 FLA; 3 HP, 4.8 FLA; 5 HP, 7.6 FLA; 7.5 HP, 11 FLA; 10 HP, 14 FLA.
  • Example: using fused disconnect
  • Imax = 175% x 14 + (11 + 7.6 + 4.8 + 1) = 48.9A
  • Fuse rating using Table NEC 240-6 = 50A
  • Minimum disconnect size = 115% x Total FLA
  • NEC 430-150 table = 115% x (14+ 11 + 7.6 + 4.8 + 1) = 44.16
  • Disconnect for 50A fuses is ok
  • NEC Table 430-150 full load current, 3ph AC motor
H.P Induction Type Motor (Squirrel Cage, Wound Type)
230V Amp 460V Amp 575V Amp
½ 2 1 0.8
¾ 2.8 1.4 1.1
1 3.6 1.8 1.4
1.5 5.2 2.6 2.1
2 6.8 3.4 2.7
3 9.6 4.8 3.9
5 15.2 7.6 6.1
7.5 22 11 9
10 28 14 11
15 42 21 17
20 54 27 22
25 68 34 27

Fuse Ratings

  • Fuses with an A-C voltage rating may be applied at system voltages below the fuse voltage rating, but not at voltages above the fuse voltage rating.
  • The other A-C fuse ratings remain the same at applied voltages below the fuse voltage rating.
  • A-C rated fuses should not be applied in D-C voltage circuits unless D-C application ratings are provided by the fuse manufacturer.
  • Except for some special purpose fuses, D-C ratings are not usually shown on fuse labels.
  • The operating frequency (Hertz) will affect fuse characteristics in various ways.
  • Time/Current Curves will not shift and fuse ratings will not change from 1-100 Hertz in normal applications. If ferrous hardware is used to mount the fuses, eddy current heating could alter the ratings.
  • Above 100 Hertz, “skin effect” could alter the fuses’ rating characteristics. This effect must be analyzed on an individual application basis.

Current Transformer


Current transformers

Principle of operation of CT

  • A current transformer is defined as “as an instrument transformer in which the secondary current is substantially proportional to the primary current (under normal conditions of operation) and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections.”
  • Current transformers are usually either “measuring” or “protective” types.

Some Definitions used for CT:

1)    Rated primary current:

  • The value of primary current which appears in the designation of the transformer and on which the performance of the current transformer is based.

2)    Rated secondary current:

  • The value of secondary current which appears in the designation of the transformer and on which the performance of the current transformer is based.
  • Typical values of secondary current are 1 A or 5 A. In the case of transformer differential protection, secondary currents of 1/ root 3 A and 5/ root 3 A are also specified.

3)    Rated burden:

  • The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current and at a specific power factor (0.8 for almost all standards)

4)    Rated output:

  • The value of the apparent power (in volt-amperes at a specified power (factor) which the current transformer is intended to supply to the secondary circuit at the rated secondary current and with rated burden connected to it.

5)    Accuracy class:

  • In the case of metering CT s, accuracy class is typically, 0.2, 0.5, 1 or 3.
  • This means that the errors have to be within the limits specified in the standards for that particular accuracy class.
  • The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor.
  • In the case of protection CT s, the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the CT.

6)    Current Ratio Error:

  • The error with a transformer introduces into the measurement of a current and which arises from the fact that actual transformation ratio is not equal to the rated transformer ratio. The current error expressed in percentage is given by the formula:
  • Current error in % = (Ka(Is-Ip)) x 100 / Ip
  • Where Ka= rated transformation ratio ,Ip= actual primary current, Is= actual secondary current when Ip is flowing under the conditions of measurement

7)    Accuracy limit factor:

  • The value of primary current up to which the CT complies with composite error requirements. This is typically 5, 10 or 15, which means that the composite error of the CT has to be within specified limits at 5, 10 or 15 times the rated primary current.

8)    Short time rating:

  • The value of primary current (in kA) that the CT should be able to withstand both thermally and dynamically without damage to the windings, with the secondary circuit being short-circuited. The time specified is usually 1 or 3 seconds.

9)    Instrument security factor (factor of security):

  • This typically takes a value of less than 5 or less than 10 though it could be much higher if the ratio is very low. If the factor of security of the CT is 5, it means that the composite error of the metering CT at 5 times the rated primary current is equal to or greater than 10%. This means that heavy currents on the primary are not passed on to the secondary circuit and instruments are therefore protected. In the case of double ratio CT’s, FS is applicable for the lowest ratio only.

10) Class PS X CT:

  • In balance systems of protection, CT s with a high degree of similarity in their characteristics is required. These requirements are met by Class PS (X) CT s. Their performance is defined in terms of a knee-point voltage (KPV), the magnetizing current (Imag) at the knee point voltage or 1/2 or 1/4 the knee-point voltage, and the resistance of the CT secondary winding corrected to 75C. Accuracy is defined in terms of the turn’s ratio.

11) Knee point voltage:

  • That point on the magnetizing curve where an increase of 10% in the flux density (voltage) causes an increase of 50% in the magnetizing force (current).
  • The ‘Knee Point Voltage’ (Vkp) is defined as the secondary voltage at which an increase of 10% produces an increase in magnetizing current of 50%. It is the secondary voltage above which the CT is near magnetic saturation.

12) Core balance CT (CBCT):

  • The CBCT, also known as a zero sequence CT, is used for earth leakage and earth fault protection. The concept is similar to the RVT. In the CBCT, the three core cable or three single cores of a three phase system pass through the inner diameter of the CT. When the system is fault free, no current flows in the secondary of the CBCT. When there is an earth fault, the residual current (zero phase sequence current) of the system flows through the secondary of the CBCT and this operates the relay. In order to design the CBCT, the inner diameter of the CT, the relay type, the relay setting and the primary operating current need to be furnished.

13) Phase displacement:

  • The difference in phase between the primary and secondary current vectors, the direction of the vectors being so chosen that the angle is zero for the perfect transformer. The phase displacement is said to be positive when the secondary current vector leads the primary current vector. It is usually express in minutes

14) Highest system voltage:

  • The highest rms line to line voltage which can be sustained under normal operating conditions at any time and at any point on the system. It excludes temporary voltage variations due to fault condition and the sudden disconnection of large loads.

15) Rated insulation level:

  • That combination of voltage values (power frequency and lightning impulse, or where applicable, lightning and switching impulse) which characterizes the insulation of a transformer with regard to its capability to withstand by dielectric stresses. For low voltage transformer the test voltage 4kV, at power-frequency, applied during 1 minute.

16) Rated short-time thermal current (Ith):

  • The rms value of the primary current which the current transformer will withstand for a rated time, with their secondary winding short circuited without suffering harmful effects.

17) Rated dynamic current (Idyn):

  • The peak value of the primary current which a current transformer will withstand, without being damaged electrically for mechanically by the resulting electromagnetic forces, the secondary winding being short-circuited.

18) Rated continuous thermal current (Un)

  • The value of current which can be permitted to flow continuously in the primary winding, the secondary windings being connected to the rated burdens, without the temperature rise exceeding the specified values.

19) Instrument security factor (ISF or Fs):

  • The ratio of rated instrument limits primary current to the rated primary current. The times that the primary current must be higher than the rated value, for the composite error of a measuring current transformer to be equal to or greater than 10%, the secondary burden being equal to the rated burden. The lower this number is, the more protected the connected instrument are against. 

20) Sensitivity

  • Sensitivity is defined as the lowest value of primary fault current, within the protected zone, which will cause the relay to operate. To provide fast operation on an in zone fault, the current transformer should have a ‘Knee Point Voltage’ at least twice the setting voltage of the relay.

21) Field Adjustment of Current Transformer Ratio:

  • The ratio of current transformers can be field adjusted to fulfil the needs of the application.  Passing

more secondary turns or more primary turns through the window will increase or decrease the turns ratio.  

Actual Turns Ratio = (Name Plate Ration- Secondary Turns Added) / Primary Turns.

Types of Current transformers (CT’s)

 According to Construction of CT:

1)    Bar Type:

  • Bar types are available with higher insulation levels and are usually bolted to the current caring device.

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  • Bar type current transformers are insulated for the operating voltage of the system.
  • Bar-type CTs operate on the same principle of window CTs but have a permanent bar installed as a primary conductor

2)    Wound CT’s:

  • Capacity: There are designed to measure currents from 1 amp to 100 amps.
  • the most common one is the wound type current transformer. The wound type provides excellent performance under a wide operating range. Typically, the wound type is insulated to only 600 volts.

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  • Since the load current passes through primary windings in the CT, screw terminals are provided for the load and secondary conductors. Wound primary CT’s are available in ratios from 2.5:5 to 100:5.
  • Wound CTs have a primary and secondary winding like a normal transformer. These CTs are rare and are usually used at very low ratios and currents, typically in CT secondary circuits to compensate for low currents, to match different CT ratios in summing applications, or to isolate different CT circuits. Wound CTs have very high burdens, and special attention to the source CT burden should be applied when wound CTs are used.

3)    Window:

  • Window CTs are the most common. They are constructed with no primary winding and are installed around the primary conductor. The electric field created by current flowing through the conductor interacts with the CT core to transform the current to the appropriate secondary output. Window CTs can be of solid or split core construc­tion. The primary conductor must be disconnected when installing solid window CTs. However, split core CTs can be installed around the primary conductor without disconnecting the primary conductor

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  • Ring Core CT’s :
  • Capacity: There are available for measuring currents from 50 to 5000 amps

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  • Size: with windows (power conductor opening size) from 1″ to 8″ diameter.
  • Split Core CT’s:
  • Capacity: There are available for measuring currents from 100 to 5000 amps.
  • Size:  with windows in varying sizes from 1″ by 2″ to 13″ by 30″.
  • Split core CT’s have one end removable so that the load conductor or bus bar does not have to be disconnected to install the CT.

4)    Bushing

  • Bushing CTs are window CTs specially constructed to fit around a bush­ing. Usually they cannot be accessed, and their nameplates are found on the transformer or circuit-breaker control cabinets.
  • The bushing type is typically used around the bushing on circuit breakers and transformers and may not have a hard protective outside cover. 
  • Donut type current transformers are typically insulated for 600 volts. To ensure accuracy, the conductor should be positioned in the center of the current transformer opening.

 According to Application of CT:

1)    Measuring CT:

  • The principal requirements of a measuring CT are that, for primary currents up to 120% or 125% of the rated current, its secondary current is proportional to its primary current to a degree of accuracy as defined by its “Class” and, in the case of the more accurate types, that a specified maximum phase angle displacement is not exceeded.
  • A desirable characteristic of a measuring CT is that it should “saturate” when the primary current exceeds the percentage of rated current specified as the upper limit to which the accuracy provisions apply. This means that at these higher levels of primary current the secondary current is less than proportionate. The effect of this is to reduce the extent to which any measuring device connected to the CT secondary is subjected to current Overload.
  • On the other hand the reverse is required of the protective type CT, the principal purpose of which is to provide a secondary current proportional to the primary current when it is several, or many, times the rated primary current. The measure of this characteristic is known as the “Accuracy Limit Factor” (A.L.F.).
  •  A protection type CT with an A.L.F. of 10 will produce a proportional current in the secondary winding (subject to the allowable current error) with primary currents up to a maximum of 10 times the rated current.
  • It should be remembered when using a CT that where there are two or more devices to be operated by the secondary winding, they must be connected in series across the winding. This is exactly the opposite of the method used to connect two or more loads to be supplied by a voltage or power transformer where the devices are paralleled across the secondary winding.
  • With a CT, an increase in the burden will result in an increase in the CT secondary output voltage. This is automatic and necessary to maintain the current to the correct magnitude. Conversely, a reduction in the burden will result in a reduction in the CT secondary output voltage.
  • This rise in secondary voltage output with an increase in burden means that, theoretically, with infinite burden as is the case with the secondary load open circuit, an infinitely high voltage appears across the secondary terminals. For practical reasons this voltage is not infinitely high, but can be high enough to cause a breakdown in the insulation between primary and secondary windings or between either or both windings and the core. For this reason, primary current should never be allowed to flow with no load or with a high resistance load connected across the secondary winding.
  • When considering the application of a CT it should be remembered that the total burden imposed on the secondary winding is not only the sum of the burden(s) of the individual device(s) connected to the winding but that it also includes the burden imposed by the connecting cable and the resistance of the connections.
  • If, for example, the resistance of the connecting cable and the connections is 0.1 ohm and the secondary rating of the CT is 5A, the burden of the cable and connections (RI2) is 0.1 x 5 x 5 = 2.5VA. This must be added to the burden(s) of the connected device(s) when determining whether the CT has an adequately large burden rating to supply the required device(s) and the burden imposed by the connections.
  • Should the burden imposed on the CT secondary winding by the connected device(s) and the connections exceed the rated burden of the CT the CT may partly or fully saturate and therefore not have a secondary current adequately linear with the primary current.
  • The burden imposed by a given resistance in ohms [such as the resistance of a connecting cable] is proportional to the square of the rated secondary current. Therefore, where long runs of cable between CT and the connected device(s) are involved, the use of a 1A secondary CT and a 1A device rather than 5A will result in a 25-fold reduction in the burden of the connecting cables and connections. All burden ratings and calculations are at rated secondary current.
  • Because of the foregoing, when a relatively long [more than a very few meters] cable run is required to connect a CT to its burden [such as a remote ammeter] a calculation should be made to determine the cable burden. This is proportional to the “round trip” resistance, i.e. twice the resistance of the length of twin cable used. Cable tables provide information on the resistance values of different sizes of conductors at 20o C per unit length.

2)    Protective CT:

  • The calculated resistance is then multiplied by the square of the CT secondary current rating [25 for 5A, 1 for 1A]. If the VA burden as calculated by this method and added to the rated burden(s) of the device(s) to be driven by the CT exceeds the CT burden rating, the cable size must be increased [to reduce the resistance and thus the burden] or a CT with a higher VA burden rating must be used, or a lower CT secondary current rating [with matching change in the current rating of the device(s) to be driven] should be substituted

 Nomenclature of CT:

  1. Ratio: input / output current ratio
  2. Burden (VA): total burden including pilot wires. (2.5, 5, 10, 15 and 30VA.)
  3. Class: Accuracy required for operation (Metering: 0.2, 0.5, 1 or 3, Protection: 5, 10, 15, 20, 30).
  4. Accuracy Limit Factor:
  5. Dimensions: maximum & minimum limits
  6. Nomenclature of CT: Ratio, VA Burden, Accuracy Class, Accuracy Limit Factor.
  7. Example: 1600/5, 15VA 5P10  (Ratio: 1600/5, Burden: 15VA, Accuracy Class: 5P, ALF: 10)
  8. As per IEEE Metering CT: 0.3B0.1 rated Metering CT is accu­rate to 0.3 percent if the connected secondary burden if imped­ance does not exceed 0.1 ohms.
  9. As per IEEE Relaying (Protection) CT: 2.5C100 Relaying CT is accurate within 2.5 percent if the secondary burden is less than 1.0 ohm (100 volts/100A).

 1)   Current Ratio of CT:

  • The primary and secondary currents are expressed as a ratio such as 100/5. With a 100/5 ratio CT, 100A flowing in the primary winding will result in 5A flowing in the secondary winding, provided the correct rated burden is connected to the secondary winding. Similarly, for lesser primary currents, the secondary currents are proportionately lower.
  • It should be noted that a 100/5 CT would not fulfil the function of a 20/1 or a 10/0.5 CT as the ratio expresses the current rating of the CT, not merely the ratio of the primary to the secondary currents.
  • The rated secondary current is commonly 5A or 1A, though lower currents such as 0.5A are not uncommon. It flows in the rated secondary load, usually called the burden, when the rated primary current flows in the primary winding.
  • Increasing or Decreasing Turns Ratio of CT: 
  • Increasing Number of Turn: Increasing the number of primary turns can only decrease the turn’s ratio. A current transformer with a 50 to 5 turn’s ratio can be changed to a 25 to 5 turn’s ratio by passing the primary twice through the window. 
  • Increasing or Decreasing Turns Ratio:
  • The turn’s ratio can be either increased or decreased by wrapping wire from the secondary through the window of the current transformer.
  • Increasing the turn’s ratio with the secondary wire, turns on the secondary are essentially increased. A 50 to 5 current transformer will have a 55 to 5 ratio when adding a single secondary turn.
  • Decreasing the turn’s ratio with the secondary wire, turns on the secondary are essentially decreased.  A 50 to 5 current transformer will have a 45 to 5 ratio when adding a single secondary turn.
  • Decreasing the turn’s ratio with the primary, accuracy and VA burden ratings are the same as the original configuration.
  • Increasing the turn’s ratio with the secondary will improve the accuracy and burden rating.
  • Decreasing the turn’s ratio with the secondary will worsen the accuracy and burden rating.
  • When using the secondary of a current transformer to change the turn’s ratio, the right hand rule of magnetic fields comes into play.  Wrapping the white lead or the X1 lead from the H1 side of the transformer through the window to the H2 side will decrease the turn’s ratio.  Wrapping this wire from the H2 side to the H1 side will increase the turn’s ratio.
  • Using the black or X2 lead as the adjustment method will do the opposite of the X1(white) lead.  Wrapping from the H1 to the H2 side will increase the turns ratio, and wrapping from the H2 to the H1 side will decrease the turns ratio.

 2)   Burden of CT:

  • Common burden ratings of CT: 2.5, 5, 10, 15 and 30VA.
  • The external load applied to the secondary of a current transformer is called the “burden”.
  • The burden of CT is the maximum load (in VA) that can be applied to the CT secondary.
  • The burden can be expressed in two ways.
  • The burden can be expressed as the total impedance in ohms of the circuit or the total volt-amperes (VA) and power factor at a specified value of current or voltage and frequency.
  • Formerly, the practice was to express the burden in terms of volt-amperes (VA) and power factor, the volt-amperes being what would be consumed in the burden impedance at rated secondary current (in other words, rated secondary current squared times the burden impedance). Thus, a burden of 0.5Ωimpedance may be expressed also as “12.5 VA at 5 amperes,” if we assume the usual 5-ampere secondary rating. The VA terminology is no longer standard, but it needs defining because it will be found in the literature and in old data.

Burden for Measuring CT:

  • Total burden of Measuring CT = Sum of Meters Burden in VA (Ammeter, Wattmeter, Transducer etc.) connected in series to the CT secondary circuit + Connecting Secondary Circuit Cable Burden in VA.
  • Cable burden = I2 x R x2 L, where I = CT secondary current, R = cable resistance per length, 2L is the tro &fro distance of cable length L from CT to metering circuits. If the proper size and short length of wire is used, cable burden can be ignored.
  • The CT secondary circuit load shall not be more than the CT VA rating. If the load is less than the CT burden, all meters connected to the measuring CT should provide correct reading.
  • In the case of Measuring Current transformer, the burden depends on the connected meters and quantity of meters on the secondary i.e. no of Ammeters, KWh meters, Kvar meters, Kwh meters, transducers and also the connection cable burden (I2 x R x2 L) to metering shall be taken into account.
  • Note Meters burden can be obtained from manufacturer catalogue.
  • Selected CT burden shall be more than the calculated burden

Burden for Protecting CT:

  • In the case of Protection CTs the burden is calculated in the same way as above except the burden of individual protective relays burden shall be considered instead of meters. The connecting cable burden is calculated in the same way as metering CT
  • Total burden of Protection CT=Connecting cable Burden in VA + sum of Protective relays Burden in VA.
  • All manufacturers can supply the burden of their individual devices. Although not used very often these days, induction disk over-current devices always gave the burden for the minimum tap setting. To determine the impedance of the actual tap setting being used, First Square the ratio of minimum divide by the actual tap setting used and, second multiply this value by the minimum impedance.
  • Suppose an impedance of 1.47 + 5.34j at the 1A tap. To apply the relay at the 4A tap the engineer would multiply the impedance at the 1A taps setting by (1/4)2. The impedance at the 4A tap would be 0.0919 + 0.3338j or 0.3462 Z at 96.4 power factor.
  • The CT burden impedance decreases as the secondary current increases, because of saturation in the magnetic circuits of relays and other devices. Hence, a given burden may apply only for a particular value of secondary current. The old terminology of volt-amperes at 5 amperes is most confusing in this respect since it is not necessarily the actual volt amperes with 5 amperes flowing, but is what the volt-amperes would be at 5 amperes
  • If there were no saturation. Manufacturer’s publications give impedance data for several values of over current for some relays for which such data are sometimes required. Otherwise, data are provided only for one value of CT secondary current.
  •  If a publication does not clearly state for what value of current the burden applies, this information should be requested. Lacking such saturation data, one can obtain it easily by test. At high saturation, the impedance approaches the DC resistance. Neglecting the reduction in impedance with saturation makes it appear that a CT will have more inaccuracy than it actually will have. Of course, if such apparently greater inaccuracy can be tolerated, further refinements in calculation are unnecessary. However, in some applications neglecting the effect of saturation will provide overly optimistic results; consequently, it is safer always to take this effect into account.
  • It is usually sufficiently accurate to add series burden impedances arithmetically. The results will be slightly pessimistic, indicating slightly greater than actual CT ratio inaccuracy. But, if a given application is so borderline that vector addition of impedances is necessary to prove that the CTÕs will be suitable, such an application should be avoided.
  • If the impedance at pickup of a tapped over current-relay coil is known for a given pickup tap, it can be estimated for pickup current for any other tap. The reactance of a tapped coil varies as the square of the coil turns, and the resistance varies approximately as the turns. At pickup, there is negligible saturation, and the resistance is small compared with the reactance. Therefore, it is usually sufficiently accurate to assume that the impedance varies as the square of the turns. The number of coil turns is inversely proportional to the pickup current, and therefore the impedance varies inversely approximately as the square of the pickup current.
  • Whether CT is connected in wye or in delta, the burden impedances are always connected in wye. With wye-connected CT the neutrals of the CT and of the burdens are connected together, either directly or through a relay coil, except when a so-called zero phase-sequence-current shunt is used.
  • It is seldom correct simply to add the impedances of series burdens to get the total, whenever two or more CT are connected in such a way that their currents may add or subtract in some common portion of the secondary circuit. Instead, one must calculate the sum of the voltage drops and rises in the external circuit from one CT secondary terminal to the other for assumed values of secondary currents flowing in the various branches of the external circuit. The effective CT burden impedance for each combination of assumed currents is the calculated CT terminal voltage divided by the assumed CT secondary current. This effective impedance is the one to use, and it may be larger or smaller than the actual impedance which would apply if no other CTÕs were supplying current to the circuit.
  • If the primary of an auxiliary CT is to be connected into the secondary of a CT whose accuracy is being studied, one must know the impedance of the auxiliary CT viewed from its primary with its secondary short-circuited. To this value of impedance must be added the impedance of the auxiliary CT burden as viewed from the primary side of the auxiliary CT; to obtain this impedance, multiply the actual burden impedance by the square of the ratio of primary to secondary turns of the auxiliary CT. It will become evident that, with an auxiliary CT that steps up the magnitude of its current from primary to secondary, very high burden impedances, when viewed from the primary, may result.
  • Burden is depending on pilot lead length
  • For Metering Class CTs burden is expressed as ohms impedance. For Protection-class CTs burden is express as volt-amperes (VA).
VA Applications
1 To 2 VA Moving iron ammeter
1 To 2.5VA Moving coil rectifier ammeter
2.5 To 5VA Electrodynamics instrument
3 To 5VA Maximum demand ammeter
1 To 2.5VA Recording ammeter or transducer
  • Burden (VA) of copper wires between instrument & current transformer for 1A and 5A secondary’s
Cross Section (mm2)

CT  1 Amp Secondary Burden in VA (Twin Wire)

Distance

10 meter 20 meter 40 meter 60 meter 80 meter 100 meter

1.0

0.35

0.71

1.43

2.14

2.85

3.57

1.5

0.23

0.46

0.92

1.39

1.85

2.31

2.5

0.14

0.29

0.57

0.86

1.14

1.43

4.0

0.09

0.18

0.36

0.54

0.71

0.89

6.0

0.06

0.12

0.24

0.36

0.48

0.6

 

Cross Section (mm2)

CT  5 Amp Secondary Burden in VA (Twin Wire)

Distance

1 meter 2 meter 4 meter 6 meter 8 meter 10 meter

1.5

0.58

1.15

2.31

3.46

4.62

5.77

2.5

0.36

0.71

1.43

2.14

2.86

3.57

4.0

0.22

0.45

0.89

1.34

1.79

2.24

6.0

0.15

0.30

0.60

0.89

1.19

1.49

10.0

0.09

0.18

0.36

0.54

0.71

0.89

 CT Burden Calculation:

  • The Actual burden is formed by the resistance of the pilot conductors and the protection relay(s). The resistance of a conductor (with a constant cross-sectional area) can be calculated from the equation:
  • R =ƿxL / A
  • where ƿ  = resistivity of the conductor material (given typically at +20°C) ,L= length of the conductor , A = cross sectional area
  • If the resistivity is given in μΩm, the length in meters and the area in mm2, the equation 1 will give the resistance directly in ohms.
  • Resistivity: Copper 0.0178 µΩm at 20 °C and 0.0216 µΩm at 75 °C

Burden of CT for 4 or 6 wire connection:

  • If 6-wire connection is used, the total length of the wire, naturally, will be two times the distance between the CT and the relay.  However, in many cases a common return conductor is used as shown in figure then, instead of multiplying the distance by two, a factor of 1.2 is typically used. This rule only applies to the 3-phase connection only.  The factor 1.2 allows for a situation, where up to 20% of the electrical conductor length, including terminal resistances, uses 6-wire connection and at least 80% 4-wire connection.

  • Example: the distance between the CT and the relay is 5 meters the total length is 2 x 5 m = 10 meter for 6-wire connection, but only 1.2 x 5 m = 6.0 meter when 4-wire connection is used.

Burden of the relay:

  • Example: The Distance between the CTs and the protection relay is 15 meters, 4 mm2 Cu conductors in 4-wire connection are used. The burden of the relay input is less than 20 mΩ (5 A inputs). Calculate the actual burden of the CT at 75°C , the input impedance is less than 0.020 Ω for a 5 A input (i.e. burden less than 0.5 VA) and less than 0.100 Ω for a 1 A input (i.e. less than 0.1 VA):
  • Solution:
  • ƿ = 0.0216 µΩm (75°C) for copper conductor.
  • R =ƿxL / A ,R = 0.0216 µΩm x (1.2 x 15 m) / 4 mm2 = 0.097 Ω
  • Burden of CT = 0.097 Ω + 0.020 Ω = 0.117 Ω.
  • Using CTs of burden values higher than required, is unscientific since it leads to inaccurate reading (meter) or inaccurate sensing of fault / reporting conditions.
  • Basically, such high value of design burden extends saturation characteristics of CT core leading to likely damage to the meter connected across it under overload condition. e.g. When we expect security factor (ISF) to be 5, the secondary current should be restricted to less than 5 times in case primary current shoots to more than 5 times its rated value.
  • In such an overload condition, the core of CT is desired to go into saturation, restricting the secondary current thus the meter is not damaged. However, when we ask for higher VA, core doesn’t go into saturation due to less load (ISF is much higher than desired) which may damage the meter.
  • To understand the effect on Accuracy aspect, let’s take an example of a CT with specified burden of 15 VA, and the actual burden is 2.5 VA:15 VA CT with less than 5 ISF will have saturation voltage of 15 Volts (15/5×5), and actual burden of 2.5 VA the saturation voltage required shall be ( 2.5/5 x 5) 2.5 Volts against 15 Volts resulting ISF = 30 against required of 5.
  • Example: Decide  Whether 5A,20VA CT is sufficient for following circuit

Untitled

  • Total instrument burden = 2 + 2 + 3 + 2 + 4 = 13V A.
  • Total pilot load resistance = 2 x 0.1 = 0.2 ohm.
  • With 5A secondary current, volt-drop in leads is 5 x 0.2 = 1 V.
  • Burden imposed by both leads = 5A x 1 V = 5V A.
  • Total burden on CT = 13 + 5 = 18V A. 
  • As the CT is rated 20V A, it has sufficient margin.

3)   Accuracy Class of CT:

  • The CT accuracy is determined by its certified accuracy class which is stamped on nameplate. For example, CT accuracy class of 0.3 means that the CT is certified by the manufacturer to be accurate to within 0.3 percent of its rated ratio value for a primary current of 100 percent of rated ratio.
  • CT with a rated ratio of 200/ 5 with accuracy class of 0.3 would operate within 0.45 percent of its rated ratio value for a primary current of 100 amps. To be more explicit, for a primary current of 100A it is certified to produce a secondary current between 2.489 amps and 2.511 amps.
  • Accuracy is specified as a percentage of the range, and is given for the maximum burden as expressed in VA.  The total burden includes the input resistance of the meter and the loop resistance of the wire and connections between the current transformer and meter.
  • Example: Burden = 2.0 VA. Maximum Voltage drop = 2.0 VA / 5 Amps = 0.400 Volts.
  •  Maximum Resistance = Voltage / Current = 04.00 Volts / 5 Amps =0.080 Ohms.
  • If the input resistance of the meter is 0.010Ω, then 0.070Ω is allowed for loop resistance of the wire, and connections between the current transformer and the meter. The length and gauge of the wire must be considered in order to avoid exceeding the maximum burden.
  • If resistance in the 5 amp loop causes the burden to be exceeded, the current will drop. This will result in the meter reading low at higher current levels.
  • As in all transformers, errors arise due to a proportion of the primary input current being used to magnetize the core and not transferred to the secondary winding. The proportion of the primary current used for this purpose determines the amount of error.
  • The essence of good design of measuring current transformers is to ensure that the magnetizing current is low enough to ensure that the error specified for the accuracy class is not exceeded.
  • This is achieved by selecting suitable core materials and the appropriate cross-sectional area of core. Frequently in measuring currents of 50A and upwards, it is convenient and technically sound for the primary winding of a CT to have one turn only.
  • In these most common cases the CT is supplied with a secondary winding only, the primary being the cable or bus bar of the main conductor which is passed through the CT aperture in the case of ring CTs (i .e. single primary turn) it should be noted that the lower the rated primary current the more difficult it is (and the more expensive it is) to achieve a given accuracy.
  • Considering a core of certain fixed dimensions and magnetic materials with a secondary winding of say 200 turns (current ratio 200/1 turns ratio 1/200) and say it takes 2 amperes of the 200A primary current to magnetize the core, the error is therefore only 1% approximately. However considering a 50/1 CT with 50 secondary turns on the same core it still takes 2 amperes to magnetize to core. The error is then 4% approximately. To obtain a 1% accuracy on the 50/1 ring CT a much larger core and/or expensive core material is required
  • Accuracy Class of Metering CT:

Metering Class CT

Class Applications
0.1 To 0.2 Precision measurements
0.5 High grade kilowatt hour meters for commercial grade kilowatt hour meters
3 General industrial measurements
3 OR 5 Approximate measurements

  

Protective System CT Secondary VA Class
Per current for phase & earth fault 1A 2.5 10P20 Or 5P20
5A 7.5 10P20 Or 5P20
Unrestricted earth fault 1A 2.5 10P20 Or 5P20
5A 7.5 10P20 Or 5P20
Sensitive earth fault 1A or 5A   Class PX use relay manufacturers formula
Distance protection 1A or 5A   Class PX use relay manufacturers formula
Differential protection 1A or 5A   Class PX use relay manufacturers formula
High impedance differential impedance 1A or 5A   Class PX use relay manufacturers formula
High speed feeder protection 1A or 5A   Class PX use relay manufacturers formula
Motor protection 1A or 5A 5 5P10
  • Accuracy Class of  Letter of CT:

Metering Class CT

Accuracy Class Applications

B

Metering Purpose

Protection Class CT

C

CT has low leakage flux.

T

 CT can have significant leakage flux.

H

 CT accuracy is applicable within the entire range of secondary currents from 5 to 20 times the nominal CT rating. (Typically wound CTs.)

L

 CT accuracy applies at the maximum rated secondary burden at 20 time rated only. The ratio accuracy can be up to four times greater than the listed value, depending on connected burden and fault current. (Typically window, busing, or bar-type CTs.)
  • Accuracy Class of Protection CT:
Class Applications
10P5 Instantaneous over current relays & trip coils: 2.5VA
10P10 Thermal inverse time relays: 7.5VA
10P10 Low consumption Relay: 2.5VA
10P10/5 Inverse definite min. time relays (IDMT) over current
10P10 IDMT Earth fault relays with approximate time grading:15VA
5P10 IDMT Earth fault relays with phase fault stability or accurate time grading: 15VA
  •  Accuracy Class: Metering Accuracy as per IEEE C37.20.2b-1994

   

Ratio B0.1 B0.2 B0.5 B0.9 B1.8 Relaying Accuracy
50:5 1.2 2.4 C or T10
75:5 1.2 2.4 C or T10
100:5 1.2 2.4 C or T10
150:5 0.6 1.2 2.4 C or T20
200:5 0.6 1.2 2.4 C or T20
300:5 0.6 1.2 2.4 2.4 C or T20
400:5 0.3 0.6 1.2 1.2 2.4 C or T50
600:5 0.3 0.3 0.3 1.2 2.4 C or T50
800:5 0.3 0.3 0.3 0.3 1.2 C or T50
1200:5 0.3 0.3 0.3 0.3 0.3 C100
1500:5 0.3 0.3 0.3 0.3 0.3 C100
2000:5 0.3 0.3 0.3 0.3 0.3 C100
3000:5 0.3 0.3 0.3 0.3 0.3 C100
4000:5 0.3 0.3 0.3 0.3 0.3 C100

Important of accuracy & phase angle

  • Current error is an error that arises when the current value of the actual transformation ratio is not equal to rated transformation ratio.
  • Current error (%) = {(Kn x Is – Ip) x 100}/Ip
  • Kn = rated transformation ratio, Ip = actual primary current, Is = actual secondary current
  • Example: In case of a 2000/5A class 1 5VA current transformer
  • Kn = 2000/5 = 400 turn, Ip = 2000A, Is = 4.9A
  • Current error = ((400 x 4.9 – 2000) x100)/2000 = -2%
  • For protection class current transformer, the accuracy class is designed by the highest permissible percentage composite error at the accuracy limit primary current prescribed for the accuracy class concerned.
  • Accuracy class includes: 5P, 10P

By phase angle

  • Phase error is the difference in phase between primary & secondary current vectors, the direction of the vectors to be zero for a perfect transformer.
  • You will experience a positive phase displacement when secondary current vector lead primary current vector.
  • Unit of scale expressed in minutes / cent radians.
  • Circular measure = (unit in radian) is the ratio of the distance measured along the arc to the radius.
  • Angular measure = (unit in degree) is obtained by dividing the angle subtended at the center of a circle into 360 deg equal division known as “degrees”.
  • Limits of current error and phase displacement for measuring current transformer (Classes 0.1 To 1)

Accuracy

Class

+/- Percentage Current (Ratio) Error at % Rated Current

+/- Phase Displacement at % Rated Current

                  Minutes

Centi radians

5

20

100

120

5

20

100

120

5

20

100

120

0.1

0.4

0.2

0.1

0.1

15

8

5

5

0.45

0.24

0.15

0.15

0.2

0.75

0.35

0.2

0.2

30

15

10

10

0.9

0.45

0.3

0.3

0.5

1.5

0.75

0.5

0.5

90

45

30

30

2.7

1.35

0.9

0.9

1.0

3

1.5

1

1

180

90

60

60

5.4

2.7

1.8

1.8

  •  limits of current error and phase displacement for measuring current transformer For special application

Accuracy

Class

+/- Percentage Current (Ratio) Error at % Rated Current

+/- Phase Displacement at % Rated Current

Minutes

Centi radians

1

5

20

100

120

1

5

20

100

120

1

5

20

100

120

0.2S

0.75

0.35

0.2

0.2

0.2

30

15

10

10

10

0.9

0.4

0.3

0.3

0.3

0.5S

1.50

0.75

0.5

0.5

0.5

90

45

30

30

30

2.7

1.3

0.9

0.9

0.9

  •  limits of current error for measuring current transformers (classes 3 and 5)

Accuracy Class

+/- Percentage Current (Ratio) Error at % Rated Current

 
 

50

120

 

3

3

3

 

5

5

5

 

 Class X Current Transformer:

  • Class X current transformer is use in conjunction with high impedance circulating current differential protection relay, eg restricted earth fault relay. As illustrated in IEC60044-1, the class X current transformer is needed.
  • The following illustrates the method to size a class X current transformer.
  • Step 1: calculating knee point voltage Vkp
  • Vkp = {2 x Ift (Rct+Rw)}/ k
  • Vkp = required CT knee point voltage, Ift = max transformer through fault in ampere
  • Rct = CT secondary winding resistance in ohms, Rw = loop impedance of pilot wire between CT and the
  • K = CT transformation ratio
  • Step 2: calculate Transformer through fault Ift
  • Ift = (KVA x 1000)/ (1.732 x V x Impedance)
  • KVA = transformer rating in kVA , V = transformer secondary voltage, Impedance = transformer impedance
  • Step 3: How to obtain Rct
  • To measure when CT is produce
  • Step 4: How to obtain Rw
  • This is the resistance of the pilot wire used to connect the 5th class X CT at the transformer star point to the relay
  • In the LV switchboard. Please obtain this data from the Electrical contractor or consultant. We provide a table to Serve as a general guide on cable resistance.
  • Example:
  • Transformer Capacity: 2500kVA
    Transformer impedance: 6%
    Voltage system : 22kV / 415V 3phase 4 wire
    Current transformer ratio: 4000/5A
    Current transformer type: Class X PR10
    Current transformer Vkp : 185V
    Current transformer Rct  : 1.02½ (measured)
    Pilot wire resistance Rw : 25 meters using 6.0mm sq cable        
    = 2 x 25 x 0.0032 = 0.16½
    Ift = (kVA x 1000) / (1.732 x V x impedance) = (2500 x 1000) / (1.732 x 415 x 0.06)= 57,968 (Say 58,000A)
    Vkp = {2 x Ift (Rct+Rw)} / k= {2 x 58000 (1.02+0.16)} / 800= 171.1½.

 4)   Accuracy Limit Factor:

  • Standard Accuracy Limit Factors:  5, 10, 15, 20 and 30.
  • Accuracy of a CT is another parameter which is also specified with CT class. For example, if a measuring CT class is 0.5M (or 0.5B10), the accuracy is 99.5% for the CT, and the maximum permissible CT error is only 0.5%.
  • Accuracy limit Factor is defined as the multiple of rated primary current up to which the transformer will comply with the requirements of ‘Composite Error’. Composite Error is the deviation from an ideal CT (as in Current Error), but takes account of harmonics in the secondary current caused by non-linear magnetic conditions through the cycle at higher flux densities.
  • The electrical requirements of a protection current transformer can therefore be defined as :
  • Selection of Accuracy Class & Limit Factor.
  • Class 5P and 10P protective current transformers are generally used in over current and unrestricted earth leakage protection. With the exception of simple trip relays, the protective device usually has an intentional time delay, thereby ensuring that the severe effect of transients has passed before the relay is called to operate. Protection Current Transformers used for such applications are normally working under steady state conditions Three examples of such protection is shown. In some systems, it may be sufficient to simply detect a fault and isolate that circuit. However, in more discriminating schemes, it is necessary to ensure that a phase to phase fault does not operate the earth fault relay.
  • Calculation of the Accuracy limit factor
  • Fa=Fn X ( (Sin+Sn) / (Sin+Sa) )
  • Fn = Rated Accuracy Limit Factor, Sin = Internal Burden of CT secondary Coil
  • Sn= Rated Burden of CT (in VA), Sa= Actual Burden of CT (in VA)
  • Example: The internal secondary coil resistance of the CT(5P20) is 0.07 Ω, the secondary burden (including wires and relay) is 0.117 Ω and the CT is rated 300/5, 5P20, 10 VA. Calculate the actual accuracy limit factor.
  • Fn = 20 (CT data 5P20), Sin = (5A)2 × 0.07 Ω =1.75 VA, Sn = 10 VA (from CT data),
  • Sa = (5A)2 × 0.117 Ω = 2.925 VA
  • Accuracy limit factor ALF (Fa) = 20 X ((1.75+10) / (1.75+2.925)) =50.3

Selection of CT:

1)    Indoors or Out Door:

  • Determine where CT needs to be used. Indoor transformers are usually less costly than outdoor transformers. Obviously, if the current transformer is going to be enclosed in an outdoor enclosure, it need not be rated for outdoor use. This is a common costly error in judgment when selecting current transformers.

2)    What do We need:

  • The first thing we need to know that what degree of accuracy is required. Example, if you simply want to know if a motor is lightly or overloaded, a panel meter with 2 to 3% accuracy will likely suit for needs. In that case the current transformer needs to be only 0.6 to 1.2% accurate. On the other hand, if we are going to drive a switchboard type instrument with 1% accuracy, we will want a current transformer with 0.3 to 0.6 accuracy. We must keep in mind that the accuracy ratings are based on rated primary current flowing and per ANSI standards may be doubled (0.3 becomes 0.6%) when 10% primary current flows. As mentioned earlier, the rated accuracies are at stated burdens. We must take into consideration not only the burden of the load (instrument) but you must consider the total burden. The total burden includes the burden of the current transformers secondary winding, the burden of the leads connecting the secondary to the load, and the burden of the load itself. The current transformer must be able to support the total burden and to provide the accuracy required at that burden. If we are going to drive a relay you must know what relay accuracy the relay will require.

3)    Voltage Class:

  • You must know what the voltage is in the circuit to be monitored. This will determine what the voltage class of the current transformer must be as explained earlier.

4)    Primary Conductor:

  • If you have selected a current transformer with a window you must know the number, type and size of the primary conductor(s) in order to select a window size which will accommodate the primary conductors.

5)    Application:

  • The variety of applications of current transformers seems to be limited only by ones imagination. As new electronic equipment evolves and plays a greater role in the generation, control and application of electrical energy, new demands will be placed upon current transformer manufacturers and designers to provide new products to meet these needs

6)    Safety:

  • For personnel and equipment safety and measurement accuracy, current measurements on conductors at high voltage should be made only with a conducting shield cylinder placed inside the CT aperture. There should be a low electrical impedance connection from one end only to a reliable local ground. An inner insulating cylinder of adequate voltage isolation should be between the shield cylinder and the conductor at high voltage. Any leakage, induced or breakdown current between the high voltage conductor and the ground shield will substantially pass to local ground rather than through the signal cable to signal ground. Do not create a “current loop” by connecting the shield cylinder to ground from both ends. Current flowing in this loop will also be measured by the CT.

7)     CT output signal termination:

  • The CT output coaxial cable should preferably be terminated in 50 ohms. CT characteristics are guaranteed only when CT is terminated in 50 ohms.  The termination should present sufficient power dissipation capability.  When CT output is terminated in 50 ohms, its sensitivity is half that when terminated in a high-impedance load.

 Installing of CT:

  • Measurements must have the same polarity to keep the power factor and direction of power flow measurements accurate and consistent.
  • Most CTs are labelled that shows which side of the CT should face either the source or the load.

 

  • Primary Side : The Primary of CT is marked with H1 and H2 ( or only marking dot on one side)
  • The label “H1” or dot defines the direction as flowing current into the CT (H1 or the dot should face the Power source side). H2 side to load facing direction
  • Secondary Side: The Secondary (The output wires) of CT is marked with X1 and X2.
  •  X1 corresponds to H1, or the input side.The X1 secondary terminal is the polarity terminal. The polarity marks of a current transformer indicate that when a primary current enters at the polarity mark (H1) of the primary, a current in phase with the primary current and proportional to it in magnitude will leave the polarity terminal of the secondary (X1).
  •  Normally CT’s should not be installed on live services. The power should be disconnected when the CT’s are installed. Many times this is not possible because of critical loads such as computers, laboratories, etc. that cannot be shut down. Split core CT’s should not be installed on live un insulated bus bars under any conditions.

Modification of Primary & Secondary Turns Ratio:

  • The nameplate current ratio of the current transformer is based on the condition that the primary conductor will be passed once through the transformer opening. If necessary, this rating can be reduced in even multiples by looping this conductor two or more times through the opening.
  • A transformer having a rating of 300 amperes will be changed to 75 amperes if four loops or turns are made with the primary cable.
  • The ratio of the current transformer can be also modified by altering the number of secondary turns by forward or back-winding the secondary lead through the window of the current transformer.
  • By adding secondary turns, the same primary amperage will result in a decrease in secondary output.
  • By subtracting secondary turns, the same primary amperage will result in greater secondary output. Again using the 300:5 example, adding two secondary turns will require 310 amps on the primary to maintain the 5 amp secondary output or 62/1p = 310p/5s.
  • Subtracting two secondary turns will only require 290 amps on the primary to maintain the 5 amp secondary output or 58s/5p = 290p/5s. The ratio modifications are achieved in the following manner:
  • To add secondary turns, the white lead should be wound through the CT from the side opposite the polarity mark.
  • To subtract turns, the white lead should be wound through the CT from the same side as the polarity mark.

1)    Modifications in Primary Turns Ratio of CT:

  • The ratio of the current transformer can be modified by adding more primary turns to the transformer. By adding primary turns, the current required to maintain five amps on the secondary is reduced.
  • Ka = Kn X (Nn/Na)
  • Ka= Actual Turns Ration.
  • Kn=Name Plate T/C Ratio.
  • Nn=Name Plate Number of Primary Turns.
  • Na=Actual Number of Primary Turns.
  • Example: 100:5 Current Transformers.

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2)    Modifications in Secondary Turns Ratio of CT:

  • Formula  : Ip/Is = Ns/Np
  • Ip = Primary Current , Is = Secondary Current , Np = No of Primary Turns, Ns = No of Secondary Turns
  • Example: A 300:5 Current Transformer.
  • The ratio of the current transformer can be modified by altering the number of secondary turns by forward or back winding the secondary lead through the window of the current transformer.
  • By adding secondary turns, the same primary current will result in a decrease in secondary output. By subtracting secondary turns, the same primary current will result in greater secondary output.
  • Again using the 300:5 example adding five secondary turns will require 325 amps on the primary to maintain the 5 amp secondary output or:  325 p / 5s = 65s / 1p
  • Deducting 5 secondary turns will only require 275 amps on the primary to maintain the 5 amp secondary output or: 275p / 5s = 55s / 1p
  • The above ratio modifications are achieved in the following manner:

  • Current Transformer Ratio Modification:

CT Ratio

Number of Primary Turns

Modified Ratio

100:5A

2

50:5A

200:5A

2

100:5A

300:5A

2

150:5A

100:5A

3

33.3:5A

200:5A

3

66.6:5A

300:5A

3

100:5A

100:5A

4

25:5A

200:5A

4

50:5A

300:5A

4

75:5A

  • A primary turn is the number of times the primary conductor passes through the CT’s window. The main advantage of this ratio modification is you maintain the accuracy and burden capabilities of the higher ratio. The higher the primary rating the better the accuracy and burden rating.
  • You can make smaller ratio modification adjustments by using additive or subtractive secondary turns.
  •  For example, if you have a CT with a ratio of 100:5A. By adding one additive secondary turn the ratio modification is 105:5A, by adding on subtractive secondary turn the ratio modification is 95:5A.
  • Subtractive secondary turns are achieved by placing the “X1” lead through the window from the H1 side and out the H2 side. Additive secondary turns are achieved by placing the “X1” lead through the window from the H2 and out the H1 side.
  • So, when there is only one primary turn each secondary turn modifies the primary rating by 5 amperes. If there is more than one primary turn each secondary turn value is changed (i.e. 5A divided by 2 primary turns = 2.5A).
  •  The following table illustrates the effects of different combination of primary and secondary turns:

CT RATIO 100:5A

PRIMARY TURNS

SECONDARY TURNS

RATIO ADJUSTMENT

1

-0-

100:5A

1

1+

105:5A

1

1-

95:5A

2

-0-

50:5A

2

1+

52.5:5A

2

2-

45.0:5A

3

-0-

33.3:5A

3

1+

34.97:5A

3

1-

31.63:5A

Advantages of using a CT having 1A Secondary:

  • The standard CT secondary current ratings are 1A & 5A,The selection is based on the lead burden used for connecting the CT to meters/Relays.5A CT can be used where Current Transformer & protective’s device are located within same Switchgear Panel.
  • 1A CT is preferred if CT leads goes out of the Switchgear.
  • For Example if CT is located in Switch Yard & CT leads have to be taken to relay panels located in control room which can be away.1A CT is preferred to reduce the load burden. For CT with very High lead length, CT with Secondary current rating of 0.5 Amp can be used.
  • In large Generator Circuits, where primary rated current is of the order of few kilo-amperes only,5A CTs are used, 1A CTs are not preferred since the turns rations becomes very high & CT becomes unwieldy.

Danger with Current Transformer:

  • When a CT secondary circuit is closed, current flows through it, which is an exact proportion of the primary current, regardless of the resistance of the burden. In the CT have a ratio of 1OOO/5A and to have 1OOOA flowing in the primary is carrying exactly 5A.

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  • If the secondary terminals S1 and S2 are short- circuited, there is no voltage between them.
  • If now the short-circuit be replaced by a resistance of, say, 0.5 ohm the same 5A will flow through, causing a volt-drop of 2.5V and a burden of 5 x 2.5 = 12.5V A. If the resistance were increased to 5 ohms the terminal voltage with 5A flowing would rise to 25V and the burden to 125V A.
  • The greater the resistance, the greater would be the voltage and burden until, as it approached infinity (the open-circuit condition), so also in theory would the voltage (and burden) become infinite. This cannot of course happen in practice because the CT would saturate or the terminals flash over due to the very high secondary voltage between them. But it does show the danger of open-circuiting the secondary of running CT. lethal voltages can be produced at the point of opening. This is why CT secondaries are never fused.
  • The danger from an open-circuited CT is twofold. It can produce lethal voltages and so is a very real danger to personnel. The high voltage across the secondary winding could also cause insulation failure in that winding, leading at best to inaccuracy and at worst to burn- out or fire.
  • Before ever an instrument or relay is removed from the secondary loop of a running CT (if such a thing had to be done), the wires feeding that instrument must first be securely short- circuited at a suitable terminal box or, better, at the CT itself. Similarly, if a running CT is ever to be taken out of circuit, it must first be firmly shorted. CTs with 1 A secondary’s are more dangerous than those with 5A, as the induced voltages are higher.
  • Ammeter resistance is very low ,the current transformer normally works short circuited.
  • If for any reason the ammeter is taken out of secondary winding then the secondary winding must be short circuited with the help of short circuit switch .
  • If this is not done, then due to high m.m.f. will set up high flux in the core and it will produces excessive core loss which produce heat and high voltage across the secondary terminals
  • Hence the secondary of current transformer is never left open

 Sizing of CT for Building:

  • New construction: size the CT to handle about 80% of the circuit breaker capacity. If the building is served by a 2000 amp breaker, use 1600 amp (2000 x 0.8) CT’s.
  • Older buildings: the peak demand can generally be determined from the power company or from past billings. In this case add 20 to 30% to the peak demand and size the CT’s for this load. If the peak demand was 500 kW, the peak current on a 480/3/60 system would be 500,000 / (480 x 1.73 x 0.9 pf) = 669 amps. This assumes a 0.9 power factor. (Peak current would be higher with a lower power Factor.) Use CT’s about 20% larger. 800:5 CT’s would be a good selection.
  • For older buildings with no demand history, size the CT’s the same as for new construction. Where possible, use multi-tap CT’s so that the ratio can be reduced if the maximum load is much less than 80% of the breaker size.
  • CT’s that are used to monitor motor loads can be sized from the nameplate full load motor amps.

 

 

 

 

 

 

Difference between Power T.C & Distribution T.C


Difference between Power Transformer & Distribution Transformer:

  • Power transformers are used in transmission network of higher voltages for step-up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA.
  • Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V) and are generally rated less than 200 MVA.

Transformer Size / Insulation Level:

  • Power transformer is used for the transmission purpose at heavy load, high voltage greater than 33 KV & 100% efficiency. It also having a big in size as compare to distribution transformer, it used in generating station and Transmission substation .high insulation level.
  • The distribution transformer is used for the distribution of electrical energy at low voltage as less than 33KV in industrial purpose and 440v-220v in domestic purpose. It work at low efficiency at 50-70%, small size, easy in installation, having low magnetic losses & it is not always fully loaded.

Iron Loss & Copper Loss:

  • Power Transformers are used in Transmission network so they do not directly connect to the consumers, so load fluctuations are very less. These are loaded fully during 24 hr’s a day, so cu losses & iron losses takes place throughout day the specific weight i.e. (iron weight)/(cu weight) is very less .the average loads are nearer to full loaded or full load and these are designed in such a way that maximum efficiency at full load condition. These are independent of time so in calculating the efficiency only power basis is enough.
  • Power Transformers are used in Distribution Network so directly connected to the consumer so load fluctuations are very high. these are not loaded fully at all time so iron losses takes place 24hr a day and cu losses takes place based on load cycle. the specific weight is more i.e. (iron weight)/(cu weight).average loads are about only 75% of full load and these are designed in such a way that max efficiency occurs at 75% of full load. As these are time dependent the all day efficiency is defined in order to calculate the efficiency.
  • Power transformers are used for transmission as a step up devices so that the I2r loss can be minimized for a given power flow. These transformers are designed to utilize the core to maximum and will operate very much near to the knee point of B-H curve (slightly above the knee point value).This brings down the mass of the core enormously. Naturally these transformers have the matched iron losses and copper losses at peak load (i.e. the maximum efficiency point where both the losses match).
  • Distribution transformers obviously cannot be designed like this. Hence the all-day-efficiency comes into picture while designing it. It depends on the typical load cycle for which it has to supply. Definitely Core design will be done to take care of peak load and as well as all-day-efficiency. It is a bargain between these two points.
  • Power transformer generally operated at full load. Hence, it is designed such that copper losses are minimal. However, a distribution transformer is always online and operated at loads less than full load for most of time. Hence, it is designed such that core losses are minimal.
  • In Power Transformer the flux density is higher than the distribution transformer.

Maximum Efficiency:

  • The main difference between power and distribution transformer is distribution transformer is designed for maximum efficiency at 60% to 70% load as normally doesn’t operate at full load all the time. Its load depends on distribution demand. Whereas power transformer is designed for maximum efficiency at 100% load as it always runs at 100% load being near to generating station.
  • Distribution Transformer is used at the distribution level where voltages tend to be lower .The secondary voltage is almost always the voltage delivered to the end consumer. Because of voltage drop limitations, it is usually not possible to deliver that secondary voltage over great distances. As a result, most distribution systems tend to involve many ‘clusters’ of loads fed from distribution transformers, and this in turn means that the thermal rating of distribution transformers doesn’t have to be very high to support the loads that they have to serve.
  • All day efficiency = (Output in KWhr) / (Input in KWhr) in 24 hrs which is always less than power efficiency.

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Transformer


Standard Transformer Accessories & Fittings:

Standard Transformer Fittings:

1)    Standard Fittings

  • Rating and terminal marking plate.
  • Tap Changing arrangement
  • Off – circuit tap changing switch
  • Off – circuit tap changing link
  • On Load tap changer
  • Two earthing terminals
  •  Lifting Lugs
  • Drain – cum filter valve
  • Pressure Relief Device
  • Silica gel dehydrating breather.
  • Oil Level Indicator.
  • Thermometer Pocket.
  • Conservator with drain plug and filling hole.
  • Air Release plug.
  • Jacking lugs (above 1600 KVA)
  • Filter valve (top tank)
  •  Under base unidirectional flat rollers.

2)    Terminal Arrangement:

  • Bare Bushings  or Cable box.
  • Compound filled for PVC cables (up to 33000 Volts)  or Air filled for PVC cable s (Up to 11000 Volts) or
  • Bus Duct  (Bare bushing enclosed in housing up to 600 Volts)
  • Disconnection chamber between cable box and transformer tank.
  •  Additional bare neutral terminal.

3)    Optional Fittings:

  •  These are optional fittings provided at an extra cost, if customer specifically orders them.
  • Winding temperature  indicator
  • Oil temperature indicator
  • Gas and oil actuated (Buchholz) relay
  • Conservator drain valve
  • Shut off valve between conservator and tank.
  • Magnetic oil level gauge
  • Explosion vent
  • Filter valve (Bottom of tank)
  • Skid under base with haulage holes
  • Junction box.

Standard Transformer Accessories:

1)    Thermometer Pockets:

  • This pocket is provided to measure temperature of the top oil in tank with a mercury in glass type thermometer. It is essential to fill the pocket with transformer oil before inserting the thermometer,  to have uniform and correct reading. One additional pocket is provided for dial type thermometer (OTI) with contacts 

2)    Air release plug:

  • Air release plug is normally provided on the tank cover for transformer with conservator. Space is provided in the plug which allows air to be escaped without removing the plug fully from the  seat. Plug should be unscrewed till air comes out from cross hole and as soon as oil flows out it should be closed. Air release plugs are also provided on radiator headers and outdoor bushings.

3)     Winding temperature Indicator

  • The windings temperature indicator indicates ‘’ Hot spot’’ temperature of the winding. This is a ‘’Thermal Image type’’ indicator. This is basically an oil temperature indicator with a heater responsible to raise the temperature equal to the ‘’Hot spot’’ gradient between winding and oil over the oil temperature. Thus, this instrument indicates the ‘’Hot Spot’’ temperature of the windings. Heater coil is fed with a current proportional to  the windings current through a current transformer mounted on the winding under measurement. Heater coil is either placed on the heater bulb enveloping the sensing element of the winding temperature indicator immersed  in oil or in the instrument. The value of the current fed to the heater is such that it raises the  temperature by an amount equal to the hot spot gradient of the winding, as described above.  Thus temperature of winding  is simulated on the dial of the instrument. Pointer is connected thought  a mechanism to indicate the hot spot temperature on dial. WTI is provided with a temperature recording dial main pointer. Maximum pointer and re setting device and two sets of contacts for alarm and trip.

4)     Oil Temperature Indicator

  • Oil temperature indicator provides local temperature of top oil. Instruments are provided with temperature sensing bulb, temperature recording dial with  the pointer and maximum reading pointer and resetting device. Electrical contacts are provided to give alarm or trip at  a required setting (on capillary tube  type thermometer).

5)    Conservator Tank:

  • It is an Expansion Vessel
  • It maintains oil in the Transformer above a Minimum Level
  •  It has a Magnetic Oil Level Gage.
  •  It can give an alarm if the oil level falls below the limit
  • A portion of the Tank is separated for use with OLTC.
  • This usually has oil level indicators
  • Main Conservator Tank can have a Bellow
  • It has an oil filling provision
  • It has an oil drain valve
  • Provision is there for connecting a Breather

6)    Silica Gel Breather:

  •  Prevents Moisture Ingress.
  • Connected to Conservator Tank
  • Silica Gel is Blue when Dry; Pink when moist
  • Oil Seal provides a Trap for Moisture before passing thro Silica Gel

7)     Cooling:

  • ONAN .. Oil Natural Air Natural
  • ONAF .. Oil Natural Air Forced
  • OFWF .. Oil Forced Water Forced
  • ODWF .. Oil directed Water Forced.
  • By Forced Cooling, the Transformer capacity can be increased by more than 50%

8)    Bushing:

  •  Insulators and Bushings are built with the best quality Porcelain shells manufactured by wet process.
  • For manufacture of electro porcelain,  high quality indigenous raw  materials viz, China Clay,  Ball  Clay,  Quartz  and  Feldspar  is  used Quartz and feldspar are ground to required finesses and then intimately mixed with ball and china clay in high speed blungers. They are then passed through electromagnetic separators, which remove  iron  and  other  magnetic  impurities.  The  slip  produced  is passed to a filter press where extra water is removed under pressure and the resulting clay cakes are aged over a period. The aged cakes are extruded to required form viz., cylinders,  on  high  vacuum  de-airing  pug  mill.  The  extruded  blanks  or  cylinders  are given  shapes  of  Insulators  /  Bushings  which  are  conditioned  and  are  shaped  on copying lathes as the case may be.
  • Testing, Assembly & packing:
  • All insulators & bushings undergo routine electrical and mechanical  tests.  The  tests before  and  after  assembly  are  carried  out  according  to  IS  Specifications, to  ensure their suitability for actual conditions of use. Porosity tests are also carried out regularly on  samples  from  every  batch,  to  ensure  that  the  insulators  are  completely  vitrified. These insulators are then visually checked  and sorted, before they are packed in sea worthy packing, to withstand transit conditions.
  • Types of Insulators & Bushings:
  • Bushing Insulators:  Hollow Porcelain Bushings up to 33 KV
  • Application : Transformers, Capacitors, Circuit Breakers
  • Solid Core Insulators:
  •  Line Post
  • Long Rod
  • Support
  • Special Type Insulators
  • C.T. up to 66 KV
  •  P.T. up to 33 KV
  • Weather Casing
  • L.T. Insulators
  • Shackel Type
  • Spool Type
  • Pin Type
  • Guy strain
  • H.V. Bushings (IS:3347)    
  •  Pin Insulators:  Up to 33 KV
  • Post type Insulators: Post  type  insulators,  complete  with  metal  fittings,  generally  IS Specifications and other  International Standards up to 33 KV
12 to17.5 KV / 250 amps 24 KV / 1000 amps            
12 to 17.5 KV / 630 amps 24 KV / 2000 to 3150 amps
12 to 17.5 KV / 1000 amps     36 KV / 250 amps              
12 to 17.5 KV / 2000 to 3150 amps 36 KV / 630 amps
24 KV / 250 amps               36 KV / 1000 amps     
24 KV / 630 amps 36 KV / 2000 to 3150 amps
  • L.V. Bushings (IS:3347) 
11 KV / 250 amps 1 KV / 2000 amps
1 KV / 630 amps 1 KV / 3150 amps
1 KV / 1000 amps  
  • H.V. Bushings (IS:8603)    
12 KV / 250 amps 36 KV / 250 amps              
12 / 630 amps 36 KV / 630 amps
12 KV / 1000 amps     6 KV / 1000 amps            
12 KV / 2000 to 3150 amps 36 KV 3150 amps
  • C.T. Bushings (IS:5612)   
11 KV 1 KV / 2000 amps
1 KV / 630 amps 1 KV / 3150 amps
1 KV / 1000 amps  
  • Epoxy Bushing:
  • All  Epoxy  Resin  Cast  Components  are  made  from  hot  setting  reins  cured  with anhydrides;  hence  these  provide  class-F  Insulation  to  the  system.  In  an  oxidizing atmosphere, certain amine cured Epoxy Resins can start to degrade at 150ºC whereas the anhydride cured systems are stable at 200ºC therefore our epoxy components are cured with anhydrides which gives them a longer life.

9)    Buchholz Relay:

  • The purpose of such devices is  to  disconnect faulty  apparatus before large scale  damage  caused  by  a  fault  to  the  apparatus  or  to  other  connected  apparatus. Such devices generally respond to a change in the current or pressure arising from the faults and are used for either signaling or tripping the circuits.
  • Considering  liquid  immersed transformer,  a  near  ideal  protective  device  is  available  in  the  form  of  gas  and  oil operated relay described  here. The relay operates on the well known fact that almost every type of electric fault in a liquid immersed transformer gives rise to a gas. This gas is collected in the body of the relay and is used in some way or the other to cause the alarm or the tripping circuit to operate.
  • In the event of fault in an oil filled transformer gas is generated, due to which buchholz relay gives warning of developing fault. Buchholz relay is provided with two elements one for minor faults (gives alarm) and other for major faults (tripping). The alarm elements operate after a specific volume gets accumulated in the relay. Examples of incipient faults which will generate gas in oil are:- Buchholz Relay
  •  i) Failure of core bolt insulation.
  • ii) Shorting of lamination and core clamp.
  • iii) Bad Electrical contact or connections.
  • iv) Excessive hot spots in winding.
  • The alarm element will also operated in the event of oil leakage. The trip element operates due to sudden oil surge in the event of more serious fault such as: –
  • i) Earth fault due to insulation failure from winding to earth.
  • ii) Winding short circuit inter turn, interlayer, inter coil etc.
  • iii) Short circuit between phases.
  • iv) Puncture of bushing.
  • The trip element will also operate if rapid loss of oil occurs. During the  operation of transformer, if there is an alarm transformer should be isolated from lines and possible reasons, listed above for the operation of relay should be checked starting with simple reason such as loss of oil due to  leaks, air accumulation in relay chamber which  may be the absorbed  air released by oil  due to change in temperature etc. Rating of contacts: – 0.5 Amps. At 230 Volts AC or 220 Volts. DC.

Pre commissioning Inspection of Transformer:

  • Sample of oil taken from the transformer and subjected to  electric test (break down value) of 50KV (RMS) as specified in IS : 335.
  • Release trapped air through air release plugs and valve fitted for the purpose on various fittings like radiators, bushing caps, tank cover, Bushing turrets etc.
  • The float lever of the magnetic oil level indicator (if provided) should be moved up and down  between the end position to check that the mechanism does not stick at any point. If the  indicator has signaling contact they should be checked  at the same time for correct operation.  Checking the gauge by draining oil is a more positive test.
  • Check whether  gas operated really (if provided) is mounted at angle by placing a spirit  level on the top of the relay. See that the conservator is filled upto the filling oil level marked on plain oil gauge side and corresponding to the pointer reading in MOG side. Check the operation of the alarm and trip contacts of the relay independently by injecting air through the  top cocks  using a dry air bottle. The air should  be released after the tests. Make sure that transformer oil runs through pert cock of Buchholz relay.
  • Check alarm and trip contacts of WTIs, Dial type thermometer, magnetic oil gauge etc. (if  provided).
  • Ensure that off circuit switch  handle is locked at the desired tap position with padlock.
  • Make sure that all valves except drain, filter and sampling valves are opened (such as radiator valves, valves on the buchholz relay pipe line if Provided).
  • Check  the condition of silicagel in the breather to ensure that silicagel in the breather is active and colour is blue. Also check that the transformer oil is filled in the silicagel breather upto the level indicated.
  • Check tightness of external electrical connections  to bushings.
  • Give a physical  check on all bushing  for any crack or any breakage of porcelain. Bushing  with cracks or any other defects should be immediately replaced.
  • Check the neutral earthing  if specified.
  • Make sure that neutrals of HV / LV are effectively earthed.
  • Tank should be  effectively earthed at two points.
  • Check that the thermometer pockets on tank cover are filled with oil.
  • If  the oil temperature indicator  is not working satisfactorily, loosen and remove the  thermometer bulb from the pocket on the cover and place it with a standard thermometer in a suitable vessel filled with transformer oil. Warm the oil slowly while  string it and take reading of the thermometers if an adjustment of the transformer  thermometer is necessary  the  same many be done. Also check signaling contacts and set for the desired temperature.
  • CT secondary terminals must be shorted and earthed if not in use.
  • Check relief vent diaphragm for breakage. See that the Bakelite diaphragm at bottom and glass diaphragm at top are not ruptured.
  • Check all the gasket joints to ensure that there is no leakage of transformer oil at any point.
  • Clear off extraneous material like tools earthling rods, pieces of clothes, waste etc.
  • Lock the rollers for accidental movement on rails.
  • Touching of paint may be done after erection.

Parts of Transformer:

1)    Transformer Oil

  • Oil is used as coolant and dielectric in the transformer and keeping it in good condition will assist in preventing deterioration of the insulation, which is immersed in oil. Transformer oil is always exposed to the air to some extent therefore in the course of time it may oxidize and form sludge if the breather is defective, oil may also absorb moisture from air thus reducing dielectric strength.

2)    Transformer Winding:

  • The primary and secondary windings in a core type transformer are of the concentric  type only, while in case of shell type transformer these could be of sand-witched type as well. The concentric windings are normally constructed in any of the following types depending on the size and application of the transformer.
  • (1)Cross over Type.
  • (2) Helical Type.
  • (3) Continuous Disc Type.
  •  Distributed.
  • Spiral.
  •  Interleaved Disc.
  •  Shielded Layer

a)    Distributed Winding :

  •  Used   for   HV windings   of   small   Distribution   Transformers where   the   current   does  not   exceed   20   amps  using   circular   cross  section conductor .

b)   Spiral:

  • Used  up   to  33  kv for  low  currents using  strip  conductor. Wound closely  on  Bakelite or press board cylinders generally without cooling ducts. However, multi layer windings are provided with cooling ducts between layers. No Transposition is necessary.

c)    Interleaved Disc:

  • Used for voltages above 145 kv . Interleaving enables the winding withstand higher impulse voltages.

d)   Shielded Layer :

  • Used up to 132 kv in star connected windings with graded insulation. Comprises of a number of concentric spiral coils arranged in layers grading   the   layers.
  • The  longest  at the  Neutral  and  the  shortest  at  the  Line Terminal. The layers are separated by cooling ducts. This type of construction ensures uniform distributed voltages.

e)    Cross-over type winding:

  • It is normally employed where rated currents are up-to about 20 Amperes or so.
  •  In this type of winding, each coil consists of number of layers having number of turns per layer. The conductor being a round wire or strip insulated with a paper covering.
  • It is normal practice to provide one or two extra lavers of paper insulation between lavers. Further, the insulation between lavers is wrapped round the end turns of the lavers there by assisting to keep the whole coil compact.
  • The inside end of a coil is connected to the outside end of adjacent coil. Insulation blocks are provided between adjacent coils to ensure free circulation of oil.

f)     Helical winding:

  •  Used for Low Voltage and high currents .The turns comprising of a number   of   conductors   are   wound axially. Could be   single, double or   multi layer   winding.   Since   each   conductor   is   not   of   the   same   length,   does not embrace the   same   flux and   of  different  impedances,   and  hence  circulating currents, the winding is Transposed.
  • The coil consists of a number of rectangular strips wound in parallel racially such that each separate turn occupies the total radial depth of the winding.
  • Each turn is wound on a number of key spacers which form the vertical oil duct and each turn or group of turns is spaced by radial keys sectors.
  • This ensures free circulation of oil in horizontal and vertical direction.
  • This type of coil construction is normally adopted for low voltage windings where the magnitude of current is comparatively large.
  • Helical Disc winding:
  • This type of winding is also termed “interleaved disk winding.”
  • Since conductors 1 – 4 and conductors 9 – 12 assume a shape similar to a wound capacitor, it is known that these conductors have very large capacitance. This capacitance acts as series capacitance of the winding to highly improve the voltage distribution for surge.
  • Unlike cylindrical windings, Helical disk winding requires no shield on the winding outermost side, resulting in smaller coil outside diameter and thus reducing Transformer dimension. Comparatively small in winding width and large in space between windings, the construction of this type of winding is appropriate for the winding, which faces to an inner winding of relatively high voltage.
  • Thus, general EHV or UHV substation Transformers employ Helical disk winding to utilize its features mentioned above.

g)   Continuous disc type of windings:

  • Used for 33kv and 132 kv for  medium currents. The coil comprises   of   a   number   of   sections   axially.   Cooling   ducts   are   provided between each section.
  • IT is consists of number of Discs wound from a single wire or number of strips in parallel. Each disc consists of number of turns, wound radically, over one another.
  • Arrangement of layers
  • The conductor passing uninterruptedly from one disc to another. With ultiple-strip conductor. Transpositions are made at regular intervals to ensure uniform resistance and length of conductor. The discs are wound on an insulating cylinder spaced from it by strips running the whole length of the cylinder and separated from one another by hard pressboard sectors keyed to the vertical strips.
  • This ensures free circulation of oil in horizontal and vertical direction and provides efficient heat dissipation from windings to the oil.
  •  The whole coil structure is mechanically sound and capable of resisting the most enormous short circuit forces.
  • This is the most general type applicable to windings of a wide range of voltage and current
  • Rectangular wire is used where current is relatively small, while transposed cable Fig. (12) is applied to large current. When voltage is relatively low, a Transformer of 100MVA or more capacity handles a large current exceeding 1000A. In this case, the advantage of transposed cable may be fully utilized
  • Since the number of turns is reduced, even conventional continuous disk construction is satisfactory in voltage distribution, thereby ensuring adequate dielectric characteristics. Also, whenever necessary, potential distribution is improved by inserting a shield between turns.
  •  According to the number of layers used the paper is applied as follows.
  •  Two layers: =Where there are two layers both of them are wound in opposite directions.
  •  More than two layers: =Where there are more than two layers all the layers are applied in the same direction, all,  except  the  outermost  layer  is  butt  wound,  and  the  outermost  layer  is  overlap wound. Within each group of papers the position of the butt joints of any layer relative to the layer below is progressively displaced by approximately 30 percent of the paper width.
  •  Note: Overlapping can also be done as per customer requirements.
  • Grade of paper
  •  The paper, before  application, is ensured to be free  from  metallic  and  other  injurious inclusions  and    have  no  deleterious  effect  on  insulating  oil.
  • The thickness  of  paper used is between 0.025 mm to 0.075 mm.
  • Enameled Conductor
  •  Apart from paper covered conductors, we have all the facilities of producing enameled conductors as per customer specified requirements.
  • Copper –  Usually in 8 – 16mm rods is drawn to the  required sizes and then insulated with paper etc..
  •  Annealing is done for softening and stress relieving in electrically heated annealing plant under vacuum upto 400-500ºC. After 48hrs when the temperature reaches ambient, the vacuum is slowly released and the material is transferred to Insulation section.
  •  Conductors are one  of  the principal materials used in  manufacturing  of  transformers. Best quality of  copper  rods are procured from indigenous as  well as foreign sources. Normally 8 mm & 11 mm rods are procured. For each supply  of input, test  certificate from suppliers is obtained and at times.
  •  After  the  wires  &  strips are drawn  as per clients  requirements they are moved  on  to paper  covering  process.
  • To  prevent  the inclusion  of  copper  dust  or other extraneous matter under paper covering the conductor is fully cleaned by felt pads or other suitable means  before  entering  the  paper  covering  machine.  As  per  the  customers requirements DPC, TPC & MPC conductors are produced. It is ensured that each layer of paper is continuous, firmly applied and substantially free from creases.
  • No bonding or adhesive material  is used except  to  anchor the ends of paper.   Any  such  bonding materials  used  to  anchor  the  ends  do  not have  deleterious  effect  on transformer  oil, insulating  paper  or  the  electric  strength  of  the  covering.    It is ensured  that  the overlapping percentage is not less than 25% of the paper width.
  • The rectangular paper-covered copper conductor is the most commonly used conductor for the windings of medium and large power transformers.
  • These conductors can be individual strip conductors, bunched conductors or continuously transposed cable (CTC) conductors.
  •  In low voltage side of a distribution transformer, where much fewer turns are involved, the use of copper or aluminum foils may find preference.
  • To enhance the short circuit withstand capability, the work hardened copper is commonly used instead of soft annealed copper, particularly for higher rating transformers
  • In the case of a generator transformer having high current rating, the CTC conductor is mostly used which gives better space factor and reduced eddy losses in windings. When the CTC conductor is used in transformers, it is usually of epoxy bonded type to enhance its short circuit strength.

3)    Transformer Core:

  • Purpose of the core: 
  • To reduce the magnetizing current. (For topologies such as Forward, Bridge etc we need the magnetizing current to be as small as possible. For fly-back topology, though the magnetizing current is used to transfer energy, the size of the transformer will be very large to get the required inductance if a core is not used.)
  • To improve the linkage of the flux within windings if  the windings are separated spatially.
  • To contain the magnetic flux within a given volume
  •  In magnetic amplifier applications a saturable core is used as a switch.
  • Core Material:
  •  Different types of material used for cores
  •  Iron-Silicon Steel- Nickel-Iron-Iron-Cobalt-Ferrite-Molybdenum-Met-glass
  •  Salient characteristics of a core material:  
  •  Permeability, Saturation flux density, Coercive force, Remnant flux, Losses due to           Hysteresis & Eddy Current.
  • The power loss is a function of frequency and the ac flux swing and is given by the equation P = K1 * (frequency)K2 * (Flux Density)K3
  •  Every transformer has a core, which is surrounded by windings. The core is made out of special cold rolled grain oriented silicon sheet steel laminations. The special silicon steel ensures low hysteretisis losses. The silicon steel laminations also ensure high resistively of core material which result in low eddy currents. In order to reduce eddy current losses, the laminations are kept as thin as possible. The thickness of the laminations is usually around 0.27 to 0.35 mm.
  • Transformer cores construction is of two types, viz, core type and shell type. In core type transformers, the windings are wound around the core, while in shell type transformers, the core is constructed around the windings. The shell type transformers provide a low reactance path for the magnetic flux, while the core type transformer has a high leakage flux and hence higher reactance.
  • The limb laminations in small transformers are held together by stout webbing tape or by suitably spaced glass fiber bends. The use of insulated bolts passing through the limb laminations has been discontinued due to number of instances of core bolt failures. The top and bottom mitered yokes are interleaved with the limbs and are clamped by steel sections held together by insulated yoke bolts. The steel frames clamping the top and bottom yokes are held together by vertical tie bolts.
  • Grain Oriented steel sheets namely ORIENTCORE, ORIENTCORE H1-B & ORIENTCORE HI-B.LS are some of the finest quality of core.
  • ORENTCORE.HI-B  is  a  breakthrough  in  that  it  offers  higher  magnetic  flux  density, lower  core loss  and  lower  magnetostriction  than  any  conventional  grain-oriented electrical steel sheet.
  • ORIENT.HI-B.LS  is  a  novel  type  with  marked  lower  core  losses,  produced  by  laser irradiation of the surface of ORIENTCORE.HI-B sheets.
  • Annealing of stacked electrical sheets
  • Annealing is to be done at 760 to 845ºC to
  • Reduce mechanical stress
  •  Prevent contamination
  • Enhance insulation of lamination coating
  • Though  ORIENTCORE  and  ORIENTCORE.HI-B  are  grain  orient  steel  sheets  with excellent  magnetic  properties,  mechanical  stress  during  such  operations  as  cutting, punching  and bending  affect their  magnetic  properties adversely.  When these stress are excessive, stress relief annealing is necessary. 
  • Following method is observed for stress relief annealing
  • Available Grades:
  1.  Stacked  electrical  steel  sheets  are  heated  thoroughly  in  the  edge-to-edge direction  rather  than  in the  face-to-face  direction,  because  heat  transfer  is  far faster in side heating.
  2.  A cover is put over sheets stacked on a flat plate. Because ORIENTCORE and ORIENTCORE.HI-B  have  extremely  low  carbon  content  and  very  easily decarburized at annealing temperatures, the base, cover and other accessories used are of very low carbon content .
  3. To prevent oxidation so as to protect the coating on the sheets, a no oxidizing atmosphere free from carbon sources is used having less than 2%hydrogen or high-purity  nitrogen  gas.  Due point of  the  atmosphere  is  maintained  at  0ºC  or less.
  4. Care  is  taken  to  the  flatness  of  annealing  base,  because  an  uneven  base distorts cores, leading to possible  distortion during assembly.
  5.  Annealing  temperature  ranging  from  780ºC  to  820ºC  is  maintained  for  more than 2 hours or more. Cooling is done upto 350ºC in about 15 hours or more.
  •  ORIENTCORE           :M1, M2, M3, M4, M5 & M6
  •  ORIENTCORE.HI-B    :23ZH90, 23ZH95, 27ZH95, 27ZH100, 30ZH100,M-0H, M-1H, M-2H, M-3H
  • ORIENTCORE.HI-B.LS: 23ZDKH90, 27ZDKH95
  •  Non-oriented silicon steel, hot rolled grain oriented silicon steel,cold rolled grain oriented (CRGO) silicon steel, Hi-B, laser scribed and mechanically scribed. The last three materials are improved versions of CRGO.
  •  Saturation flux density has remained more or less constant around 2.0 Tesla for CRGO; but there is a continuous improvement in watts/kg and volt-amperes/kg characteristics in the rolling direction.
  •  The core building technology has improved from the non-mitred to mitred and then to the step-lap construction
  • The better grades of core steel not only reduce the core loss but they also help in reducing the noise level by few decibels
  •  Use of amorphous steel for transformer cores results in substantial core loss reduction (loss is about one-third that of CRGO silicon steel). Since the manufacturing technology of handling this brittle material is difficult, its use in transformers is not widespread
  •  In the early days of transformer manufacturing, inferior grades of laminated steel (as per today’s standards) were used with inherent high losses and magnetizing volt-amperes. Later on it was found that the addition of silicon content of about 4 to 5% improves the performance characteristics significantly, due to a marked reduction in eddy losses (on account of the increase in material resistivity) and increase in permeability. Hysteresis loss is also lower due to a narrower hysteresis loop. The addition of silicon also helps to reduce the aging effects.
  •  Although silicon makes the material brittle, it is well within limits and does not pose problems during the process of core building.
  •  The cold rolled manufacturing technology in which the grains are oriented in the direc tion of rolling gave a new direction to material development for many decades, and even today newer materials are centered around the basic grain orientation process.
  •  Important stages of core material development are: non-oriented, hot rolled grain oriented (HRGO), cold rolled grain oriented (CRGO), high permeability cold rolled grain oriented (Hi-B), laser scribed and mechanically scribed.
  •  Laminations with lower thickness are manufactured and used to take advantage of lower eddy losses. Currently the lowest thickness available is 0.23 mm, and the popular thickness range is 0.23 mm to 0.35 mm for power transformers.
  •  Maximum thickness of lamination used in small transformers can be as high as 0.50 mm.
  •  Inorganic coating (generally glass film and phosphate layer) having thickness of 0.002 to 0.003 mm is provided on both the surfaces of laminations, which is sufficient to withstand eddy voltages (of the order of a few volts).
  •  Since the core is in the vicinity of high voltage windings, it is grounded to drain out the statically induced voltages. While designing the grounding system, due care must be taken to avoid multiple grounding, which otherwise results into circulating currents and subsequent failure of transformers.

4)    Transformer Core:

a)    Core Type Construction: (Mostly Used):

  • Generally in  India, Core  type  of construction  with Two/Three/Five limbed cores are used. Generally five limbed cores are used where the dimensions of the Transformer is to be limited due to Transportation difficulties. In three limbed core the cross section of the Limb and the Yoke are the same where as in five Limbed core, the cross section of the Yoke and the Flux return  path  Limbs are  ver y  less (58%  and  45%  of  the  principal  Limb). 
  • Limb:which is surrounded by windings, is called a limb or leg? 
  • York: Remaining part of the core, which is not surrounded by windings, but is essential for completing the path of flux, is called as yoke.
  • Advantage:
  • Construction is simpler, cooling is better and repair is easy.
  •  The yoke and end limb area should be only 50% of the main limb area for the same operating flux density.
  • Zero-sequence impedance is equal to positive-sequence impedance for this construction (in a bank of single-phase transformers).
  • Sometimes in a single-phase transformer windings are split into two parts and placed around two limbs as shown in figure (b). This construction is sometimes adopted for very large ratings. Magnitude of short-circuit forces are lower because of the fact that ampere-turns/height are reduced. The area of limbs and yokes is the same. Similar to the single-phase three-limb transformer.
  •  The most commonly used construction, for small and medium rating transformers, is three-phase three-limb construction as shown in figure (d).For each phase, the limb flux returns through yokes and other two limbs (the same amount of peak flux flows in limbs and yokes).
  •  limbs and yokes usually have the same area. Sometimes the yokes are provided with a 5% additional area as compared to the limbs for reducing no-load losses.
  •  It is to be noted that the increase in yoke area of 5% reduces flux density in the yoke by 5%, reduces watts/kg by more than 5% (due to non-linear characteristics) but the yoke weight increases by 5%. Also, there may be additional loss due to cross-fluxing since there may not be perfect matching between lamination steps of limb and yoke at the joint. Hence, the reduction in losses may not be very significant.
  • In large power transformers, in order to reduce the height for transportability, three-phase five-limb construction depicted in figure (e) is used. The magnetic length represented by the end yoke and end limb has a higher reluctance as compared to that represented by the main yoke. Hence, as the flux starts rising, it first takes the path of low reluctance of the main yoke. Since the main yoke is not large enough to carry all the flux from the limb, it saturates and forces the remaining flux into the end limb. Since the spilling over of flux to the end limb occurs near the flux peak and also due to the fact that the ratio of reluctances of these two paths varies due to non-linear properties of the core.
  • Fluxes in both main yoke and end yoke/end limb paths are non-sinusoidal even though the main limb flux is varying sinusoidal [2,4]. Extra losses occur in the yokes and end limbs due to the flux harmonics. In order to compensate these extra losses, it is a normal practice to keep the main yoke area 60% and end yoke/end limb area 50% of the main limb area.
  • The zero-sequence impedance is much higher for the three-phase five-limb core than the three-limb core due to low reluctance path (of yokes and end limbs) available to the in-phase zero-sequence fluxes, and its value is close to but less than the positive-sequence impedance value.

b)   Shell-type construction:

  • Cross section of windings in the plane of core is surrounded by limbs and yokes, is also used.
  • Shell   type   of   construction   of   the   core   is   widely   used   in   USA.
  • Advantage:
  • One can use sandwich construction of LV and HV windings to get very low impedance, if desired, which is not easily possible in the core-type construction.
  • Analysis of overlapping joints and building factor:
  • While building a core, the laminations are placed in such a way that the gaps between the laminations at the joint of
  • limb and yoke are overlapped by the laminations in the next layer.
  • This is done so that there is no continuous gap at the joint when the laminations are stacked one above the other (figure). The overlap distance is kept around 15 to 20 mm.
  • There are two types of joints most widely used in transformers: non-mitred and mitred joints.
  • Non-mitered joints:
  • In which the overlap angle is 90°, are quite simple from the manufacturing point of view, but the loss in the corner joints is more since the flux in the joint region is not along the direction of grain orientation. Hence, the on-mitred joints are used for smaller rating transformers. These joints were commonly adopted in earlier days when non-oriented material was used
  • Non-mitered joints:
  • In which the overlap angle is 90°, are quite simple from the manufacturing point of view, but the loss in the corner joints is more since the flux in the joint region is not along the direction of grain orientation. Hence, the on-mitred joints are used for smaller rating transformers. These joints were commonly adopted in earlier days when non-oriented material was used
  • Mitered joints:
  •  The joint where these laminations meet could be Butt or Mitred. In CRGO, the Mitred  Joint is preferred  as it reduces the  Reluctance  of  the  Flux  path  and reduces the No Load Losses and the No Load current (by about 12% & 25% respectively).
  •  The Limb and  the Yoke are made of a number  of Laminations in Steps. Each step  comprises of  some  number  of  laminations  of  equal  width. The  width   of  the  central  strip  is Maximum   and  that at  the  circumference  is Minimum. The   cross  section   of  the  Yoke  and  the   Limb  are  nearly Circular. Mitred  joint  could  be at 35/45/55  degrees but the  45  one  reduces wastage.
  • The angle of overlap (a) is of the order of 30° to 60°, the most commonly used angle is 45°. The flux crosses from limb to yoke along the grain orientation in mitred joints minimizing losses in them. For airgaps of equal length, the excitation requirement of cores with mitred joints is sin a times that with non-mitred joints.
  • Better grades of core material (Hi-B, scribed, etc.) having specific loss (watts/kg) 15 to 20% lower than conventional CRGO material (termed hereafter as CGO grade, e.g., M4) are regularly used. However, it has been observed that the use of these better materials may not give the expected loss reduction if a proper value of building factor is not used in loss calculations
  • The building factor generally increases as grade of the material improves from CGO to Hi-B to scribed (domain refined). This is a logical fact because at the corner joints the flux is not along the grain orientation, and the increase in watts/kg due to deviation from direction of grain orientation is higher for a better grade material.
  • The factor is also a function of operating flux density; it deteriorates more for better grade materials with the increase in operating flux density. Hence, cores built with better grade material may not give the expected benefit in line with Epstein measurements done on individual lamination. Therefore, appropriate building factors should be taken for better grade materials using experimental/test data.
  • Also the loss contribution due to the corner weight is higher in case of 90° joints as compared to 45° joints since there is over-crowding of flux at the inner edge and flux is not along the grain orientation while passing from limb to yoke in the former case. Smaller the overlapping length better is the core performance; but the improvement may not be noticeable.
  •  The gap at the core joint has significant impact on the no-load loss and current. As compared to 0 mm gap, the increase in loss is 1 to 2% for 1.5 mm gap, 3 to 4% for 2.0 mm gap and 8 to 12% for 3 mm gap. These figures highlight the need for maintaining minimum gap at the core joints.
  •  Lesser the laminations per lay, lower is the core loss. The experience shows that from 4 laminations per lay to 2 laminations per lay, there is an advantage in loss of about 3 to 4%. There is further advantage of 2 to 3% in 1 lamination per lay. As the number of laminations per lay reduces, the manufacturing time for core building increases and hence most of the manufacturers have standardized the core building with 2 laminations per lay.
  • Joints of limbs and yokes contribute significantly to the core loss due to cross-fluxing and crowding of flux lines in them. Hence, the higher the corner area and weight, the higher is the core loss.
  • The corner area in single-phase three-limb cores, single-phase four-limb cores and three-phase five-limb cores is less due to smaller core diameter at the corners, reducing the loss contribution due to the corners. However, this reduction is more than compensated by increase in loss because of higher overall weight (due to additional end limbs and yokes).
  • Building factor is usually in the range of 1.1 to 1.25 for three-phase three-limb cores with mitred joints. Higher the ratio of window height to window width, lower is the contribution of corners to the loss and hence the building factor is lower.
  • Step-lap joint :
  •  It is used by many manufacturers due to its excellent performance figures. It consists of a group of laminations (commonly 5 to 7) stacked with a staggered joint as shown in figure.
  •  Its superior performance as compared to the conventional mitred construction.
  •  It is shown that, for a operating flux density of 1.7 T, the flux density in the mitred joint in the core sheet area shunting the air gap rises to 2.7 T (heavy saturation), while in the gap the flux density is about 0.7 T. Contrary to this, in the step-lap joint of 6 steps, the flux totally avoids the gap with flux density of just 0.04 T, and gets redistributed almost equally in laminations of other five steps with a flux density close to 2.0 T. This explains why the no-load performance figures (current, loss and noise) show a marked improvement for the step-lap joints.
  • The   assembled   core   has  to   be   clamped  tightly not  only  to   provide   a  rigid mechanical   structure   but   also   required   magnetic   characteristic.   Top   and Bottom Yokes are clamped by   steel sections using Yoke Studs. These studs do not pass through the core  but held  between steel sections. Of late Fiber Glass Band tapes are wound round the Limbs tightly upto the desired tension and  heat treated. These laminations , due to elongation and contraction  lead to magnetostriction, generally called Humming which can be reduced by using higher  silicon  content  in   steel   but  this  makes  the  laminations become   very brittle.
  •  The choice of operating flux density of a core has a very significant impact on the overall size, material cost and performance of a transformer.
  •  For the currently available various grades of CRGO material, although losses and magnetizing volt-amperes are lower for better grades, viz. Hi-B material (M0H, M1H, M2H), laser scribed, mechanical scribed, etc., as compared to CGO material (M2, M3, M4,M5, M6, etc.), the saturation flux density has remained same (about 2.0 T).
  • The peak operating flux density (Bmp ) gets limited by the over-excitation conditions specified by users.
  • The slope of B-H curve of CRGO material significantly worsens after about 1.9 T (for a small increase in flux density, relatively much higher magnetizing current is drawn). Hence, the point corresponding to 1.9 T can be termed as knee-point of the B-H curve.
  • It has been seen in example 1.1 that the simultaneous over-voltage and under-frequency conditions increase the flux density in the core. Hence, for an over-excitation condition (over-voltage and under-frequency).
  • When a transformer is subjected to an over-excitation, core contains an amount of flux sufficient to saturate it. The remaining flux spills out of the core. The over-excitation must be extreme and of a long duration to produce damaging effect in the core laminations
  • The laminations can easily withstand temperatures in the region of 800°C (they are annealed at this temperature during their manufacture), but insulation in the vicinity of core laminations, viz. press-board insulation (class A: 105°C) and core bolt insulation (class B: 130°C) may get damaged. Since the flux flows in air (outside core) only during the part of a cycle when core gets saturated, the air flux and exciting current are in the form of pulses having high harmonic content which increases the eddy losses and temperature rise in windings and structural parts.

Winding Insulation in Transformer:

  •  Requirement of Insulating Oil:
  •  1.0 lit / kva for Trs from 400 – 1600 Kva
  • 0.6 lit / kva for Trs from 1600 – 80,000 kva
  • 0.5 lit / Kva for Trs above 80,000 Kva.
  • In Transformers, the insulating oil provides an insulation medium as well as a heat transferring medium that carries away heat produced in the windings and iron core. Since the electric strength and the life of a Transformer depend chiefly upon the quality of the insulating oil, it is very important to use a high quality insulating oil
  • Provide a high electric strength.
  • Permit good transfer of heat.
  •  Have low specific gravity-In oil of low specific gravity particles which have become suspended  in the oil will settle down on the bottom of the tank more readily and at a faster rate, a property aiding the oil in retaining its homogeneity.
  •  Have a low viscosity- Oil with low viscosity, i.e., having greater fluidity, will cool Transformers at a much better rate.
  • Have low pour point- Oil with low pour point will cease to flow only at low temperatures.
  • Have a high flash point. The flash point characterizes its tendency to evaporate. The lower the flash point the greater the oil will tend to vaporize When oil vaporizes, it loses in volume, its viscosity rises, and an explosive mixture may be formed with the air above the oil
  • The Core Insulation is:
  •  SRBP- Synthetic Resin Bonded Paper
  •  OIP   – Oil Impregnated Paper
  • RIP   – Resin Impregnated Pape
  • Resin Coated Paper/ Kraft Paper/ Crepe Kraft Paper are used for making core for the above It is Hermetically Sealed.
  •  Pre-compressed pressboard is used in windings as opposed to the softer materials used in earlier days. The major insulation (between windings, between winding and yoke, etc.)
  •  Mineral oil has traditionally been the most commonly used electrical insulating medium and coolant in transformers. Studies have proved that oil-barrier insulation system can be used at the rated voltages greater than 1000 Kv.
  • A high dielectric strength of oil-impregnated paper and pressboard is the main reason for using oil as the most important constituent of the transformer insulation system.
  •  Manufacturers have used silicon-based liquid for insulation and cooling. Due to non-toxic dielectric and self-extinguishing properties, it is selected as a replacement of Askarel. High cost of silicon is an inhibiting factor for its widespread use.
  • Super-biodegradable vegetable seed based oils are also available for use in environmentally sensitive locations.
  • SF6 gas has excellent dielectric strength and is non-flammable. Hence, SF6 transformers find their application in the areas where fire-hazard prevention is of paramount importance.
  • Due to lower specific gravity of SF6 gas, the gas insulated transformer is usually lighter than the oil insulated transformer. The dielectric strength of SF6 gas is a function of the operating pressure; the higher the pressure, the higher the dielectric strength.
  • However, the heat capacity and thermal time constant of SF6 gas are smaller than that of oil, resulting in reduced overload capacity of SF6 transformers as compared to oil-immersed transformers. Environmental concerns, sealing problems, lower cooling capability and present high cost of manufacture are the challenges.
  • Dry-type resin cast and resin impregnated transformers use class F or C insulation. High cost of resins and lower heat dissipation capability limit the use of these transformers to small ratings.
  • The dry-type transformers are primarily used for the indoor application in order to minimize fire hazards. Nomex paper insulation, which has temperature withstand capacity of 220°C, is widely used for dry-type transformers. The initial cost of a dry-type transformer may be 60 to 70% higher than that of an oil-cooled transformer at current prices, but its overall cost at the present level of energy rate can be very much comparable to that of the oil-cooled transformer.

Transformer Noise:

  •  Transformers located near a residential area should have sound level as low as possible.
  • Levels specified are 10 to 15 dB lower than the prevailing levels mentioned in the international standards.
  • Core, windings and cooling equipment are the three main sources of noise.
  • The core is the most important and significant source of the transformer noise.
  • The core vibrates due to magnetic and magnetostrictive forces. Magnetic forces appear due to non-magnetic gaps at the corner joints of limbs and yokes
  •  These magnetic forces depend upon the kind of interlacing between the limb and yoke; these are highest when there is no overlapping (continuous air gap).
  • The magnetic forces are smaller for 90° overlapping, which further reduce for 45°overlapping. These are the least for the step-lap joint due to reduction in the value of flux density in the overlapping region at the joint.
  • The forces produced by the magnetostriction phenomenon are much higher than the magnetic forces in transformers.
  • Magnetostriction is a change in configuration of magnetizable material in a magnetic field, which leads to periodic changes in the length of material. An alternating field sets the core in vibration.
  • This vibration is transmitted, after some attenuation, through the oil and tank structure to the surrounding air. This finally results in a characteristic hum.
  • The magnetostriction force varies with time and contains even harmonics of the power frequency (120, 240, 360, —Hz for 60 Hz power frequency). Therefore, the noise also contains all harmonics of 120 Hz.
  •  The amplitude of core vibration and noise increase manifold if the fundamental mechanical natural frequency of the core is close to 120 Hz.
  • The value of the magnetostriction can be positive or negative, depending on the type of the magnetic material, and the mechanical and thermal treatments.
  • Magnetostriction is generally positive (increase in length by a few microns with increase in flux density) for CRGO material at annealing temperatures below 800°C, and as the annealing temperature is increased (=800°C), it can be displaced to negative values.The mechanical stressing may change it to positive values
  • Magnetostriction is minimum along the rolling direction and maximum along the 90° direction.
  • Most of the noise transmitted from a core comes principally from the yoke region because the noise from the limb is effectively damped by windings (copper and insulation material) around the limb.
  • The quality of yoke clamping has a significant influence on the noise level.
  •  Apart from the yoke flux density, other factors which decide the noise level are: limb flux density, type of core material, leg center (distance between the centers of two adjacent phases), core weight, frequency, etc.
  • The higher the flux density, leg centers, core weight and frequency of operation, the higher is the noise level.
  • The noise level is closely related to the operating peak flux density and core weight.
  •  If core weight is assumed to change with flux density approximately in inverse proportion, for a flux density change from 1.6 T to 1.7 T, the increase in noise level is 1.7 dB
  • Hence, one of the ways of reducing noise is by designing transformer at lower operating flux density. For a flux density reduction of 0.1 T, the noise level reduction of about 2 dB is obtained. This method results into an increase of material content and it may be justified economically if the user has specified a lower no-load loss, in which case the natural choice is to use a lower flux density.
  • Use of step-lap joint gives much better noise reduction (4 to 5 dB).
  • Some manufacturers also use yoke reinforcement (leading to reduction in yoke flux density); the method has the advantage that copper content does not go up since the winding mean diameters do not increase. Bonding of laminations by adhesives and placing of anti-vibration/damping elements between the core and tank can give further reduction in the noise level.
  • The use of Hi-B/scribed material can also give a reduction of 2 to 3 dB. When a noise level reduction of the order of 15 to 20 dB is required, some of these methods are necessary but not sufficient.

Transformer Protection:

Internal Protection:

(1) Bucholtz Relay:  

  • This Gas operated relay is a protection for minor  and major  faults that  may develop inside  a Transformer  and  produce  Gases.
  •  This   relay   is   located   in   between   the   conservator   tank   and   the   Main Transformer tank in the pie line which is mounted at an inclination of 3 to 7 degrees.
  •  A shut off valve is located in between the Bucholtz relay and the Conservator.
  • The relay comprises of  a cast housing    which contains two pivoted   Buckets  counter   balanced  weights.
  • The   relay  also  contains  two mercury y  switches   which   will   send   alarm   or   trip   signal   to   the   breakers controlling the Transformer. In healthy condition, this relay will be full of oil and the   buckets will   also  be   full  of  oil   and   is  counter   balanced  by  the weights.
  • In the event of a fault inside the transformer, the gases flow up to the conservator via the relay and pushes the  oil in the relay down. Once the oil level falls below the bottom level of the  buckets, the bucket due to the weight of oil inside tilts and closes the mercury switch and causes the Alarm or trip to be actuated and isolate the transformer from the system.

(2) Oil Surge /  Bucholtz   Relay for OLTC: 

  • This   relay   operating   on   gas produced  slowly or  in a  surge  due  to  faults inside  the  Diverter  Switch of OLTC protects the Transformer and isolates it from the system.

(3) Pressure Relief Valve for Large Transformers:

  •  In   case of  a   serious fault   inside   the   Transformer,   Gas   is   rapidly   produced.
  • This   gaseous pressure must be relieved immediately otherwise it will damage the Tank and  cause damage to neighboring equipment.
  • This relay is mounted on the  top  cover  or  on  the  side   walls  of  the  Transformer.  The  valve  has a corresponding  port which  will be  sealed by a  stain less steel  diaphragm .
  • The   diaphragm   rests on   a   O   ring   and   is   kept   pressed   by two   heavy springs. If a high pressure is developed inside, this diaphragm lifts up and releases   the   excessive   gas.
  •  The   movement   of   the   diaphragm   lifts the spring  and  causes  a  micro   switch  to  close  its contacts to  give  a  trip signal  to   the  HV  and  LV  circuit  breakers  and  isolate   the  transformer.
  •  A visual   indication can  also  be   seen   on   the  top   of  the  relay.   For   smaller capacity   transformer,   an   Explosion   vent   is   used   to   relieve   the   excess pressure but it cannot isolate the Transformer.

(4) Explosion Vent Low & Medium Transformers  : 

  •  For smaller capacity Transformers, the excessive pressures inside a Transformer due to  major faults inside  the  transformer  can  be  relieved by Explosion vents. But this cannot isolate the Transformer.

(5) Winding /Oil Temperature   Protection   :

  • These   precision   instruments operate on the principle of liquid expansion.
  •  These record the hour to hour temperatures and also record the Maximum temperature over  a  period of time  by a  resettable pointer.
  • These in conjunction with mercury switches provide   signals for   excessive   temperature   alarm   annunciation   and   also isolate   the   Transformer   for   very   excessive   temperatures.
  • These   also switch on the cooler fans and cooler pumps if the temperature exceeds the set values. Normally two separate instruments are used for oil and winding temperatures.
  •  In   some   cases   additional   instruments     are   provided separately for  HV,LV  and  Tertiary winding  temperatures.
  • The  indicator   is provided  with  a  sensing  bulb  placed  in  an  oil  pocket  located  on  the  top cover  of the  Transformer  tank.  The  Bulb  is connected  to  the  instrument housing  by means of flexible connecting  tubes consisting of two capillary tubes.
  •  One   capillary   tube   is   connected   to   an   operating   Bellow   in   the instrument. The  other  is  connected  to  a compensating  Bellow  .
  • The  tube follows the same path as the one with the Bulb but the other  end, it does not   end   in   a   Bulb   and   left   sealed.   This   compensates   for   variations in Ambient Temperatures.
  • As the temperature varies, the volume of the liquid in the  operating  system  also  varies   and  operates the  operating  Bellows transmitting its movements to the pointer and also the switching disc. This disc is  mounted  with  mercury  float  switches which  when  made  provides signals to alarm/trip/cooler controls.
  • Oil and winding temperature indicators work   on   the   same   principles except   that   the   WTI is   provided   with   an additional   bellows     heating   element.   This  heating   element   is  fed   by  a current transformer  with  a current  proportional  to  the load  in the  winding whose   temperature   is   to   be   measured/monitored.   The   tem premature increase of the heating  element is proportional to  the temperature rise of winding over top oil  temperature.
  • The operating bellow gets an additional movement   simulating   the   increase   of   winding   temperature   over   top   oil temperature and represents the Winding Hot Spot. This is called Thermal Imaging process.

(6) Conservator   Magnetic Oil Level Protection   : 

  • Inside   the   conservator tank, a float is used to sense the levels of oil and move. This is transmitted to a switch mechanism by means of magnetic coupling. The Float and the Magnetic   mechanism   are   totally   sealed.   The   pointer   connected   to   the magnetic   mechanism   moves   indicating   the   correct   oil   level   and   also provision is m ade for Low oil level alarm by switch.

(7) Silica gel Breather: 

  • This is a means to preserve the dielectric strength of insulating oil and prevent  absorption of moisture, dust etc. The breather is connected to the Main conservator tank. It is provided with an Oil seal. The breathed in air is passed through the oil seal to retain moisture before the air   passes through  the silica  gel cr ystals which  absorbs moisture  before breathing  into  the  conservator   tank.  In  latest  large  transformers,  Rubber Diaphragm or Air cells are used to reduce contamination of oil.

 (8) Transformer Earthing :

  •  For  Distribution Transformers, normally Dy11 vector Group, the LT Neutral  is Earthed  by a separate   Conductor  section of at least half  the section of the conductor used for phase wire and connected to a Separate Earth whose Earth Resistance must be less than 1 ohm.
  •  The Body of the Tank has two different earth connections, which should be connected to two distinct earth electrodes by GI flat of suitable section.
  • For   Large   Power   Transformers,   Neutral   and   Body  Connections  are   made separately but all the Earth Pits are connected in parallel so that the combined Earth   Resistance   is  always  maintained  below  0.1   ohm.
  • The   individual  and combined   earth   resistance   is   measured   periodically   and   the   Earth   Pits maintained regularly and electrodes replaced if required.

External Protection:

  • Lightning Arrestors on HV & LV for Surge Protection
  • HV / LV Over Current  Protection(Instantaneous /IDMT- Back up)
  • Earth Fault Protection ( Y connected side)
  • REF (HV & LV) ( For internal fault protection)
  • Differential  Protection (for internal fault protection)
  • Over Fluxing Protection (against system Kv & HZ variations)
  • HG Fuse Protection for Small Capacity Transformers.
  • Normally Each Power   Transformers   will   have   a   LV   Circuit Breaker.  For   a Group  of  Transformers  up to  5  MVA  in  a  substation, a  Group  control  Circuit Breaker   is   provided.   Each   Transformer   of   8   MVA   and   above  will   have   a Circuit Breaker on the HV side.

Transformer Cooling:

  •  The Heat in a transformer is produced due to I square R in the  windings and in the core due to Eddy Current and Hysteresis Loss.
  •  In Dry type Transformer the Heat is directly dissipated into the atmosphere but in Oil filled Transformer, the   Heat   is   dissipated   by   Thermosyphon   and   transmitted   to   the   top   and dissipated   into   the   atmosphere   through   Radiators   naturally   or   by   forced cooling   fans   or   by   Oil   pumps   or   through   Water   Coolers.
  •  The   following Standard symbols are adopted to denote the Type of Cooling:
  •   A =Air Cooling
  •  N =Natural Cooling by Convection
  •   B= Cooling by Air Blast Fans
  •  O=Oil (mineral) immersed cooling
  •  W= Water Cooled
  •  F =Forced Oil Circulation by Oil Pumps
  •  S=Synthetic Liquid used  instead of Oil
  •  G =Gas Cooled (SF6 or N2)
  •  D=Forced (Oil directed)
  •  ONAF=Oil immersed Transformer with natural oil circulation and forced air external cooling is designated.
  •  ONAN= Oil Immersed Natural cooled
  •  ONAF= Oil Immersed Air Blast
  •  ONWN=Oil Immersed Water Cooled
  •  OFAF=Forced Oil Air Blast Cooled
  • OFAN=Forced Oil Natural Air Cooled
  •  OFWF=Forced Oil Water Cooled
  • ODAF=Forced Directed Oil and Forced Air Cooling.
  • Cooling e.g., ONAN/ONAF or ONAN/OFAF or sometimes three systems e.g., ONAN/ONAF/ OFAF