Electrical Thumb Rules-(Part-15)


Selection of MCB

MCB curve Type of Load Residential Commercial
B curve Resistive Loads Incandescent lights Incandescent lights
Geyser  Boilers
 Heater  Heaters
Fan blower heaters Oil radiator heaters
Slight Inductive Loads Florescent Lights Florescent lights
Small motors (FHP) High pressure mercury vapor lamps
C curve Slight Inductive Loads Fans & small pumps Sodium vapor lamps
Window / Split ACs
Lights with ballasts
 General household equipment
D curve Inductive Loads Water lifting pumps Florescent lights
UPS ID & FD fans
  Small control transformers
  Medium size motors
  Refrigerators for commercial use


Type of MCB

MCB Curve Type of Load Response Tripping Application Uses
B curve Resistive loads MCBs react quickly to overloads 3 To 5 times F.L current (0.04 To 13 Sec) Domestic & Commercial applications Suitable for incandescent lighting, socket-outlet, bulbs, heaters etc. Protection of DG sets (since DG sets have low short-circuit capacity)
C curve Slightly inductive loads MCBs react more slowly, 5 To 10 times F.L current  (0.04 To 5 Sec) Commercial and Industrial applications


Highly Inductive loads such as motors, air conditioners, fluorescent lighting lights, fans & household electrical appliances.
D curve Inductive loads MCBs are slower 10 To 20 times F.L current  (0.04 To 3 Sec) Commercial and Industrial applications


Very high inrush Inductive currents, Small transformers, welding machines. UPS, small motor & pumps, x-ray machines etc. Note, however, that MCBs with Type K characteristics may provide better protection in some applications of this type.
K curve Inductive loads MCBs are slower 8 To 10 times F.L current  (0.04 To 3 Sec)   Placing them between the traditional Type C and Type D breakers. In most cases, they allow improved cable protection to be provided in circuits that include motors, capacitors and transformers, where it would previously have been necessary to use Type D devices. This enhanced protection is achieved without increasing the risk of nuisance tripping.


Selection of RCCB

Type of RCCB Sensitive Application
Type AC Sensitive to AC Currents Only Suitable for most domestic and commercial applications.
Type A Sensitive to AC Currents + Pulsating DC Currents (Produced by Rectifier, Thyristors) Used where there are a lot of “electronic” loads, such as computer equipment or lighting systems with electronic ballasts.
Type B Sensitive to AC Currents + Pulsating DC Currents+ Pure DC Currents Use in photovoltaic (PV) solar energy installations because the PV panels produce a DC Output and some types of fault can result in the leakage of DC Currents to Earth.
Type B+ Similar to Type B, but respond to ac leakage currents over a wider frequency range Type B and Type B+ devices can be used wherever a Type AC or Type A device is specified, as they provide the same functionality as these types and more.



TYPE AC Current 50Hz AC Current 50Hz To 1KHZ Pulsating Current with DC Component Multi Frequency Current Generated By 1Phase Inverter Multi Frequency Current Generated By 3Phase Inverter


Sensitivity of RCCB

RCCBs Application
30 mA personal protection domestic installation / direct contact
100 mA limited personal protection / indirect contact
300 mA building / fire protection


MCB Enclosure Size

MCB Rating (A) Min. Enclosure Size
Height Width Depth
100A 370mm 216mm 72mm
125A 310mm 180mm 83mm
225A 370mm 217mm 72mm
250A 380mm 195mm 83.5mm
300A to 400A 506mm 381mm 153mm
600A to 800A 520mm 420mm 200mm
1000A to 1200A 704mm 554mm 173mm
1600A to 3000A 1016mm 608mm 615mm


Switch Gear Protection

Switch Gear Protection Isolation Control
Over Load Short Circuit
Circuit Breaker YES YES YES YES
Contactor NO NO NO YES
Disconnector NO NO YES NO


Type of Faults

Types of Fault Reason Consequences  Protective Device to be used
Overload When Equipment tries to run beyond its rated capacity, or there is a fault in the equipment E.g. When you keep a heater on without any water in it. It can lead to reduction in life of equipment, Failure of insulation and hence damaging the equipment. MCB / RCBO
Short Circuit Insulation Failure, Shorting of the Phase to Phase or Phase and Neutral Wires. High Inrush Current, causing permanent damage to equipment and may lead to a Fire. MCB
Earth Fault Short circuit between Phase and Earth Conductor. Can result in Fire due to sparking. RCBO / RCCB
Earth Leakage Human Body Touching Live Wires. Insulation failure   RCBO / RCCB
Over Voltage Opening of Neutral Connection increase in Phase To Phase Voltage of 440V, Surge through Lighting or transients, Over voltage from Utility. Damage to sensitive Electronic Equipment. OV protection Device
Under Voltage Drop in supply voltage, starting of heavy loads Damage of Equipment, Flickering of Lights. UV relays


 MCB Type (BS EN 60898-2)

Trip Type instantaneous
Trip (< 0.1 s)
Load Type Typical Load
B 3 to 5 In (AC) Resistive Heaters, showers, cookers, socket outlets.
4 to 7 In (DC)
C 5 to 10 In (AC) Inductive Motors, general lighting circuits, power supplies.
7 to 15 In (DC)
D 10 to 20 In High Inductive Transformers, motors, discharge lighting circuits, computers


Relays for Transformer 

Capacity of Transformer Relays on HV Side Relays on LV Side Common Relays
Generator Transformer 3 Nos Non-Directional O/L Relay – – Differential Relay or
1 no Non-Directional E/L Relay Overall differential Relay
and/or standby E/F + REF Relay Over flux Relay
  Buchholz Relay
  OLTC Buchholz Relay
  PRV Relay
  OT Trip Relay
  WT Trip Relay
220 /6.6KV Station Transformer 3 Nos Non-Directional O/L Relay 3 Nos Non-Directional O/L Relay Differential Relay
1 no Non-Directional E/L Relay Over flux Relay
and/or standby E/F + REF Relay Buchholz Relay
  OLTC Buchholz Relay
  PRV Relay
  OT Trip Relay
  WT Trip Relay
132/33/11KV up to 8 MVA 3 Nos O/L Relay 2 Nos O/L Relays Buchholz Relay
1 no E/L Relay 1 no E/L Relay OLTC Buchholz Relay
    PRV Relay
    OT Trip Relay
    WT Trip Relay
132/33/11KV up to 8 MVA to 31.5 MVA 3 Nos O/L Relay 3 Nos O/L Relay Differential Relay
1 no Directional E/L Relay 1 no E/L Relay Buchholz Relay
    OLTC Buchholz Relay
    PRV Relay
    OT Trip Relay
    WT Trip Relay
132/33KV, 31.5 MVA & above 3 Nos O/L Relay 3 Nos O/L Relay Differential Relay
1 no Directional E/L Relay 1 no E/L Relay Over flux Relay
    Buchholz Relay
    OLTC Buchholz Relay
    PRV Relay
    OT Trip Relay
    WT Trip Relay
220/33 KV, 31.5MVA & 50MVA , 220/132KV, 100 MVA 3 No O/L Relay 3 Nos O/L Relay Differential Relay
1 no Directional E/L Relay 1 no Directional E/L Relay Over flux Relay
    Buchholz Relay
    OLTC Buchholz Relay
    PRV Relay
    OT Trip Relay
    WT Trip Relay
400/220KV 315MVA 3 Nos Directional O/L Relay 3 Nos Directional O/L Relay Differential Relay
1 no Directional E/L relay. 1 no Directional E/L relay. Over flux Relay
Restricted E/F relay Restricted E/F relay Buchholz Relay
3 Nos Directional O/L Relay for action   OLTC Buchholz Relay
  PRV Relay
    OT Trip Relay
    WT Trip Relay
    Over Load (Alarm) Relay


Relays for Transmission & Distribution Lines Protection

Lines to be protected Relays to be used
400 KV Transmission Line Main-I: Non switched or Numerical Distance Scheme
Main-II: Non switched or Numerical Distance Scheme
220 KV Transmission Line Main-I : Non switched distance scheme (Fed from Bus PTs)
Main-II: Switched distance scheme (Fed from line CVTs)
With a changeover facility from bus PT to line CVT and vice-versa.
132 KV Transmission Line Main Protection : Switched distance scheme (fed from bus PT).
Backup Protection: 3 Nos. directional IDMT O/L Relays and
1 No. Directional IDMT E/L relay.
33 KV lines Transmission Line Non-directional IDMT 3 O/L and 1 E/L relays.
11 KV lines Transmission Line Non-directional IDMT 2 O/L and 1 E/L relays.


Selection Chart for 3Ph Induction Motor

Motor Rating,415V,3Ph Full Load Current (A) CONTACTOR (A) OVER LOAD RELAY (A) BACK UP FUSE (A) Cable Size
DOL Starter STAR-DELTA Starter
0.75 0.52 1.6 16   1.0 To 1.6   4 1.5 1.5    
1 0.75 2 16   1.6 To 2.5   6 1.5 1.5    
2 1.5 3.5 16   3.0 To 4.5   10 1.5 1.5    
3 2.2 5 16   4.5 To 7.0   10 1.5 1.5    
5 3.7 7.5 16   6.5 To 10   16 1.5 1.5    
7.5 5.5 11 16 16 10 To 15 4.5 To 7.0 16 2.5 1.5 2.5 1.5
10 7.5 14 16 16 13 To 20 6.5 To 10 20 2.5 2.5 2.5 2.5
12.5 9.3 18 25 16 13 To 20 10 To 15 25 4 2.5 4 2.5
15 11 21 25 16 15 To 22 13 To 20 25 6 4 6 4
20 15 28 32 18 24 To 30 13 To 20 32 10 6 10 6
25 18.5 35 40 25 25 To 30 15 To 22 50 16 10 16 10
30 22.5 40 50 25 32 To 50 24 To 30 50 16 16 16 16
35 26 47 70 32 32 To 50 25 To 30 63 25 16 25 16
50 37 66 70 40 57 To 70 32 To 50 80 35 25 35 25
60 45 80 95 50 70 To 105 32 To 50 100 50 35 50 35
75 55 100 125 70 100 To 150 40 To 57 100 70 50 70 50
90 67.5 120 140 70 100 To 150 57 To 70 160 95 70 95 70
100 75 135 140 95 100 To 150 70 To 105 160 95 70 95 70
125 90 165   125   70 To 105 160     150 95
150 110 200   125   100 To 150 200     185 150


MCB Selection Chart For Motor Protection

Kw HP 1Phase 230V DOL
3Phase 400V DOL
3 Phase 400V Star Delta
Full Load Current MCB Selection Full Load Current MCB Selection Full Load Current MCB Selection MCB Selection
0.18 0.24 2.8 10 0.9 2  —  —  —
0.25 0.34 3.2 10 1.2 2  —  —  —
0.37 0.5 3.5 10 1.2 2  —  —  —
0.55 0.74 4.8 16 1.8 3  —  —  —
0.75 1.01 6.2 20 2 3  —  —  —
1.1 1.47 8.7 25 2.6 6  —  —  —
1.5 2.01 11.8 32 3.5 10  —  —  —
2.2 2.95 17.5 50 4.4 10  —  —  —
3 4.02 20 63 6.3 16 6.3 16 10
3.75 5.03 24 80 8.2 20 8.2 20 10
5.5 7.37 26 80 11.2 25 11.2 32 16
7.5 10.05 47 125 14.4 40 14.4 40 25
10 13.4  —  — 21 50 21 50 32
15 20.11  — 27 100 27 63 40
18.5 24.8  —  — 32 125 32  — 50
22 29.49  —  — 38 125 38 63
30 40.21  —  — 51 125 51  — 63



Code Type of Relay
1 Master Element
2 Time-delay Starting or Closing Relay
3 Checking or Interlocking Relay
4 Master Contactor
5 Stopping Device
6 Starting Circuit Breaker
7 Rate of Change Relay
8 Control Power Disconnecting Device
9 Reversing Device
10 Unit Sequence Switch
11 Multifunction Device
12 Over speed protection
13 Synchronous-Speed Device
14 Under speed Device
15 Speed or Frequency Matching Device
16 Data Communications Device
17 Shunting or Discharge Switch
18 Accelerating or Decelerating Device
19 Starting-to-Running Transition Contactor
20 Electrically-Operated Valve
21 Distance protection Relay
21G Ground Distance
21P Phase Distance
22 Equalizer circuit breaker
23 Temperature control device
24 Volts per hertz relay
25 Synchronizing or synchronism-check device
26 Apparatus thermal device
27 Under voltage relay
27P Phase Under voltage
27S DC under voltage relay
27TN Third Harmonic Neutral Under voltage
27TN/59N 100% Stator Earth Fault
27X Auxiliary Under voltage
27 AUX Under voltage Auxiliary Input
27/27X Bus/Line Under voltage
27/50 Accidental Generator Energization
28 Flame Detector
29 Isolating Contactor
30 Annunciator Relay
31 Separate Excitation Device
32 Directional Power Relay
32L Low Forward Power
32N Watt metric Zero-Sequence Directional
32P Directional Power
32R Reverse Power
33 Position Switch
34 Master Sequence Device
35 Brush-Operating or Slip-ring Short Circuiting Device
36 Polarity or Polarizing Voltage Device
37 Undercurrent or Under power Relay
37P Under power
38 Bearing Protective Device / Bearing Rtd
39 Mechanical Condition Monitor
40 Field Relay / Loss of Excitation
41 Field Circuit Breaker
42 Running Circuit Breaker
43 Manual Transfer or Selector Device
44 Unit Sequence Starting Relay
45 Atmospheric Condition Monitor
46 Reverse-Phase or Phase Balance Current Relay or Stator Current Unbalance
47 Phase-Sequence or Phase Balance Voltage Relay
48 Incomplete Sequence Relay / Blocked Rotor
49 Machine or Transformer Thermal Relay / Thermal Overload
49RTD RTD Biased Thermal Overload
50 Instantaneous Overcurrent Relay
50BF Breaker Failure
50DD Current Disturbance Detector
50EF End Fault Protection
50G Ground Instantaneous Overcurrent
50IG Isolated Ground Instantaneous Overcurrent
50LR Acceleration Time
50N Neutral Instantaneous Overcurrent
50NBF Neutral Instantaneous Breaker Failure
50P Phase Instantaneous Overcurrent
50SG Sensitive Ground Instantaneous Overcurrent
50SP Split Phase Instantaneous Current
50Q Negative Sequence Instantaneous Overcurrent
50/27 Accidental Energization
50/51 Instantaneous / Time-delay Overcurrent relay
50Ns/51Ns Sensitive earth-fault protection
50/74 Ct Trouble
50/87 Instantaneous Differential
51 Phase Inverse Time Overcurrent IDMT (Time delay phase overcurrent )
51G Ground Inverse Time Overcurrent
51LR AC inverse time overcurrent (locked rotor) protection relay
51N Neutral Inverse Time Overcurrent
51P Phase Time Overcurrent
51R Locked / Stalled Rotor
51V Voltage Restrained Time Overcurrent
51Q Negative Sequence Time Overcurrent
52 AC circuit breaker
52a AC circuit breaker position (contact open when circuit breaker open)
52b AC circuit breaker position (contact closed when circuit breaker open)
53 Exciter or Dc Generator Relay
54 Turning Gear Engaging Device
55 Power Factor Relay
56 Field Application Relay
57 Short-Circuiting or Grounding Device
58 Rectification Failure Relay
59 Overvoltage Relay
59B Bank Phase Overvoltage
59P Phase Overvoltage
59N Neutral Overvoltage
59NU Neutral Voltage Unbalance
59P Phase Overvoltage
59X Auxiliary Overvoltage
59Q Negative Sequence Overvoltage
60 Voltage or current balance relay
60 Voltage or Current Balance Relay
60N Neutral Current Unbalance
60P Phase Current Unbalance
61 Density Switch or Sensor
62 Time-Delay Stopping or Opening Relay
63 Pressure Switch Detector
64 Ground Protective Relay
64F Field Ground Protection
64R Rotor earth fault
64REF Restricted earth fault differential
64S Stator earth fault
64S Sub-harmonic Stator Ground Protection
64TN 100% Stator Ground
65 Governor
66 Notching or Jogging Device/Maximum Starting Rate/Starts Per Hour/Time Between Starts
67 AC Directional Overcurrent Relay
67G Ground Directional Overcurrent
67N Neutral Directional Overcurrent
67Ns Earth fault directional
67P Phase Directional Overcurrent
67SG Sensitive Ground Directional Overcurrent
67Q Negative Sequence Directional Overcurrent
68 Blocking Relay / Power Swing Blocking
69 Permissive Control Device
70 Rheostat
71 Liquid Switch
72 DC Circuit Breaker
73 Load-Resistor Contactor
74 Alarm Relay
75 Position Changing Mechanism
76 DC Overcurrent Relay
77 Telemetering Device
78 Phase Angle Measuring or Out-of-Step Protective Relay
78V Loss of Mains
79 AC Reclosing Relay / Auto Reclose
80 Liquid or Gas Flow Relay
81 Frequency Relay
81O Over Frequency
81R Rate-of-Change Frequency
81U Under Frequency
82 DC Reclosing Relay
83 Automatic Selective Control or Transfer Relay
84 Operating Mechanism
85 Pilot Communications, Carrier or Pilot-Wire Relay
86 Lock-Out Relay, Master Trip Relay
87 Differential Protective Relay
87B Bus Differential
87G Generator Differential
87GT Generator/Transformer Differential
87L Segregated Line Current Differential
87LG Ground Line Current Differential
87M Motor Differential
87O Overall Differential
87PC Phase Comparison
87RGF Restricted Ground Fault
87S Stator Differential
87S Percent Differential
87T Transformer Differential
87V Voltage Differential
88 Auxiliary Motor or Motor Generator
89 Line Switch
90 Regulating Device
91 Voltage Directional Relay
92 Voltage And Power Directional Relay
93 Field-Changing Contactor
94 Tripping or Trip-Free Relay
Abbreviation Code
AFD Arc Flash Detector
CLK Clock or Timing Source
CLP Cold Load Pickup
DDR Dynamic Disturbance Recorder
DFR Digital Fault Recorder
DME Disturbance Monitor Equipment
ENV Environmental data
HIZ High Impedance Fault Detector
HMI Human Machine Interface
HST Historian
LGC Scheme Logic
MET Substation Metering
PDC Phasor Data Concentrator
PMU Phasor Measurement Unit
PQM Power Quality Monitor
RIO Remote Input/output Device
RTD Resistance Temperature Detector
RTU Remote Terminal Unit/Data Concentrator
SER Sequence of Events Recorder
TCM Trip Circuit Monitor
LRSS  Local/Remote selector switch
VTFF  Vt Fuse Fail

Suffixes Description

_1 Positive-Sequence
_2 Negative-Sequence
A Alarm, Auxiliary Power
AC Alternating Current
AN Anode
B Bus, Battery, or Blower
BF Breaker Failure
BK Brake
BL Block (Valve)
BP Bypass
BT Bus Tie
BU Backup
C Capacitor, Condenser, Compensator, Carrier Current, Case or Compressor
CA Cathode
CH Check (Valve)
D Discharge (Valve)
DC Direct Current
DCB Directional Comparison Blocking
DCUB Directional Comparison Unblocking
DD Disturbance Detector
DUTT Direct Under reaching Transfer Trip
E Exciter
F Feeder, Field, Filament, Filter, or Fan
G Ground or Generator
GC Ground Check
H Heater or Housing
L Line or Logic
M Motor or Metering
MOC Mechanism Operated Contact
N Neutral or Network
O Over
P Phase or Pump
PC Phase Comparison
POTT  Pott: Permissive Overreaching Transfer Trip
PUTT Putt: Permissive Under reaching Transfer Trip
R Reactor, Rectifier, or Room
S Synchronizing, Secondary, Strainer, Sump, or Suction (Valve)
SOTF Switch On To Fault
T Transformer or Thyratron
TD Time Delay
TDC Time-Delay Closing Contact
TDDO Time Delayed Relay Coil Drop-Out
TDO Time-Delay Opening Contact
TDPU Time Delayed Relay Coil Pickup
THD Total Harmonic Distortion
TH Transformer (High-Voltage Side)
TL Transformer (Low-Voltage Side)
TM Telemeter
TT Transformer (Tertiary-Voltage Side)
U Under or Unit
X Auxiliary
Z Impedance


Harmonic Effects

Harmonic R Phase Y Phase B Phase Phase Rotation Sequence Harmonic Effect
Rotation Rotation Rotation
Fundamental 120° 240° R-Y-B  
3th 3×0°= 3×120°=360°=0° 3×240°=720°=0° No Rotation          (In Phase) Adds Voltages or Currents in Neutral Wire causing Heating
9th 9×0°= 9×120°=1080°=0° 9×240°=2160°=0°
15th 15×0°= 15×120°=1800°=0° 15×240°=3600°=0°
21th 21×0°= 21×120°=2520°=0° 21×240°=5040°=0°
5th 5×0°= 5×120°=600°=(600-720)=(-120°) 5×240°=1200°=(1200-2400)=(-240°) Rotate Against Fundamental (-) (B-Y-R) Motor Torque Problems
11th 11×0°= 11×120°=1320°=(1320-1400)=(-120°) 11×240°=2640°=(2880-2640)=(-240°)
17th 17×0°= 17×120°=2040°=(2040-2160)=(-120°) 17×240°=4080°=(4320-4080)=(-240°)
23th 23×0°= 23×120°=2760°=(2760-2880)=(-120°) 23×240°=5520°=(5760-5520)=(-240°)
7th 7×0°= 7×120°=840°=(840-720)=(+120°) 7×240°=1680°=(1680-1440)=(+240°) Rotate with Fundamental (+) (R-Y-B) Excessive Heating Effect
13th 13×0°= 13×120°=1560°=(1560-1440)=(+120°) 13×240°=3120°=(3120-2880)=(+240°)
19th 19×0°= 19×120°=2280°=(2280-2160)=(+120°) 19×240°=4560°=(4560-4320)=(+240°)
25th 25×0°= 25×120°=3000°=(3000-2880)=(+120°) 25×240°=6000°=(6000-5760)=(+240°)

Difference Between High Bay-Low Bay and Flood Light Fixture


  • In the lighting industry, the term “bay” means to illuminate any large area.
  • High Bay fixtures and Low Bay fixtures are used to for illumination in Buildings with higher ceilings like warehouse lighting, industrial lighting, Commercial lighting, retail lighting, and gym lighting.
  • High Bay Lighting and Low Bay Lighting are mounted at high level via a pendant, chain, or directly to a ceiling or ceiling girder.

Type of Lighting Fixture for Larger Area Illumination:

  • There are three type of lighting fixture to illuminate large open Area
  • Low Bay Lighting Fixtures
  • High Bay Lighting Fixtures
  • Flood Lights

 (1) Low Bay Light Fixture.

  • As the name says, these bay lights are often used with lower ceilings in open areas.
  • Low bay lights are designed to illuminate open areas with ceilings Between 12 foot to 20 foot.
  • Anything use over this height treat as high bays, and anything lower is very uncommon in large open area facilities, and would require a different type of light fixture.
  • The reflectors or lens for low bays also spread the light far out to maintain a desired lighting level.



  • Ware House.
  • Petrol Station.
  • Retail Store.

(2) High Bay Light Fixture.

  • As their name implies, high-bay lights are used to illuminate spaces with high ceilings. That usually means ceilings ranging from 20 feet to 45 feet.
  • These light is effective at high Ceiling Level to provide well distributed and uniform light for open areas.
  • They need specifically reflectors (for HPS / MH bulbs) or lens angles to ensure light reaches the floor evenly and reduces wasted light. Different kinds of reflectors can accomplish different kinds of illumination tasks for high-bay lights. Aluminum reflectors make light from the fixtures flow directly downward to the floor, while prismatic reflectors create a more diffused lighting useful for illuminating shelves and other elevated objects in a space.
  • High-ceiling location has more space to fill, hence a high-bay by definition is a powerful light source that can brighten up a large area.
  • High-bay lighting is provides clear, uniform lighting of what’s below it with little glare.
  • Numerous types of fixtures can be used as a high-bay lights like LED lights, induction lights, metal halide lights, and fluorescent lights.
  • For instance, LED lights offer extremely long life and energy efficiency but require a bigger initial investment, while traditional incandescent lights are less expensive to purchase initially but don’t last as long and use more energy.
  • There are several types of fixtures available for high-bay lights. Round high-bay lights, linear high-bays, architectural high-bays and grid-mount high-bays.



  • Whenever a large indoor space needs to be illuminated, high bay lighting is usually appropriate. These area is typically vast and cover a lot of vertical as well as horizontal space. This need powerful lighting to provide the appropriate Lux levels to adequately illuminate.
  • High bay lighting fixtures typically hang from the ceiling via hooks, chains or pendants, or they may be fixed to the ceiling directly (similar to troffer lights).  
  • Various industries and facilities require high bay lighting. Some of the most common are
  • Industrial facilities.
  • Manufacturing facilities.
  • School and university gymnasiums.
  • Municipal facilities like community centers or recreation centers.
  • Commercial applications like department stores.
  • Airport hangar or any large open area industrial and commercial space with relatively high ceilings

Choosing the Correct High Bay & Low Bay Fixture

  • Choosing the right High Bay fixture can make the difference between a successful lighting project or
  • A light designed for a warehouse is a totally different than a light designed for a gymnasium or a factory floor. In gymnasium or a factory floor, a light can distribute in the area evenly while in a warehouse, a light can light up the face of the shelves and on the path way between two shelves.

(A) Lumen Output of Lamps:

  • We cannot be assumed that 100% of the lamp output will be emitted from the fitting or that the light output will be constant over its operational lifetime.
  • The actual total illumination levels that can be provided by an installed commercial light fitting will depend on the Light Output Ratio:
  • As an example, an industrial or warehouse high bay light fitting with a LOR of 70%, this indicates that 30% of the lamp’s light output is lost due to the design of the fitting.
  • The light output ratio is need to be consider in commercial lighting installation because when a lamp is positioned in a light fitting (such as an industrial 400W metal halide high bay) losses of light occur within the fitting itself.

(B) Beam Angle:

  • For maximum light coverage, we need to select a beam configuration that matches the height of the high bay light.


  • The common beam angles used for high-bay lighting are 60°, 90° and 120°.
  • The narrow beam angle creates a more focused beam enabling a high lux level on the floor or the platform.
  • The wider beam angle ensures large open areas with lower roof heights receive an excellent spread of light.


Beam Angle

Beam Angle

Ceiling Height


Up to 4 meter


4 to 6 meter


6 to 8 meter


8 to12 meter


Beam Angle & Applications

Beam Angle



Spot Lights Stadium Lights


Spot Lights Stadium Lights


Residential and Architectural Lighting


Commercial and Industrial Lighting


Commercial and Industrial Lighting


Low Ceiling Gas Stations and Public Spaces


Industrial Lighting Parking Garages


Beam Angle & Fitting Type

Beam Angle

Type of Fitting

4° To 9°

Spot Light

20° To 35°

Flood Light

36° To 49°

Wide Flood Light

More than 60°

Very Wide Flood Light

(C) Glare:

  • When there is an excessive contrast between the dark areas and bright areas in the direction of viewing, then glare can occur. When there is too much light, it will cause glare.
  • Glare can happen during daytime and nighttime. Examples of where glare can occur includes moving from a shaded location into bright sunlight, and the reflection of light from a surface which is shiny.


(D) Fixtures Shape

  • Circular fixtures creates circular beams; rectangular fixtures creates rectangular beams.
  • Round LED high bays certainly have their universal application, but if we are going to illuminate a long workbenches or a production line, we may get more efficient results from a rectangular linear high bay

(3) Flood Light Fixture.

  • A floodlight is called Flood Light because it illuminate evenly a large area with high intensity of Light.
  • Flood lights are a general method for illuminating areas where a conventional mounting arrangement of Fixtures may or may not be an available and we can also change direction Light or tilt Angle.


  • The flood light have an asymmetric throw of light which can be angled into the space to be illuminate.
  • Flood light illuminate uniformly in all directions and its exposure range can be adjusted.
  • Flood Lights utilizing light bulbs of high power to illuminate a big outdoor location.
  • Flood light is able to equably shine in all directions. Besides, the shine angels could change freely and is able to generate shadow. It is most widely used to illuminate the whole area.
  • When we install floodlights, we should need to care about glare because the brightness of the fitting is high and it angled close to horizontal.
  • Flood Light is different from spot light. Its light beam is highly diffuse without direction. Therefore, its shadow is gentle and transparency.


  • Floodlights are broad beamed, high intensity lights often used to illuminate outdoor playing fields while an outdoor sports event.
  • Flood light is a good choice for lighting and decoration of construction sites, squares, parks, arts venues.
  • Flood light also use as a object Lighting.
  • Factory buildings, stadiums, golf courses, shops, hotels, subway stations, gas stations, buildings.
  • Sculptures and other indoor and outdoor applications.

Difference between High Bay and Low Bay Lighting.

  • Normally, there is a confusion between high bay light and low bay light because both looks like same and having same applications except installation height and intensity of illumination and lumen output.
  • High bay and low bay fixtures both are typically suspension mounted using chains or hooks, but they may also have the option of being surface mounted depending on the fixture.
  • Actually both are not same lights. There are some differences between them.
  • The wattage
  • The wattage or applications of both are different. The wattage and application determines whether to call them high bay or low bay.
  • If the wattage used is above 100 Watts then it is called  high bay. Those using below 100 Watts are called low bay fixtures.
  • The Mounting Height
  • The  low bay light fixtures are used in areas where the bottom of the fixture is up to 20 feet or less above the floor.
  • They are usually spread the light evenly. They also contain optical refractors which cover the lamp thereby reducing glare. Their widespread distribution improves the vertical illumination and also permits spacing as much as 2 or more times the mounting height.
  • High bay lighting fixtures, they are mostly used in areas where the bottom of the fixture is 20 feet or more above the floor.
  • They allowing for a more concentrated beam spreading with a prominent downward component. High wattage is needed so as to illuminate the space properly.


Spacing between lights



15 feet

12 feet to 15 feet
20 feet

15 feet to 18 feet

30 feet

20 feet to 25 feet


Height and lumens


10 to 15 feet

 10,000 to 15,000 lumens

15 to 20 feet

16,000 to 20,000 lumens
25 to 30 feet

 33,000 lumens


Low Bay / High Bay Lighting Fixtures


Installation height Distance Fixture To Fixture
50 Watt 3 Meter

3 To 6 Meter

90 Watt

4 Meter 6 Meter

120 To 150 Watt

5 Meter 6 To 8 Meter
200 Watt 7 Meter

9 To 10 Meter

300 Watt 8 Meter

More than 10 Meter

Difference between Spot Light and Flood Light.

  • A spotlight casts a narrow beam of light, usually no wider than 45°. This beam is more concentrated and easier to point and control.
  • A floodlight can have a beam spread of up to 120°. It can illuminate a larger amount of space with the same wattage and lumen output as a spotlight.
  • Flood Lights is generally utilized for highlighting the architectural appearance of an outstanding or historically Building.
  • By utilizing flood lights, we can boost the in-depth framework of a building.

Determining beam width:

  • The width of a light’s beam in degrees is not always helpful. It should be much easier to know the beam width in feet, from a given distance away.
  • There is a simple formula to know Beam width
  • Beam Width =Angle of Beam x 0.018 x Distance from Light Bulb
  • If we have an 80 degree floodlight, and we want to know how wide the beam will be from 10 feet away.
  • Beam Width = 80 degrees x 0.018 x 10 feet = 14.4 feet wide

LED Vs Metal Halide

LED Watt

Metal Halide Watt
20W to 50W


30W to 75W

40W to 125W


50W to 175W


60W to 225W


80W to 250W


100W to 350W


120W to 400W

150W to 500W



Electrical Thumb Rules-(Part-15).


Luminous Efficacy, Lumen Maintenance and Color Rendition (Table-8) NBC

Light Source  Wattage Efficacy (lm/W ) Average Life Maintenance Color Rendition
Incandescent lamps  15 to 200  12 to 20  500 to 1000  Fair to good  Very good
Tungsten halogen     300 to 1500  20 to 27  200 to 2000  Good to very good  Very good
Standard fluorescent lamps       20 to 80 55 to 65 5000 Fair to good  Good
Compact fluorescent lamps (CFL)       5 to 40  60 to 70 7500 Good Good to very good
Slim line fluorescent      18 to 58 57 to 67 5000  Fair to good Good
High pressure mercury vapor lamps      60 to 1000  50 to 65 5000  Very low to fair  Federate
Blended – light lamps    160 to 250  20 to 30 5000 Low to fair  Federate
High pressure sodium vapor lamps  50 to 1000  90 to 125  10000 to 15000  Fair to good  Low to good
Metal halide lamps       35 to 2000  80 to 95 4000 to 10000 Very low  Very good
Low pressure sodium       10 to 180 100 to 200 10000 to 20000 Good to very good  Poor
LED  0.5 to 2.0  60 to 100  10000 Very good  Good for white LED


Approximate Cable Current Capacity

Cable Size Current Capacity MCB Size
1.5 Sq.mm 7.5 To 16 A 8A
2.5 Sq.mm 16 To 22 A 15A
4 Sq.mm 22 To 30 A 20A
6 Sq.mm 39 To 39 A 30A
10 Sq.mm 39 To 54A 40A
16 Sq.mm 54 To 72A 60A
25 Sq.mm 72 To 93A 80A
50 Sq.mm 117 To 147A 125A
70 Sq.mm 147 To 180A 150A
95 Sq.mm 180 To 216A 200A
120 Sq.mm 216 To 250A 225A
150 Sq.mm 250 To 287A 275A
185 Sq.mm 287 To 334A 300A
240 Sq.mm 334 To 400A 350A


Requirements  for  Physical  Protection  of Underground Cables  (As per NBC)

Protective  Element Specifications
Bricks  (a) 100 mm minimum  width 
(b) 25 mm thick 
(c) sand cushioning 100  mm  and  sand  cover 100 mm 
Concrete slabs At least 50 mm thick
Plastic  slabs (polymeric cover  strips) Fiber  reinforced plastic depending on properties  and has to be matched with the protective cushioning and cover
PVC  conduit  or  PVC  pipe  or stoneware  pipe or Hume pipe The  pipe  diameter should  be  such  so  that the  cable  is  able  to easily slip down the pipe
Galvanized pipe  The  pipe  diameter should  be  such  so  that the  cable  is  able  to easily slip down the pipe
The Trench : The trench shall be back filled to cover the cable initially by 200 mm of sand fill; and then a plastic marker strip  hall be put over the full length of cable in the trench.
The Marker Signs: The marker signs shall be provided where any cable enters or leaves a building. This will identify that there is a cable located underground near the building.
 The trench shall then be completely filled. If the cables rise above ground to enter a building or other structure, a mechanical protection such as a GI pipe or PVC pipe for the cable from the trench depth to a height of 2.0 m above ground shall be provided.



Capacity  kVA Area m2 Clear Height below the Soffit of the Beam m
25 56 3.6
48 56 3.6
100 65 3.6
150 72 3.6
248 100 4.2
350 100 4.2
480 100 4.2
600 110 4.6
800 120 4.6
1010 120 6.5
1250 120 6.5
1600 150 6.5
2000 150 6.5


Low Voltage Cabling for Building (As per NBC)

Low Voltage Cable Cables/wires, such as fiber optic cable, co-axial cable, etc. These shall be laid at least at a distance of 300 mm from any power wire or cable. The distance may be reduced only by using completely closed earthed metal trucking with metal separations for various kind of cable. Special care shall be taken to ensure that the conduit runs and wiring are laid properly for low voltage signal to flow through it.
The power cable and the signal or data cable may run together under floor and near the equipment. However, separation may be required from the insulation aspect, if the signal cable is running close to an un-insulated conductor carrying power at high voltage. All types of signal cables are required to have insulation level for withstanding 2 kV impulse voltages even if they are meant for service at low voltage.
Conduit Color Scheme Power conduit=Black
Security conduit=Blue
Fire alarm conduit=Red
Low voltage conduit=Brown
UPS conduit Green


Sub Station Guideline (As per NBC)

Substation Location Location of substation in the basement should be avoided, as far as possible.
If there is only one basement in a building, the substation/switch room shall not be provided in the basement and the floor level of the substation shall not be lowest point of the basement.
Substation shall not be located immediately above or below plumbing water tanks or sewage treatment plant (STP) water tanks at the same location
Substation Door/Shutter All door openings from substation, electrical rooms, etc, should open outwards
Vertical shutters (like rolling shutters) may also be acceptable provided they are combined with a single leaf door opening outwards for exit in case of emergency
For large substation room/electrical  room  having  multiple equipment,  two  or more  doors  shall  be provided which shall be remotely located from each other
No services or ventilation shafts shall open into substation or switch room unless specific to substation or switch room
Transformer Location In case of HV panel and transformers located at different floors or at a distance more than 20 m, HV isolator shall be  provided  at transformer end
In case transformer and main MV/LV panel room are located at different floors or are at a distance more than 20 m, MV/LV isolator shall be provided at  transformer  end
In  case  of  two  transformers  (dry  type  or transformers with oil quantity less than 2 000 liter)  located  next  to  each  other without intermittent wall, the distance between the two shall  be minimum  1 500 mm  for  11  kV, minimum 2 000 mm for 22 kV and minimum 2 500 mm for 33 kV. Beyond 33 kV, two transformers shall be separated by baffle wall of 4 h fire rating.
If dry type transformer is used, it may be located adjacent to medium voltage switch gear in the form of unit type substation. In such a case, no separate room or fire barrier for the transformer is required either between transformers or between transformer and the switch gear, thereby decreasing the room space requirement; however, minimum distances as specified.
Oil Filled Equipment (Transformer / C.B) Substations with oil-filled equipment/apparatus transformers and high voltage panels shall be either located in open or in a utility building
They shall not be located in any floor other than the ground floor or the first basement of a utility building  not be located below first basement slab of utility building.
They shall have direct access from outside the building for operation and maintenance of the equipment.
It shall be separated from the adjoining buildings including the main building by at least 6 m clear distance to allow passage of fire tender between the substation/utility building and adjoining building/main building.
Substation equipment having more than 2 000 liter of oil whether located indoors in the utility building or outdoors shall have  baffle walls  of  4  h  fire  rating between apparatus.
Provision of  suitable oil soak-pit, and where use of more than 9 000 liter of oil in any one oil tank, receptacle or chamber is involved, provision shall be made for the draining away or removal of any oil which may leak or escape from the tank, receptacle or chamber containing the same
Power Supply Voltage supply  is  at  240  V  single  phase  up  to  5  kVA, 415/240 V 3-phase from 5 kVA to 100 kVA, 11 kV (or 22 kV) for loads up to 5 MVA and 33 kV or 66 kV for consumers of connected load or contract demand more than 5 MVA.
In case of connected load of 100 kVA and above, the relative advantage of high voltage three-phase supply should be considered.
In case of single point high voltage metering, energy meters shall  be  installed  in  building  premise,such a place which is readily accessible to the owner/operator of the building and the Authority. The supplier or owner of the installation shall provide at the point of commencement of supply a suitable isolating device fixed in a conspicuous position at not more than 1.7 m above the ground so as to completely isolate the supply to the building in case of emergency
Trench Drain In case of cable trench in substation/HV switch room/MV switch room, the same shall be adequately drained to ensure no water is stagnated at any time with live cables.
Fence for Substation Enclose any part of the substation which is open to the air, with a fence (earthed efficiently at both ends) or wall not less than 1800 mm (preferably not less than 2400 mm) in height
HV Distribution in Building The power supply HV cables voltage shall not be more than 12 kV and a separate dedicated and  fire  compartmented  shaft  should  be provided for carrying such high voltage cables to upper floors in a building. These shall not be mixed with any other shaft and suitable fire detection and suppression measures shall be provided throughout the length of the cable on each floor.
Switch Room / MV switch room Switch room / MV switch room shall be arrived at considering 1200 mm clearance requirement from top of the equipment to the below of the soffit of the beam .In case cable entry/exit is from above the  equipment  (transformer,  HV switchgear, MV  switchgear),  height  of substation room/HV switch room/MV switch room shall also take into account requirement of space for turning radius of cable above the equipment height.



What is Correct Method of MCB Connections


  • MCB is a mechanical switching device which can carry and break currents under normal circuit conditions and also under specified abnormal conditions, such as overload and short circuit.
  • The MCB can provide protection until and unless we have install input power (LINE) connection and Output (LOAD) connections in proper Terminals of MCB.
  • Electrical engineers seem to be confused to indentify where is the Line and Load terminal of an MCB (on the top or on the bottom).

Terminal Marking of MCB:

  • There are two type of MCB available in market.
  • MCB having terminal marking (LINE / LOAD Marking) (Polarized MCB)
  • MCB having No terminal marking (No any Marking) (Non Polarized MCB)
  • Some manufacture clearly indicates where to apply Input Power and where to connect Load on MCB while some manufacture does not indicate such Terminal Marking.
  • The constructions of both MCB are almost same even though we need to understand difference between them.

(1) LINE / LOAD Terminal Marking on MCB (Polarized MCB)

  • For AC Circuit:
  • If manufactures indicate Input (LINE) making on MCB then we have to give Supply at “LINE” Terminal and Load at “LOAD” Terminal for perfect operation of MCB.
  • If we do wrong connection than MCB may or may not give proper protection in fault Condition.
  • As Per UL 489 Paragraph It is clearly indicate that “Circuit breakers shall be marked “Line” and “Load” unless the construction and test results are acceptable with the line and load connections reversed. This marking requirement specifies that UL MCB shall be marked with the word “Line” on one end of the circuit breaker and the word “Load” on the other end”, as shown in Figure


  • If MCB is not live (ON) from long time (in Cold state) than there is possibility of MCB to not operate in fault conditions.
  • In MCB ,The fixed contact is encompassed by the arc chute, and the arc products are de ionized, cooled and ejected uneventfully when the incoming power is on “Line” Terminal (when the fixed contact is ‘live’ or ‘hot’).There is less chance to re strike arc again.
  • If the power is applied to moving contact ,”Load” Terminal, the flexible connector, the trip system, everything is live/hot after the arc is quenched. Chances of restrike/flashover are much higher.
  • For DC Circuit:
  • The polarized DC MCB have a marking of ‘+’ and ‘–‘ symbol
  • If Polarized DC MCB are wired incorrectly, they are a possibility of hazard and When we turned off under load, the MCB might not be able to extinguish the arc and the circuit breaker will burn out.
  • Polarized DC MCB use a small magnet to direct the arc away from the contacts and up into the arc shoot and arc disrupter cage. If the direction of current flow through the unit is reversed, then the magnet directs the arc away from the arc shoot and into the mechanism of the unit thus destroying it.


(2) No Terminal Marking on MCB (Non Polarized MCB)

  • For AC Circuit:
  • If manufacture has not indicated any Terminal Marking than we are free to connect line or load at any side as we wish.
  • If construction / Operating principle of both MCB are same then what are the different between them.
  • Without Terminal Marking MCB has following additional features.
  • (1) By Design improvement (Manufacture has provided some more provision for quenching of arc (So it cannot reproduce it again).
  • (2) By doing some more extra test as per IEC 60947-2 and UL 489


  • The performance of single-break circuit breakers is slightly different when the “LINE” and “LOAD” feed either from the bottom or Top hence IEC 60947-2 specifies that one additional SC test be carried out with connections required when the terminals are not specifically marked ‘Line’ and/or ‘Load’

Table 10- Number of samples for test (IS / IEC 60947-2)

Test Sequences

Terminal Marking (Line / Load) No of Sample for Testing


Sample For *





1 1
Ics (Rated service short-circuit breaking capacity)  (Ics=25%Icu)

2 1

3 1

3 1

4 1
Icu  (Rated ultimate short-circuit breaking capacity)

2 1

3 1

3 1

4 1
* Sample For Indications
1 In of a given frame size.
2 This sample is omitted in the following cases:
A circuit-breaker having a single non-adjustable current setting for a given frame size;
A circuit-breaker provided only with a shunt release (i.e. without an integral over current release);
A circuit-breaker with electronic over current protection, of a given frame size, having an adjustable current rating by electronic means only (i.e. without change of current sensors).
3 Connections reversed.
4 Connections reversed, if terminals unmarked.
  • As Per UL 489, Paragraph “if a circuit breaker is not marked “Line” and “Load,” one sample of each set tested, or one additional sample, shall be connected with the line and load connections reversed during the overload, endurance and interrupting tests”.
  • This UL test requirement specifies that for MCC to be UL Listed for reverse-feed applications, samples shall be tested with the line and load terminals reverse-fed, as shown in Figure, and that the test results shall be the same as those of “normally” fed circuit breakers. Depending on the design configuration and construction, the circuit breaker may or may not be affected by the application of power in a reverse-feed connection during these tests.


  • If Line / Load are not marked, we can connect Line or Load either on Top or bottom of MCB. However, it is a good practice to keep the fixed contact side connected to the bus bar.
  • For DC Circuit:
  • The Non polarized DC MCB have a No marking as ‘+’ and ‘–‘ symbol
  • Non polarized DC MCB operate safely as load breaking isolators and for fault current protection regardless of the direction of current flow through them.



  • When a MCB are marked “Line” and “Load,” the power supply conductors must be connected to the marked “Line.” These MCB cannot be reverse-fed.
  • If “Line” and “Load” are not marked on MCB, the power supply conductors may be connected to either end. These devices are suitable for reverse-feed applications.

Pirating of Technical Works-2

It has been observed that some website totally copy paste of this blog and parallel republished all posts of this blog again on their commercial website  .

Lots of time has been spent to read Books,Manuals,Handbooks and combined it with  practical experience to serve Handy Electrical tools,Notes to serve the Electrical Community.This Blog is a fusion of Theoretical and Practically knowledge to make all technical things easier to understand.

Please look at following totally copy paste material of  this  Blog. 

Originally published

(1) https://electricalnotes.wordpress.com/2016/10/04/how-to-select-mcb-mccb-part1/

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Totally copy paste link on website ( http://controlmakers.ir/en/)

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Totally Copy & Paste word by word


Calculate Size of Contactor / Fuse / CB / OL Relay of Star-Delta Starter

  • Calculate Size of each Part of Star-Delta starter for 10HP, 415 Volt Three Phase Induction Motor having Non Inductive Type Load, Code A, Motor efficiency 80%, Motor RPM 600, Power Factor 0.8. Also Calculate Size of Overload Relay if O/L Relay Put in the wingdings (overload is placed after the Winding Split into main and delta Contactor) or in the line (Putting the overload before the motor same as in DOL).


Basic Calculation of Motor Torque & Current:

  • Motor Rated Torque (Full Load Torque) =5252xHPxRPM
  • Motor Rated Torque (Full Load Torque)=5252x10x600=88 lb-ft.
  • Motor Rated Torque (Full Load Torque) =9500xKWxRPM
  • Motor Rated Torque (Full Load Torque)=9500x(10×0.746)x600 =119 Nm
  • If Motor Capacity is less than 30 KW than Motor Starting Torque is 3xMotor Full Load Current or 2X Motor Full Load Current.
  • Motor Starting Torque=3x Motor Rated Torque (Full Load Torque).
  • Motor Starting Torque==3×119=356 Nm.
  • Motor Lock Rotor Current =1000xHPx figure from below Chart/1.732×415
Locked Rotor Current
Code Min Max
A 1 3.14
B 3.15 3.54
C 3.55 3.99
D 4 4.49
E 4.5 4.99
F 5 2.59
G 2.6 6.29
H 6.3 7.09
I 7.1 7.99
K 8 8.99
L 9 9.99
M 10 11.19
N 11.2 12.49
P 12.5 13.99
R 14 15.99
S 16 17.99
T 18 19.99
U 20 22.39
V 22.4
  • As per above chart Minimum Locked Rotor Current =1000x10x1/1.732×415=14 Amp
  • Maximum Locked Rotor Current =1000x10x3.14/1.732×415=44 Amp.
  • Motor Full Load Current (Line) =KWx1000/1.732×415
  • Motor Full Load Current (Line) = (10×0.746)x1000/1.732×415=13 Amp.
  • Motor Full Load Current (Phase) =Motor Full Load Current (Line)/1.732.
  • Motor Full Load Current (Phase) ==13/1.732=7 Amp.
  • Motor Starting Current (Star-Delta Starter) =3xFull Load Current.
  • Motor Starting Current (Line)=3×13=39 Amp

(1) Size of Fuse:

Fuse  as per NEC 430-52
Type of Motor Time Delay Fuse Non-Time Delay Fuse
Single Phase 300% 175%
3 Phase 300% 175%
Synchronous 300% 175%
Wound Rotor 150% 150%
Direct Current 150% 150%
  • Maximum Size of Time Delay Fuse =300% x Full Load Line Current.
  • Maximum Size of Time Delay Fuse =300%x13= 39 Amp.
  • Maximum Size of Non Time Delay Fuse =1.75% x Full Load Line Current.
  • Maximum Size of Non Time Delay Fuse=1.75%13=23 Amp.

(2) Size of Circuit Breaker:

Circuit Breaker as per NEC 430-52
Type of Motor Instantaneous Trip Inverse Time
Single Phase 800% 250%
3 Phase 800% 250%
Synchronous 800% 250%
Wound Rotor 800% 150%
Direct Current 200% 150%
  • Maximum Size of Instantaneous Trip Circuit Breaker =800% x Full Load Line Current.
  • Maximum Size of Instantaneous Trip Circuit Breaker =800%x13= 104 Amp.
  • Maximum Size of Inverse Trip Circuit Breaker =250% x Full Load Line Current.
  • Maximum Size of Inverse Trip Circuit Breaker =250%x13= 32 Amp.

(3) Thermal over Load Relay:

Thermal over Load Relay (Phase):

  • Min Thermal Over Load Relay setting =70%xFull Load Current(Phase)
  • Min Thermal Over Load Relay setting =70%x7= 5 Amp
  • Max Thermal Over Load Relay setting =120%xFull Load Current(Phase)
  • Max Thermal Over Load Relay setting =120%x7= 9 Amp

Thermal over Load Relay (Line):

  • For a star-delta starter we have the possibility to place the overload protection in two positions, in the line or in the windings.
  • If O/L Relay Placed in Line: (Putting the O/L before the motor same as in DOL).Supply>Over Load Relay>Main Contactor
  • If Over Load Relay supply the entire motor circuit and are located ahead of where the power splits to the Delta and Star contactors, so O/L Relay size must be based upon the entire motor Full Load Current.
  • Thermal over Load Relay setting =100%xFull Load Current (Line).
  • Thermal over Load Relay setting =100%x13= 13 Amp
  • Disadvantage: O/L Relay will not give Protection while Motor runs in Delta (Relay Setting is too High for Delta Winding)
  • If O/L Relay Placed In the windings: (overload is placed after the Winding Split into main and delta Contactor).Supply>Main Contactor-Delta Contactor>O/L Relay
  • If overload is placed after the Point where the wiring Split into main and delta Contactor, Size of over load relay at 58% (1/1.732) of the motor Full Load Current because we use 6 leads going to the motor, and only 58% of the current goes through the main set of conductors (connected to the main contactor).
  • The overload then always measures the current inside the windings, and is thus always correct. The setting must be x0.58 FLC (line current).
  • Thermal over Load Relay setting =58%xFull Load Current (Line).
  • Thermal over Load Relay setting =58%x13= 8 Amp.
  • Disadvantage: We must use separate short-circuit and overload protections

(4) Size and Type of Contactor:

  • Main and Delta Contactor:

  • The Main and Delta contactors are smaller compared to single contactor used in a Direct on Line starter because they Main and Delta contactors in star delta starter are controlling winding currents only. The currents through the winding are 1/√3 (58%) of the current in the line. These two contactors (Main contactor and Delta Contactor) are close during run. These rated at 58% of the current rating of the motor.
  • Star Contactor:

  • The third contactor is the star contactor and that only carries star current while the motor is connected in star in starting. The current in star winding is 1/√3= (58%) of the current in delta, so this contactor can be rated at 1/3 (33%) of the motor rating. Star contactor can be selected smaller than the others, providing the star contactor pulls first before the main contactor. Then no current flows when third contactor pulls.
  • In star connection at start, the motor draws and delivers 1/3 of its full rated power.
  • When the starter switches over to Delta, the motor draws full power, but since the contactors and the overload relay are usually wired within the Delta, you need to use contcators and relay which are only rated 1/√3 =58% of the full rated power of the motor.
Application Contactor Making Cap
Non-Inductive or Slightly Inductive ,Resistive Load AC1 1.5
Slip Ring Motor AC2 4
Squirrel Cage Motor AC3 10
Rapid Start / Stop AC4 12
Switching of Electrical Discharge Lamp AC5a 3
Switching of Electrical Incandescent Lamp AC5b 1.5
Switching of Transformer AC6a 12
Switching of Capacitor Bank AC6b 12
Slightly Inductive Load in Household or same type load AC7a 1.5
Motor Load in Household Application AC7b 8
Hermetic refrigerant Compressor Motor with Manual O/L Reset AC8a 6
Hermetic refrigerant Compressor Motor with Auto O/L Reset AC8b 6
Control of Restive & Solid State Load with opto coupler Isolation AC12 6
Control of Restive Load and Solid State with T/C Isolation AC13 10
Control of Small Electro Magnetic Load ( <72VA) AC14 6
Control of Small Electro Magnetic Load ( >72VA) AC15 10
  • As per above Chart
  • Type of Contactor= AC1
  • Making/Breaking Capacity of Contactor= Value above Chart x Full Load Current (Line).
  • Making/Breaking Capacity of Contactor=1.5×13= 19 Amp.
  • Size of Star Contactor (Starting Condition) = 33%X Full Load Current (Line).
  • Size of Star Contactor =33%x13 = 4 Amp.
  • Size of Main Contactor (Starting-Transition-Running) = 58%X Full Load Current (Line).
  • Size of Main Contactor =58%x13 = 8 Amp.
  • Size of Delta Contactor (Running Condition) = 58%X Full Load Current (Line).
  • Size of Delta Contactor =58%x13 = 8 Amp.


  •  Type of Contactor= AC1
  • Making/Breaking Capacity of Contactor=19 Amp.
  • Size of Star Contactor =4 Amp.
  • Size of Main Contactor = 8 Amp.
  • Size of Delta Contactor =8 Amp.

Effects of unbalanced Electrical Load (Part:2)

  • Harmonics in system by UPS:

  • UPS or inverter supplies also perform with poor efficiency and inject more harmonic currents in case of unbalances in the system
  • Decrease Life cycle of Equipment:

  • Unbalanced Voltage increase I2R Losses which increase Temperature. High temperatures, exceeding the rated value of a device, will directly decrease the life cycle of the device and speed up the replacement cycle for the device, and significantly increase the costs of operation and maintenance.
  • Relay malfunction

  • Unbalanced Voltage flows Negative and Unbalanced Voltage of Voltage or Current.
  • The high zero-sequence current in consequence of voltage imbalance may bring about malfunctions of relay operation or make the ground relay less sensitive. That may result in serious safety problems in the system.
  • Inaccurate Measurement

  • Negative and zero-sequence components of voltages or currents will give rise to inaccurate measurements in many kinds of meters.
  • The imprecise measured values might affect the suitability of settings and coordination of relay protection systems and the correctness of decisions by some automated functions of the system.
  • Decrease Capacity of transformers, cables and lines

  • The capacity of transformers, cables and lines is reduced due to negative sequence components. The operational limit is determined by the RMS rating of the total current, due to ‘useless’ non-direct sequence currents the capacity of equipment is decrease.
  • Increase Distribution Losses

  • Distribution network losses can vary significantly depending on the load unbalance.
  • Unbalance load increase I2R Losses of distribution Lines.
  • Increase Energy Bill by increasing Maximum Demand

  • Unbalanced Load increase maximum Demand of Electrical supply which is significantly effects on energy bill. By load balancing we can reduce energy bill.
  • For Energy Consumption Energy Supply Company does not charge on kVA but on kW for Residential customers. This means that they are charged for the “actual” energy used and not charged for the “total” energy supplied. Thus the power factor and Maximum Demand do not impact residential customers.
  • But Commercial, Industrial and H.T Connection charged by its maximum demand . We have to specify the maximum “demand“(in kVA) at the time of connection. During the month if you exceed your maximum “demand” you have to pay penalty (or extra price) for the same. That is the MDI penalty that appears on electricity bills.
  • Let’s assume That Two Company has same approved load of 40 KW and runs 30KW for 100 hours.
  • Electricity charge = 65 Rs per kWh
  • Demand charge = 210Rs per kW
  • Example 1: Company A runs a 30 KW loads continuously for 100 hours but It’s Maximum Demand is 50KW
  • 30 KW x 100 hours = 3,000 KWh
  • Energy Consumption Charge =3000×65=195000Rs
  • Demand difference = 50 KW-40KW=10KW
  • Demand Charges = 10X210=2100Rs
  • Total Bill:  195000+2100=197100Rs
  • Example 2: Company A runs a 30 KW loads continuously for 100 hours but It’s Maximum Demand is 40W
  • 30 KW x 100 hours = 3,000 KWh
  • Energy Consumption Charge =3000×65=195000Rs
  • Demand difference = 40 KW-40KW=0KW
  • Demand Charges = 0X210=00Rs
  • Total Bill:  195000+0=195000Rs
  • Failure of Transformer

  • Three-phase voltage with high unbalanced may cause the flux inside the transformer core to be asymmetrical.
  • This asymmetrical flux will cause extra core loss, raise the winding temperature and may even cause transformer failure in a severe case.
  • Ideally any distribution transformer gives best performance at 50% loading and every electrical distribution system is designed for it. But in case of unbalance the loading goes over 50% as the equipments draw more current.
  • The efficiency of transformer under different loading conditions
  • Full Load- 98.1%
  • Half Load- 98.64%
  • Unbalanced loads- 96.5%
  • For a distribution transformer of 200KVA rating, the eddy currents accounts for 200W but in case of 5% voltage unbalance they can rise up to 720W.
  • Bad / Loose connection of neutral wire

  • In balance Load condition Bad connection of Neutral wire does not make more impact on distribution System but in unbalance load condition such type of Bad neutral connection make worse impact on distribution.
  • The Three Phase power supplies a small a three-floor building. Each floor of this three-floor building is serviced by a single-phase feeder with a different phase. That is the first, second and third floor are serviced by phase R, Y and B. The external lighting load is connected only on R Phase.
  • The supply transformer is rated at 150 kVA and connected delta-grounded wye to provide for 430/220 V three-phase four-wire service.
  • This Transformer has a loose or Bad Neutral connection with the earth.
  • The transformer delivers a load of 35 kVA at 220 V with 0.9 power factor lagging to each floor.
  • During the daytime on, most of the Load of the Building are distributed equally over the three floors which is R Phase=30A, Y Phase =32A, B Phase=38A.
  • In Daytime The Bad connection of Neutral does not effected the Distribution system due to equal load distribution of the System
  • However it is not case in Nighttime. the Load on Y Phase and B Phase are negligible but R Phase Load is high compare to Y and B Phase.
  • In R Phase due to High Electrical Load and The fluorescent lamps flash frequently during the Nighttime of external Lighting Load
  • In Night time a bad electrical contact of the neutral wire of the supply makes the high contact resistance between the neutral wire and connector .which was about 15 kΩ.
  • This extra high impedance caused an unusually high voltage drop in the phase a circuit. In this case, the voltage of phase a dropped from the normal 220V to 182.5V, about 17% based on the nominal voltage. If the contact impedance goes higher than 20 kΩ, it may result in more serious conditions such as extinguishing all lamps.
  • This problem can be removed by fixing the bad connection and keeping the contact impedance near to zero.
Neutral Wire Contact Resistance Voltage across  bad Connection Point Voltage across  Transformer Secondary Side
Day Time Night Time Day Time Night Time
Proper Connection (0Ω) 0v 0v 0v 0v 0v 0v 220v 220v 220v 220v 220v 220v
Bad Connection (15Ω) 0v 0v 0v 40v 0v 0v 220v 220v 220v 182v 220v 220v
  • Neutral wire broken

  • The effect of a broken neutral makes voltage imbalance in a Three Phase Four Wire System.
  • For a Three Phase Four Wire System, high neutral wire impedance might enlarge a voltage imbalance (Some Phase Voltage increase while some Phase Voltage decreases).
  • High Voltage damage the equipment connected and even destroy on other hand low voltage effect operation of equipments.
  • The Three Phase star connected lighting loads are fed by a 430 V balanced three-phase voltage source. The fluorescent lamps are all rated at 220 V, 100 W each. The lamps are not equally in R Phase 5 No’s of Bulbs are connected, in Y Phase 3 No’s of Bulbs are connected and on B Phase 3 No’s of Bulbs are Connected. And, the normal impedance of the neutral wire is 1Ω
  • In unbalanced three phase load arrangement, high neutral wire impedance will enlarge the voltage across the neutral wire. The voltages of phases B and C at the load terminal raised to 255 V and 235 V, respectively, and gaining 16.15% and 5.77% based on rated voltage. These abnormally high phase voltages might damage the lamps in phase B and C.
  • On the other hand, the voltage in phase A was reduced from 220V to 185V. That might cause the lamps to flash.
  • If the broken neutral line problem is fixed, then the three phase voltages will go back to normal in near balanced status .however, if the loads are distributed equally to the three phases this problem can also be removed or minimized.
Conditions Voltage across the neutral wire Voltage at  the load terminal
Normal Condition 1v 1v 1v 220v 220v 220v
Neutral Broken 0v 0v 0v 182v 255v 235v
  • Unsuitable capacitor bank installation

  • For reducing energy loss, utilities always force their customers to maintain the power factor within a limit. Penalty will be applied to the customers if their loads’ power factors run outside the limits.
  • Installation of shunt capacitor banks is the most common and cheapest manner to improve the power factor. However, unsuitable installation (single Phase Capacitor instead of Three Phase Capacitor ) may make it worse.
  • The supply transformer is rated at 150 kVA, 11kV/430 V, and supplies a three-phase load of 105 kVA with power factor 0.7 lagging.
  • A single-phase 20KVAR capacitor bank is connected to B phase to improve system power. The impedance of the shunt capacitor bank is 1.805Ωper phase.
  • This kind of single phase Capacitor installation should make the system unbalanced. This unsuitable installation consumes extra real power of 44355 W.
  • The extra real power consumption = 1.732x2XV(RB) / 4xXc =(1.732x2x430) / (4×1.805) =44355W
  • This case shows that the system balance should be considered when installing a capacitor bank to correct the system power factor for a three-phase power distribution system.

 Remedial Action to prevent unbalances Load:

  • All the single phase loads should be distributed on the three phase system such that they put equal load on three phases.
  • Replacing the disturbing equipments i.e. with unbalanced three phase reactance.
  • Reducing the harmonics also reduces the unbalance, which can be done by installing reactive or active filters. These filters reduce the negative phase sequence currents by injecting a compensating current wave.
  • In case the disturbing loads cannot be replaced or repaired, connect them with high voltage side this reduces the effects in terms of percentage and even controlled disturbance in low voltage side.
  • Motors with unbalanced phase reactance should be replaced and re-winded.
  • Distribution of single-phase loads equally to all phases.
  • Single-phase regulators have been installed that can be used to correct the unbalance but care must be exercised to ensure that they are controlled carefully not to introduce further unbalance.
  • Passive network systems and active power electronic systems such as static var compensators and line conditioners also have been suggested for unbalance correction.
  • Load balancing.
  • Use of passive networks and static VAR compensators.
  • Equipment that is sensitive to voltage unbalance should not be connected to systems which supply single-phase loads.
  • Effect of voltage unbalance on ac variable speed drives can be reduced by properly sizing ac side and dc link reactors
  • Tight all Neutral Connections of the System.
  • Install Proper size of Capacitor Bank to the System.
  • Load Scheduling, where the loads in an electrical network are scheduled in a way to turn on and off at precise times to prevent the overloading of any one phase.
  • Manual Load Shifting, where an electrician opens a breaker panel and physically removes the loads from one phase and inserts them onto another phase.
  • Load Shedding, where the loads in an electrical network are immediately turned off in order to instantly “rebalance” the phases. This is usually done by ranking the loads in a network by how long they can be turned off before it affects operations


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