Minimum Electrical Clearance.

April 22, 2011 313 Comments


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

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Fuse

April 21, 2011 10 Comments


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.

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Type and Specification of Fuse

April 21, 2011 10 Comments


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.

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Electrical Clearance in Substation

April 6, 2011 59 Comments


Minimum Clearance in Substation:

Voltage Highest Voltage Lighting Impulse Level (Kvp) Switching Impulse Level (Kvp) Minimum Clearance Safety Clearance(Mt) Ground Clearance(Mt)
Phase-Earth Phase-Phase
11KV 12KV 70   0.178 0.229 2.600 3.700
33KV 36KV 170   0.320 0.320 2.800 3.700
132KV 145KV 550   1.100 1.100 3.700 4.600
650   1.100 1.100 2.700 4.600
220KV 245KV 950   1.900 1.900 4.300 5.500
1050   1.900 1.900 4.300 5.500
400KV 420KV 1425 1050(P-E) 3.400 4.200 6.400 8.000 

 Electrical Clearance in Substation:

Voltage Height of I Bay From Ground (Mt) Height of II Bay From Ground (Mt) Bay Width (Mt) Phase-Phase (Mt) BetweenEquipment Earth Wire From Ground
132KV (Single) 8.0 – 11.0 3.0 3.0 10.5
220KV (Single) 12.5 – 18 4.5 4.5 15.5
220KV (Double) 18.5 25 25 4.5 4.5 28.5
400KV 15.6 22 22.0 7.0 >6.0 30.0

Standard Bay Widths in Meters:

Voltage Bay Width (Meter)
11KV 4.7 Meter
33KV 4.7 Meter
66KV 7.6 Meter
132KV 12.2 Meter
220KV 17 Meter
400KV 27 Meter

 Standard Bus and Equipment Elevation

Voltage Equipment live Terminal Elevation (Meter) Main Bus Take of Elevation (Meter)
Low High
11 KV/33KV 2.8To 4 5.5  To6.5 9 6.5To8.5
66KV 2.8To 4 6To8 9To 10.5 9.5
132KV 3.7To5 8To9.5 13.5To14.5 12To12.5
220KV 4.9To5.5 9To13 18.5 15To18.5
400KV 8.0 15.5 – 23

 Phase spacing for strung Bus:

Voltage Clearance
11KV 1300 mm
33KV 1300 mm
66KV 2200 mm
132KV 3000 mm
220KV 4500 mm
400KV 7000 mm

 Minimum Clearance of Live Parts from Ground:

Voltage Minimum Clearance to Ground (Mt) Section Clearance (Mt)
11KV 3.700 2.600
33KV 3.700 2.800
66KV 4.600 3.000
132KV 4.600 3.500
220KV 5.500 4.300
400KV 8.000 7.000

 Insulator String:

 Voltage No of Suspension String Length (mm) No of Disc for Tension String Length in (mm)
66KV 5 965 6 1070
132KV 9 1255 10 1820
220KV 14 1915 15 2915
400KV 23 3850 2 X 23 5450

 Nominal Span:

Voltage Normal Span (Meter)
66KV 240-250-275
132KV 315-325-335
220KV 315-325-335
400KV 315-325-335

 Minimum Ground Clearance:

Voltage Ground (Meter)
66KV 5.5
132KV 6.1
220KV 7.0
400KV 8.0
800KV 12.4

 Indoor Substation Minimum Clearances

 

Distance Descriptions
0.9 Meter Horizontally between any item of equipment and thesubstation wall
0.6 Meter Horizontally between any Two items of equipment
1.2 Meter Horizontally in front of any HV switchgear

 Clearance of Conductor on Tower

Voltage Tower Type Vertical Space (Mt) Horizontal Space(Mt) Total Height From Ground(Mt)
66KV A 1.03 4.0 15.91
  B 1.03 4.27 15.42
  C 1.22 4.88 16.24
132KV A 7.140 2.17 23.14
  B 4.2 6.29 22.06
  C 4.2 7.15 22.68
  D 4.2 8.8 24.06
220KV A 5.2 8.5 28.55
  B 5.25 10.5 29.08
  C 6.7 12.6 31.68
  D      

 NORMS OF PROTECTION FOR EHV CLASS POWER TRANSFORMERS

Voltage ratio & capacity HV Side LV Side Common relays
 132/33/11KV up to 8 MVA   3 O/L relays + 1 E/L relay 2 O/L relays + 1 E/L relay Buchholz, OLTC Buchholz, OT, WT
 132/33/11KV above 8 MVA and below 31.5 MVA   3 O/L relays + 1 dir. E/L relay 3 O/L relays + 1 E/L relay Differential, Buchholz, OLTC Buchholz, OT, WT
 132/33KV, 31.5 MVA & above   3 O/L relays + 1 dir. E/L relay 3 O/L relays + 1 E/L relay Differential, Over flux,Buchholz, OLTC PRV, OT, WT
 220/33 KV, 31.5MVA & 50MVA 220/132KV, 100 MVA   3 O/L relays + 1 dir. E/L relay 3 O/L relays + 1 dir. relay Differential, Over flux,Buchholz, OLTC PRV, OT, WT
 400/220KV 315MVA   3 directional O/L relays (with dir. High set) +1 directional E/L relays. Restricted E/F relay + 3 Directional O/L relays for action 3 directional O/L relays (with dir. High set)+1 directional E/L relays. Restricted E/F relay Differential, Over flux,Buchholz, OLTC PRV, OT, WT and overload (alarm) relay

 The bottom most portion of any insulator or bushing in service should be at a minimum height of 2500 mm above ground level.

 Location of L.A (From T.C Bushing):

Voltage BIL KV Peak Distance (Mt)
11KV 75 12
33KV 200 15
66KV 325 24
132KV 550 35
220KV 900 To 1050 Close To T.C
400KV 1425 To 1550

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Typical Limiting Values of SubStation Equipments.

March 31, 2011 15 Comments


Typical Limiting Values of Sub Station Electrical Equipments.

1. Transformer / Reactor:

Sr. No. Equipment / test data Permissible limits Reference
A) Transformer oil    
a) BDV    
-At the time of first charging 600 kV (Gap – 2.5 mm) – Minimum IS – 1866
-During O&M 50 kV (Gap – 2.5 mm) – Minimum IS – 1867
b) Moisture content   IS – 1868
-At the time of first charging 15 PPM (Max.) IS – 1869
-During O&M 25 PPM (Max.) IS – 1870
c) Resistivity at 90 degree C 0.1-1012 Ohm-CM (Min.) IS – 1871
d) Acidity 0.2 mg KOH/gm (Max.) IS – 1872
e) IFT at 27 degree C 0.018 N/M (Min.) IS – 1873
f) Tan delta at 90 degree C 0.20 (Max.) IS – 1874
g) Flash point 126 Deg. C (Min.) IS – 1875
B) Vibration level for reactors 200 Microns (Peak to Peak) IS – 1876
  60 Microns (Average) IS – 1877
C) Tan delta for bushing at 20 Deg. C 0.007* IEC – 137
D) Capacitance for bushing + 5% variation IEC – 138
E) IR value for winding 1000 M-Ohm By 5.0/10.0 kV Megger IEC – 139
F) Tan delta for windings at 20 Deg. C 0.007* IEEE/C57.12.90.1980
G) Contact resistance of bushing terminal connectors 10 M. Ohm / Connector NGC.UK Recommendations
H) Turret Neutral CT ratio errors 39 IS – 2705

 2. Circuit Breakers 

Sr. No. Equipment / test data Permissible limits Reference
A) Dew point of SF6 gas Dew point values as per Annexure – II  
B) Dew point of operating air -45 Deg. C at ATM. Pressure  
C) CB Operating timings 400 kV               220KV  
a) Closing time (Max.) 150 MS              200MS  
b) Trip time (Max.) 25 MS                35MS  
c) Close/trip time, Pole discrepancy    
– Phase to Phase (Max.) 3.33 MS            3.33MS  
-Break to break (Max.) of same phase 2.5 MS              2.5MS  
D) PIR time    
BHEL make 12-16 MS Manufacturers Recommendations
     
ABB make 8-12 MS Manufacturers Recommendations
NGEF make 8-12 MS Manufacturers Recommendations
M&G make 8-12 MS Manufacturers Recommendations
TELK make 8-12 MS Manufacturers Recommendations
ABB make (HVDC) 8-12 MS Manufacturers Recommendations
E) PIR opening time prior to opening of main contacts (ABB, CGL, NGEF make CBs) 5 MS (Min.) at rated pressure Manufacturers Recommendations
F) Pir and main contacts overlap time [BHEL, M&G, ABB (imported) make CBs] 6 MS (Min.) at rated pressure Manufacturers Recommendations
G) Tan delta of grading capacitors 0.007 at 20 Deg. C  
H) Capacitance of grading capacitors Within + 10% / -5% of the rated value IEC 359
I) Contact resistance of CB 150 M. Ohm  
J) Contact resistance of CB terminal connector 10 M. Ohm per connector NGC, UK recommendations
K) IR value:    
1. Phase – earth 1000 M Ohm (Min.) by 5.0 / 10.0 kV Megger  
2. Across open contacts 1000 M Ohm (Min.) by 5.0/10.0 kV Megger  
3. Control cables 50 M Ohm (Min.) by 0.5 kV Megger  
L) Pressure switch settings    
-SF6 gas pressure switches Within + 0.1 Bar of set value  
-Operating air pr. Switches Within + 0.1 Bar of set value  
-Operating oil pr. Switches Within + 0.1 Bar of set value  
M) BDV of oil used for MOCB    
-At the time of filling 40 kV at 2.5 mm Gap (Min.) Mfgs. Recommendation
-During O&M 20 kV at 2.5 mm Gap. (Min.) Mfgs. Recommendation

 3. Current Transformer

Sr. No. Equipment / test data Permissible limits Reference
A) IR value    
1. Primary – earth 1000 M – Ohm (Min.) by 5.0/10.0 kV Megger  
2. Secondary – earth 50 M – Ohm (Min.) by 0.5 kV Megger  
3. Control cables 50 M-Ohm (Min.) by 0.5 kV Megger  
B) Tan delta value 0.007* at 20 Deg. C  
C) Terminal Connector 10 M-Ohm per connector NGC, UK Recommendations
D) CT ratio errors + 3% -Protection cores IS – 2705
D) CT ratio errors + 1% -Metering cores IS – 2706

 4. Capacitive Voltage

Sr. No. Equipment / test data Permissible limits Reference
A) Tan Delta 0.007* at 20 Deg. C  
B) Capacitance Within +10%/-5% of the rated value IEC – 358
C Contact resistance of terminal connector 10 M-Ohm per connector NGC, UK Recommendations
D) IR Value IR Value  
1. Primary – earth 1000 M – Ohm (Min.) by 5.0/10.0 kV Megger  
2. Secondary – earth 50 M – Ohm (Min.) by 0.5 kV Megger  
3. Control cables 50 M-Ohm (Min.) by 0.5 kV Megger  
E) EMU tank oil parameters EMU tank oil parameters  
a) BDV (Min.) 30 kV (Gap. –2.5 mm) IS – 1866
b) Moisture content (Max.) 35 ppm -do-
c) Resistivity at 90 Deg. C 0.1 – 1012 Ohm. – CM -do-
d) Acidity 0.5 mg kOH /gm (Max.) -do-
e) IFT at 27 Deg. C 0.018 N/M (Min.) -do-
f) Tan delta at 90 Deg. C 1.0 Max. -do-
g) Flash point 125 Deg. C (Min.) -do-
F) CVT voltage ratio errors + 5% protection cores IEEE/C93.1.1990
F) CVT voltage ratio errors + 0.5% metering cores IEC 186

 5. Isolators

Sr. No. Equipment / test data Permissible limits Reference
A) Contact resistance 300 M-Ohm. (Max.)  
B) Contact resistance of terminal connector 10 M – Ohm per connector NGC, UK Recommendations
C) IR value    
1. Phase – earth 1000 M – Ohm (Min.) by 5.0/10.0 kV Megger NGC, UK Recommendations
2. Across open contacts 1000 M – Ohm (Min.) by 5.0/10.0 kV Megger NGC, UK Recommendations
3. Control cables 50 M-Ohm (Min.) by 0.5 kV Megger  

 6. Surge Arrester

Sr. No. Equipment / test data Permissible limits Reference
A) Leakage current 500 M-Amp. (Resistive) Hitachi, Japan Recom.
B) IR value 1000 M-Ohm. (Min.) Hitachi, Japan Recom.

 7. Miscellaneous

Sr. No. Equipment / test data Permissible limits Reference
A) Station earth resistance 1.0 Ohm (Max.)  
B) Thermo vision scanning    
Temp. up to 15 Deg. C (above ambient) Normal  
Temp. above 15-50 Deg. C (above ambient) Alert  
Temp. above 50 Deg. C (above ambient) To be immediately attended  
C) Terminal connectors – Contact resistance 10 M-Ohm per connector HGC, UK Recommendations
D) IR values    
1. All electrical motors 50 M-Ohm (Min.) by 0.5 kV Megger  
2. Control cables 50 M-Ohm (Min) by 0.5 kV Megger  
3. Lt. Transformers 100 M.-Ohm (Min.) by Megger  
4. Lt. Switchgears 100 M – Ohm (Min.) by 0.5 kV Megger  

 8. Batteries

Sr. No. Equipment / test data Permissible limits Reference
A) Terminal connector resistance 10 M – Ohm + 20% ANSI/IEEE – 450 1989
B) Specific gravity 1200 + 5 GM/L at 27 Deg. C  

 Temperature Correction Factor for Tan Delta Measurement

 Sr. No.  Oil temperature Deg. C  Correction factor(K)
 1  10  0.8
 2  15  0.9
 3  20  1.0
 4  25  1.12
 5  30  1.25
 6  35  1.40
 7  40  1.55
 8  45  1.75
 9  50  1.95
 10  55  2.18
 11  60  2.42
 12  65  2.70
 13  70  3.00

If Tan Delta of bushing/winding/CVT/CT is measured at oil temperature T Deg. C. Then Tan Delta at 20 Deg. C shall be as given below:

Tan Delta at 20 Deg. C = Tan Delta at Temp T Deg. C / Factor K.

Dew Point Limits for SF6 Gas in EHV Circuit Breakers

Sr. No Make of C.B Dew point at rated Pr. Deg. C  Corresponding dew point at Atmospheric .Pr.  Remarks
1 BHEL   -15  -36  At the time of commissioning
 -7  -29  During O&M
 -5  -27  Critical
2 M&G  —  -39  At the time of commissioning
   -32  During O&M
3 CGL  -15  -35  At the time of commissioning
 -10  -31  During O&M
4 ABB  -15  -35  At the time of commissioning
 -5  -26 During O&M
5 NGEF  -15  -36 At the time of commissioning
 -7  -29  During O&M

 Note: Dew point of SF6 gas varies with pressure at which measurement is carried out. So it is to be ensured that if measurement is done at pressure other than atmospheric pressure, it needs to be converted to the atmospheric pressure.

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Standard Makes for Electrical Equipments

March 28, 2011 6 Comments


Standard Makes for Electrical Equipments:

Sr. No. Item IS No. Makes
1 Transformer IS: 2026/1977 Crompton Greaves, NGEF, Kirloskar, BHEL, Andrewyule, Bharat Bijlee, Alsthom, ABB, Voltamp, Siemens, GEC, Voltas, TELK
2 11 kV/HT Vacuum Circuit Breaker, SF-6/11kV gas filled Circuit Breaker IS: 3427 for VCB GEC, Siemens, Andrew yule, Crompton Greave, Alsthom(Areva), Jyoti, ABB, BHEL, Alind, L&T,  Schneider, Biecco Lawrie.
3 ACB(11kV) IS: 13118/1991 Siemens, L&T, Crompton Greave, Schneider, Jyoti, GEC, MEI, ABB, Merlin Gerin, Moeller, Biecco Lawrie.
IS: 13947/1993
4 PSS/CSS with HT/LT switch gear, transformer and connected accessories IS:11171/85 for dry type transformer ABB, Siemens, L&T, Crompton Greave, BHEL, GEC, Kirloskar, Alsthom(Areva), Schneider
5 MCCBs, MCBs, ELCBS, RCCBs, DB, ICTPN, TP, HRC fuse, Changing over switch, Switch Fuse Unit IS: 8828/96 for MCB L&T, Crompton Greave, Siemens, Legrand, Jyoti, GEC, Andrew yule, BCH, Schneider, ABB, Moeller, Merlin Gerin
IS: 13947(Part-1&5/Sec1)/93for MCCB
IS: 12640(Part-1)/2000 for RCCB
IS: 13703(Part-2/Sec1)/93 for HRC fuse
IS: 13947(Part-3)/93 for SFU
6 XLPE Cable 11/33kV grade IS:7098(Part-2)/1985 Asian, NICCO, Universal, CCI, Torrent, Fort Gloster, INCAB, Industrial Cable, Polycab
7 PVC/XLPE Power Cables up to 1.1kV grade IS: 694/1990 for PVC cable CCI, Universal Cable,  Polycab, NICCO, Torrent, Asian, Fort Gloster, Finolex, Incab, Industrial Cable
IS: 1554(Part1)/1988 for heavy duty PVC cable
IS:7098(Part-1)/1988 for XLPE cable
8 Instrument Voltmeter, Ammeter, PF meter IS:1248 for Analog Automatic Electric, Meco, Industrial Meter, Motwani, Toshniwal, L&T, Siemens, Rishab, IMP, Shanti, Moeller(HPL).
9 11kV Cable End Termination & Jointing kits IS: 13573/92 Raychem, M-Seal, Safe system, Xicon brand of CCI, Hari consolidated(Cable seal brand), Densons(Yamuna)
10 Relays IS: 3231/65 Siemens, L&T, Alsthom, EA SUN REY Roll, ABB, BHEL, Jyoti, Alind, GE, BCH, Minilec, Enercon
11 Luminaries, MH, HPSV, T-5 fittings, CFL & related accessories IS: 9974(Part-1)/83 for HPSV Phillips, Crompton, Bajaj, GE, Osram, Wipro.
IS:10322(Part-5), 10322(Part2&3)/84 for Luminaries
IS:15111 for CFL
12 PVC insulated Elect. Wires Sheathed/ unsheathed, PVC flexible LT cable, multicore, single core, Flat cable for submersible pumps IS: 694/1990 for PVC cable Finolex, Asian,  Fort Gloster, CCI, NICCO, Universal,  Incab, Torrent, Uniflex, RPG, Polycab, ICL, Unistar
13 Current Transformer IS: 2705(Part3)/92 Automatic Electric, CGL, C&S, MECO, KAPPA, Siemens, L&T, Schneider
14 On line UPS, Servo Stabilizer, Inverter, CVT IS:13314/92 for Inverter AEI, BHEL, Hind Rectifier, L&T, NGEF, Siemens, Hi-rel, Autometer,  Enertech, Pyramid, APC, Dubas, Luminous, Microtech , TATA Libert
IS:11260/85 for voltage Stabilizer
15 Rotary Switches. Selector Switches Relevant IS Kaycee, L&T, Salzer, GE, ABB, C&S, Siemens, HPL, Moeller
16 Exhaust fan/Air Circulator/ Bracket & Pedestal fans/Ceiling fan IS: 374/79 for ceiling fan Crompton, Khaitan, GEC, Usha, Philips, Bajaj, Polar, Orient, Almonard.
IS: 2312/63 for Exhaust fan
IS:875(Part3), BSTN-10025/1993, CPE III TRT/1996 of ILE UK.
Octagonal Pole
S355JO.
Galvanization
IS: 2629
BSEN ISO-1461
18 Electronic Energy Meter IS:13779/1999 L&T, IMP,HPL, Secure, ABB, Enercon,
IEC:62053-21
19 Central Air Conditioning Plants & Package type plant IS:8148 for package type. Voltas, Blue Star, Sidwal, Fedder Lloyd, Videocon, Amtrex, Carrier, Frick, Shriram, Hitachi, O General, Mitsubhisi
IS:1391 for Room Air Conditioners.
20 Capacitors- PF correction for Electrical General Services IS:13340/93 ABB, BHEL, Indian Capacitors, Khatau Junker, Shreem, Unistar, WS Insulators, L&T, Hind Rectifier, Voltas, Siemens, Schneider, Indian Condenser, EPCOS
IS:13341/92
21 DG Sets- Portable IS: 13364(Part-1)/92 for Alternator Birla Yamaha, CGL, Shriram Honda
IS:1001/91 for Diesel Engine
22 DG Engine IS:13364 Cummins, Kirloskar, Caterpillar, Ashok Leylend, Penta-volvo
23 Alternator for DG set IS:4722/2001 KEC, CGL, Stampford, Leroy-somer, Kirloskar-Green
IS:4728/1975
24 Induction Motor IS:235/96 Bharat Bijlee, BHEL, CGL, GE, Jyoti, Kirloskar, Siemens, ABB, NGEF, Alsthom
IS:12615/2004
25 LT Switchgear & control gears- Contactors & motor starters, Energy Efficient Soft Starter panel/ Earthing Switch, Single phase preventer IS:13947(Part1)/1993 ABB, CGL, Jyoti, L&T, MEI,  NGEF,  Siemens, Telemecanique & control (India) (TC), Legrand(MDS), BCH, Standard, GEC, BHEL, Minilec, Enercon, Andrewyule, C&S, N.N. Planner, Power Boss, Schneider.
IS:13947(Part4)/1993
26 Pumps- Submersible IS: 8034/2002 for submersible pumpsets IS: 9283/1995 for motors of submersible pumpsets Calama, CGL, Jyoti, Kirloskar, KSB, TEXMO, SABAR
IS: 14220/1994 for open well submersible pumpsets
27 Timers- electronic solid state Relevant IS ABB, BHEL, GE, Jyoti, L&T, BCH, Siemens, Minilec, Legrand
28 Water Coolers IS: 1475/2001 Blue Star, Fedders, Kelvinator, Shriram, Sidwal, Voltas
29 Electrical accessories (Piano switch, Plugs & sockets, ceiling rose, Angle holder, holders,) IS: 3854/88 for switches SSK(Top line),  Anchor(Penta-ornet), Precision(Prime), Vinay(Clair-30), CONA(Nice-Indian), Leader, Legrand, ABB
IS: 1293/88 for plugs& sockets
IS: 371/79 for ceiling rose
IS: 1258/79 for lamp holder bakelite
30 Bell Buzzer IS:2268/1988 or latest CONA, MAX, Anchor, Leader, SSK
31 Electronic fan regulator IS:11037/1984 Anchor, Usha, ERIK, Rider
32 Solar cell/Module system Relevant IS TATA BP, BEL, BHEL, REIL, MOSER BEAR, CEL, Sharp Business Systems (India) Ltd.
33 Solar Lighting system Relevant IS DGS&D approved vendors on RC
34 GI/MS Pipe IS: 1239(Part-1)/90 TATA, Jindal, TT Swastik, Prakash Surya
35 Geysers IS:2082/93 Bajaj, Usha, Crompton , Spherehot, Recold, Venus.
36 Lifts & Escalators IS:14665/2000 OTIS, Thysson Krup, Shindler, KONE, Mitsubhisi.
37 LEDs Relevant IS BHEL, Indo Asian Fusegear Ltd., Switch control (India), Siemens, GE, ABB, L&T, Moeller.
38 Solar Water Heaters Relevant IS As per MNES approved sources.
39 Solar Distilled Water Plants Relevant IS Makes can be approved on the recommendation of divisions.
41 Air Cooling Plants Relevant IS for its concern equipments Voltas, Blue Star, Carrier
42 Battery Charger for other than battery room for Train Lighting IS:2026 Hind Rectifier, Usha Rectifier, Suresh Electrical, Pyramid, Automatic Electric, Delta Elect., Trinity Elect., Universal Ind. Products, Venus Engg., RS Power.
IS:3895
43 Battery Charger for battery room As per RDSO specification having re-generation facility Amar Raja, Exide, RS Power
44 PVC Conduit pipe & Casing capping for electrical wiring IS:9537/93 Precision, A.K.G., Polycab, Finolex , Prestoplast
45 Aluminum Ladders IS:4571/1977 Sumer, Beatfire
46 LT Panels As per Relevant IEC From CPRI tested firms.
47 Air Curtain Relevant IS Aircon, ALMONARD, Technocrate, Thermadyne, Mitzwak
48 Lighting Arrestor IS:5621/1980 with latest amendment/ revision (specification for hollow insulators for use in electrical equipment) (a) Jayshree Make – JSI (b) WSI (c) Shesashayee (d) BHEL (e) Luster Ceramics (f) Shakti make (g) Oblum make. Any other make approved by the PGVCL/GUVNL.
Creepage Distance : 11KV – 320 mm. 22KV – 600 mm.
49 Power transformer   BHEL, Bharat Bijlee, EMCO, Kanohar
50 66KV Circuit Breaker   ABB, Siemens, AREVA
51 66KV Isolator   ABB, AREVA.
52 66KV Current Transformer   ABB, CGL, BHEL, TELK
53 66KV CVT   ABB, TELK, CGL, BHEL
54 66KV LA   ABB, CGL
55 Control and Relay Panel   ABB, AREVA, Siemens
56 1.1KV Grade control and power Cable   Universal, KEI, GEMSCAB, Polycab, Paramount, CCI, Torrent, Krishna, HVPL
57 Batteries   Exide, Amco, Amaraja, HBL nife
58 Battery Charger   Chhabi, HBL, Caldyne, Masstech, DUBAS, Statcon
59 Station Transformer Transformer   CGL, Kotson, Voltamp, Kanohar, EMCO, AREVA,
60 Fire Protection system   Vijay, unitech, Technofab, Mather & Platt, Steelage.
61 11KV switchgear   ABB, AREVA, Siemens
       

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Power Quality

March 26, 2011 6 Comments


Power Quality:

  • In the present scenario of power utilization and consumption, the importance of power quality is vital for a continuous and effective power supply. The features of power quality play a major role in the effective power utilization along with the control & improvement measures for various factors affecting it.
  • Power quality is defined as the ability of a system to

1.     Deliver electric power service of sufficiently high quality so that the end-use equipment will operate within their design specifications and

2.     It should be of sufficient reliability so that the operator of end-use equipment will be continuous.

  • In other words it may be defined as the concept of powering, grounding and protecting electric equipment in a manner that is suitable to the operation of that equipment.

Why is it a concern?

  • Power Quality has been a problem since the conception of electricity, but only over the last 2 decades has it gotten considerable attention with the introduction of large numbers of computers & microprocessors in business and homes; and the network revolution and ever increasing equipment capability and speed.
  • There are various factors that really make us think about it.

1.     Power quality problems can cause equipment malfunctions, excessive wear or premature, failure of equipment, increased costs, increased maintenance, repair time and expense & outside consultant expense.

2. Electronic equipments are more sensitive to minor fluctuations. We rely on the equipment more and have higher expectations. New electronic devices are more sensitive than the equipment being replaced as well.

Power Quality Affecting Factor:

  • Many electronic devices are susceptible to power quality problems and a source of power quality problems. Some of the important concerns are

1.     Waveform Distortions like Harmonics

2.     Transients

3.     Voltage Fluctuations such as Voltage Sags & Swells

4.     Interruptions e.g. Outages & Blinks

1. Waveform Distortions -Harmonics

  • Due to substantial increase of non-linear loads such as the use of power electronics circuits and devices, the ac power system suffers from harmonic problems. In general, we may classify sources of harmonics into three categories i.e.

1.     Domestic loads,

2.     Industrial loads,

3.     Control devices.

  • A harmonic is “a sinusoidal component of a periodic wave or quantity having a frequency i.e. an integral multiple of fundamental frequency”. Pure or clean power is referred as those without harmonics. But this only exists in laboratories. The frequencies of the harmonics are different, depending on the fundamental frequency. Due to high harmonic voltage and/or current levels, there are a number of equipments that can have miss operation or failures.
  • The main sources of harmonic current are the phase angle controlled rectifiers and inverters.
  • Although the applied voltage to a transformer is sinusoidal, the magnetization current related to the flux through the lamination magnetization curve is non-sinusoidal. These harmonics have their maximum effect during the first hours of the day (when the system is lightly loaded and the voltage is higher).

2. Transients

  • Transients occur in Distribution System due to factors like Lightning, Switching Operations, and Fault Clearing/Breaker Operations etc. The various causes of transients in Customer System are Lightning, Arcing Devices, Starting & Stopping Motors, Breaker Operations, and Capacitor Switching etc.

3. Voltage Fluctuations such as Voltage Sags & Swells

  • In Sags, Voltage falls below 90% of normal but stays above 10% of normal for any amount of time. In Swells, Voltage rises above 110% of normal but below 180% of normal for any amount of time. If it’s long enough, you notice lights dimming or getting brighter. Sags are much more common than swells.

4. Interruptions e.g. Outages and Blinks

  • Interruptions may be defined as the interrupts that hampers the normal flow of voltage or power quality. When Voltage falls below 10% of normal circuit voltage for any length of time the power supply is off. The outages can be of microseconds to hours or days. When interruptions occur there is a chance of blinking as well.

Control & improvement of The System:

  • In order to overcome the various affecting factors, we need to implement some control and improvement measures. They are discussed as follows:

1. Harmonics

  • Several techniques are adopted to minimize harmonic effects like increasing pulse number, passive filters and active filters. By use of these techniques we get higher pulse, trap the harmonics and convert the non-linear ac line current into a sinusoidal wave respectively.
  • Power quality analysis is really a matter of concern as it is quite evident how important supply of power is especially in organizations where critical loads need continuous supply of clean power and that too without any disruption.
  • Technological advancements are developing in this sector in order to manage the advanced and sophisticated power systems with utmost proficiency.

2. Transients

  • We can use power enhancers like Surge Suppressors, Lightning Protection/Arrestors, Power Conditioning, Line Reactors/Chokes etc. Power Synthesizers such as Standby Power Systems, UPS & Motor Generator Set can be utilized. Simplest, least expensive way to condition power by clamping voltage when it exceeds a certain level and sending it away from the equipment it protects.
  • Transient voltage surge suppressors (TVSS) can be installed at the terminals of the sensitive electronic loads. Power line filters limit noise and transients to a safe level by slowing down the rate of change of these problems and keeping electronic systems safer than surge protectors can.

3. Voltage Fluctuations such as Voltage Sags & Swells

  • Use of Power Enhancers like Reduced Voltage Starters on large offending motors, Voltage Regulators, Constant Voltage Transformers (CVTs), Power Conditioners; as well as Power Synthesizers like UPS, Motor-Generator Sets can minimize voltage fluctuations.
  • Voltage Regulators can be utilized to maintain voltage output within a desired limit or tolerance regardless how much input voltage varies. They can also be utilized for protection against swells or noise and limited protection from fast voltage changes depending upon the response time of the regulator. Voltage regulators respond best to slow changes in voltage.
  • Constant Voltage Transformers (CVT’s), also known as Ferro resonant transformers are used for sags, swells, longer term over and under-voltages, especially attractive for constant, low-power loads like electronic controllers (PLC’s) where they provide ride-through capability. Variable loads, especially those with high inrush currents, (Drives) present more of a problem for CVT’s.

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Electrical Motor Connection

March 26, 2011 2 Comments


ELECTRICAL MOTOR CONNECTION:

 

How to Change Rotation of Motor in Clockwise Direction

No Present Motor Connection: Change Direction in Clockwise
1 R Phase Connected to U1 W2 R Phase Connected to U1 V2
Y Phase Connected to V1 U2 Y Phase Connected to V1 W2
B Phase Connected to W1 V2 B Phase Connected to W1 U2
2 R Phase Connected to W1 V2 R Phase Connected to W1 U2
Y Phase Connected to U1 W2 Y Phase Connected to U1 V2
B Phase Connected to V1 U2 B Phase Connected to V1 W2
3 R Phase Connected to V1 U2 R Phase Connected to V1 W2
Y Phase Connected to W1 V2 Y Phase Connected to W1 U2
B Phase Connected to U1 W2 B Phase Connected to U1 V2

Change Rotation in Anticlockwise Direction

No Present Motor Connection: Change Direction in Anticlockwise
1 R Phase Connected to U1 V2 R Phase Connected to U1 W2
Y Phase Connected to W1 U2 Y Phase Connected to W1 V2
B Phase Connected to V1 W2 B Phase Connected to V1 U2
2 R Phase Connected to W1 U2 R Phase Connected to W1 V2
Y Phase Connected to V1 W2 Y Phase Connected to V1 U2
B Phase Connected to U1 V2 B Phase Connected to U1 W2
3 R Phase Connected to V1 W2 R Phase Connected to V1 U2
Y Phase Connected to U1 V2 Y Phase Connected to U1 W2
B Phase Connected to W1 U2 B Phase Connected to W1 V2

Thumb Rule :

Check Phase Winding Starting Phase and Connected ending Connection of That Phase winding to the one Phase after the Phase where Phase winding Starting lead is connected. (Ex If U1 is connected to R Phase than Connect U2 to B Phase, If V1 is connected to Y Phase than V2 should be connected to R Phase)

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Ferranti Effect

March 26, 2011 25 Comments


What is Ferranti Effect

  • A long transmission line draws a substantial quantity of charging current. If such a line is open circuited or very lightly loaded at the receiving end, Receiving end voltage being greater than sending end voltage in a transmission line is known as Ferranti effect. All electrical loads are inductive in nature and hence they consume lot of reactive power from the transmission lines. Hence there is voltage drop in the lines. Capacitors which supply reactive power are connected parallel to the transmission lines at the receiving end so as to compensate the reactive power consumed by the inductive loads.
  • As the inductive load increases more of the capacitors are connected parallel via electronic switching. Thus reactive power consumed by inductive loads is supplied by the capacitors thereby reducing the consumption of reactive power from transmission line. However when the inductive loads are switched off the capacitors may still be in ON condition. The reactive power supplied by the capacitors adds on to the transmission lines due to the absence of inductance. As a result voltage at the receiving end or consumer end increases and is more than the voltage at the supply end. This is known as Ferranti effect.

Why does voltage rise on a long, unloaded transmission line?

  • The Ferranti Effect occurs when current drawn by the distributed capacitance of the transmission line itself is greater than the current associated with the load at the receiving end of the line. Therefore, the Ferranti effect tends to be a bigger problem on lightly loaded lines, and especially on underground cable circuits where the shunt capacitance is greater than with a corresponding overhead line. This effect is due to the voltage drop across the line inductance (due to charging current) being in phase with the sending end voltages. As this voltage drop affects the sending end voltage, the receiving end voltage becomes greater. The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied.
  • The Ferranti Effect is not a problem with lines that are loaded because line capacitive effect is constant independent of load, while inductance will vary with load. As inductive load is added, the VAR generated by the line capacitance is consumed by the load.

How to Reduce Ferranti Effect:

Shunt Reactors and Series Capacitors:

  • The need for large shunt reactors appeared when long power transmission lines for system voltage 220 kV & higher were built. The characteristic parameters of a line are the series inductance (due to the magnetic field around the conductors) & the shunt capacitance (due to the electrostatic field to earth).

  • Both the inductance & the capacitance are distributed along the length of the line. So are the series resistance and the admittance to earth. When the line is loaded, there is a voltage drop along the line due to the series inductance and the series resistance. When the line is energized but not loaded or only loaded with a small current, there is a voltage rise along the line (the Ferranti-effect)
  • In this situation, the capacitance to earth draws a current through the line, which may be capacitive. When a capacitive current flows through the line inductance there will be a voltage rise along the line.
  • To stabilize the line voltage the line inductance can be compensated by means of series capacitors and the line capacitance to earth by shunt reactors. Series capacitors are placed at different places along the line while shunt reactors are often installed in the stations at the ends of line. In this way, the voltage difference between the ends of the line is reduced both in amplitude and in phase angle.
  • Shunt reactors may also be connected to the power system at junctures where several lines meet or to tertiary windings of transformers.
  • Transmission cables have much higher capacitance to earth than overhead lines. Long submarine cables for system voltages of 100 KV and more need shunt reactors. The same goes for large urban networks to prevent excessive voltage rise when a high load suddenly falls out due to a failure.
  • Shunt reactors contain the same components as power transformers, like windings, core, tank, bushings and insulating oil and are suitable for manufacturing in transformer factories. The main difference is the reactor core limbs, which have non-magnetic gaps inserted between packets of core steel.
  • 3-phase reactors can also be made. These may have 3- or -5-limbed cores. In a 3-limbed core there is strong magnetic coupling between the three phases, while in a 5-limbed core the phases are magnetically independent due to the enclosing magnetic frame formed by the two yokes and the two unwound side-limbs.
  • The neutral of shunt reactor may be directly earthed, earthed through an Earthing-reactor or unearthed.
  • When the reactor neutral is directly earthed, the winding are normally designed with graded insulation in the earthed end. The main terminal is at the middle of the limb height, & the winding consists of two parallel-connected halves, one below & one above the main terminal. The insulation distance to the yokes can then be made relatively small. Sometimes a small extra winding for local electricity supply is inserted between the main winding & yoke.
  • When energized the gaps are exposed to large pulsation compressive forced with a frequency of twice the frequency of the system voltage. The peak value of these forces may easily amount to 106 N/m2 (100 ton /m2). For this reason the design of the core must be very solid, & the modulus of elasticity of the non-magnetic (& non-metallic) material used in gaps must be high (small compression) in order to avoid large vibration amplitudes with high sound level consequently. The material in the gaps must also be stable to avoid escalating vibration amplitudes in the end.
  • Testing of reactors requires capacitive power in the test field equal to the nominal power of the reactor while a transformer can be tested with a reactive power equal to 10 – 20% of the transformer power rating by feeding the transformer with nominal current in short –circuit condition.
  • The loss in the various parts of the reactor (12R, iron loss & additional loss) cannot be separated by measurement. It is thus preferable, in order to avoid corrections to reference temperature, to perform the loss measurement when the average temperature of the winding is practically equal to the reference temperature.

How does a phase shifting transformer help operators load and unload transmission lines?

  • Power flow between two buses can be expressed as:
  • Power Flow = (Vs*Vr / X) * Sine of the Power Angle.
  • In other words: power flow (in watts) between two buses will be equal to the voltage on the sending bus multiplied by the voltage on the receiving bus divided by the line reactance, multiplied by the sine of the power angle between the two buses.
  • This leaves grid operators with at least two options for making a path more conducive to power flow, or if desired, making a path look less conducive to power flow. The two options are to (1) adjust line reactance and (2) adjust power angle. The Phase Shifting Transformer (PST) affects the second option, i.e. adjusting power angle.
  • The physical appearance of the PST device is noteworthy, being one of the few transformer types where the physical height and construction of the primary bushings is the same as the secondary bushings. This makes sense since both bushing sets are at the same potential. Internally, the primary voltage of a PST is bussed directly to the secondary bushings, with one important addition. The primary voltage is applied to a delta-wound transformer primary that has adjustable taps that inject “opposing phase” signals. For instance the A-B primary winding has a C phase injection, the B-C winding is injected with A, and the C-A winding is injected with B. These injection points are simultaneously adjustable taps that result in an adjustable shift of power angle.
  • Since power angle is a direct contributor to the Power Flow formula provided above (in the numerator, not the denominator), changing the PST tap settings can increase power angle making the path more conducive to power flow. The PST tap settings can also decrease power angle making the path less conducive to power flow. (Remember that “power flows downhill on angle”.)
  • Why is this important? Many transmission paths naturally have less impedance by virtue of their construction and length, and these paths can carry scheduled flow as well as unscheduled flow from parallel (but higher impedance) paths. In some cases these low impedance paths become congested and PST devices and other devices and techniques may be used to relieve the congestion. This is particularly the case in regions where transmission paths are less densely developed.

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What is Corona Effect

March 23, 2011 50 Comments


Introduction:

One of the phenomena associated with all energized electrical devices, including high-voltage transmission lines, is corona. The localized electric field near a conductor can be sufficiently concentrated to ionize air close to the conductors. This can result in a partial discharge of electrical energy called a corona discharge, or corona.

  What is Corona?

  •  Electric transmission lines can generate a small amount of sound energy as a result of corona.
  • Corona is a phenomenon associated with all transmission lines. Under certain conditions, the localized electric field near energized components and conductors can produce a tiny electric discharge or corona that causes the surrounding air molecules to ionize, or undergo a slight localized change of electric charge.
  • Utility companies try to reduce the amount of corona because in addition to the low levels of noise that result, corona is a power loss, and in extreme cases, it can damage system components over time.
  • Corona occurs on all types of transmission lines, but it becomes more noticeable at higher voltages (345 kV and higher). Under fair weather conditions, the audible noise from corona is minor and rarely noticed.
  • During wet and humid conditions, water drops collect on the conductors and increase corona activity. Under these conditions, a crackling or humming sound may be heard in the immediate vicinity of the line.
  • Corona results in a power loss. Power losses like corona result in operating inefficiencies and increase the cost of service for all ratepayers; a major concern in transmission line design is the reduction of losses.

 Source of Corona:

  • The amount of corona produced by a transmission line is a function of the voltage of the line, the diameter of the conductors, the locations of the conductors in relation to each other, the elevation of the line above sea level, the condition of the conductors and hardware, and the local weather conditions. Power flow does not affect the amount of corona produced by a transmission line.
  • The electric field gradient is greatest at the surface of the conductor. Large-diameter conductors have lower electric field gradients at the conductor surface and, hence, lower corona than smaller conductors, everything else being equal. The conductors chosen for the Calumet to the line were selected to have large diameters and to utilize a two conductor bundle. This reduces the potential to create audible noise.
  • Irregularities (such as nicks and scrapes on the conductor surface or sharp edges on suspension hardware) concentrate the electric field at these locations and thus increase the electric field gradient and the resulting corona at these spots. Similarly, foreign objects on the conductor surface, such as dust or insects, can cause irregularities on the surface that are a source for corona.
  • Corona also increases at higher elevations where the density of the atmosphere is less than at sea level. Audible noise will vary with elevation. An increase in 1000 feet of elevation will result in an increase in audible noise of approximately 1 dB (A). Audible noise at 5000 feet in elevation will 5 dB (A) higher than the same audible noise at sea level, all other things being equal. The new Calumet to Comanche 345 kV double circuit line was modeled with an elevation of 6000 feet.
  • Raindrops, snow, fog, hoarfrost, and condensation accumulated on the conductor surface are also sources of surface irregularities that can increase corona. During fair weather, the number of these condensed water droplets or ice crystals is usually small and the corona effect is also small.
  • However, during wet weather, the number of these sources increases (for instance due to rain drops standing on the conductor) and corona effects are therefore greater.
  • During wet or foul weather conditions, the conductor will produce the greatest amount of corona noise. However, during heavy rain the noise generated by the falling rain drops hitting the ground will typically be greater than the noise generated by corona and thus will mask the audible noise from the transmission line.
  • Corona produced on a transmission line can be reduced by the design of the transmission line and the selection of hardware and conductors used for the construction of the line. For instance the use of conductor hangers that have rounded rather than sharp edges and no protruding bolts with sharp edges will reduce corona. The conductors themselves can be made with larger diameters and handled so that they have smooth surfaces without nicks or burrs or scrapes in the conductor strands. The transmission lines proposed here are designed to reduce corona generation.

 TYPES OF CORONA:

There are three types of corona.

  • A glow discharge occurs at a gradient of approximately 20 kV rms/cm. Glow discharge is a light glow off sharp points that does not generate objectionable RIV/TVI or cause any audible noise.
  • At about 25 kV rms/cm, negative polarity “brush” discharges occur. So named because the appearance is similar to the round ends of a bottle brush. The audible noise associated with brush corona is generally a continuous background type of hissing or frying noise.
  • At a gradient of around 30 kVrms/cm positive polarity plume corona is generated; so named because of its general resemblance to a plume. When viewed in the dark it has a concentrated stem that branches and merges into a violet-colored, tree-like halo. The audible noise associated with plume corona is a rather intense snapping and hissing sound. Plume corona generates significant RIV/TVI.
  • These observations are based on fair weather conditions. Under wet conditions virtually all energized electrodes will be in corona of one form or another.
  • Many are under the impression that the dielectric strength of air is greater under dry conditions. That is not true. In fact, the dielectric strength of air increases with increased moisture up to the dew point when moisture begins to condense on the surface of insulators and other components of the line.

 Physical Parameters of Corona:

  • Corona is caused by the ionization of the media (air) surrounding the electrode (conductor)
  • Corona onset is a function of voltage
  • Corona onset is a function of relative air density
  • Corona onset is a function of relative humidity

1. Corona and the Electric Field

  • Corona is NOT solely a function of the Electric Field
  • Corona is a function of the electric field on the surface of the electrode (conductor)
  • Corona is also a function of the radius of curvature of the electrode (conductor)
  • Corona is also a function of the rate of decay of the electric field away from the electrode (conductor)
  • For the preceding reasons, selecting the conductor with the smallest electric field at its surface is not correct.

2. Corona and the Relative Air Density

  • Corona has an inverse relationship with air density
  • Standard line designs that perform well at sea level, may have significant corona issues if used on lines that are installed over mountainous areas

3. Corona and the Humidity

  • Corona has an inverse relationship with humidity at power frequencies
  • Fair weather corona is more prevalent in low humidity environments

 4. Corona is Dependent Surface Condition of the Conductors

  • Corona is enhanced by irregularities on the conductor surface
  • Irregularities include: dust, insects, burrs and scratches and water drops present on new conductors
  • Corona will generally be greater on new conductors and will decrease to a steady-state value over a period of approximately one year in-service
  • Corona is significantly increased in foul weather.

What’s The Fuss?

  • Corona from conductors and hardware may cause audible noise and radio noise
  • Audible noise from conductors may violate noise standards
  • Radio noise from conductors may interfere with communications or navigation
  • Corona loss may be significant when compared with resistive loss of conductors
  • Corona can cause possible damage to polymeric insulators

Methods to reduce Corona Discharge Effect:

  •  Corona can be avoided
    1.  By minimizing the voltage stress and electric field gradient.: This is accomplished by using utilizing good high voltage design practices, i.e., maximizing the distance between conductors that have large voltage differentials, using conductors with large radii, and avoiding parts that have sharp points or sharp edges.
    2. Surface Treatments: Corona inception voltage can sometimes be increased by using a surface treatment, such as a semiconductor layer, high voltage putty or corona dope.
    3. Homogenous Insulators: Use a good, homogeneous insulator. Void free solids, such as properly prepared silicone and epoxy potting materials work well.
    4. If you are limited to using air as your insulator, then you are left with geometry as the critical parameter. Finally, ensure that steps are taken to reduce or eliminate unwanted voltage transients, which can cause corona to start.
    5. Using Bundled Conductors: on our 345 kV lines, we have installed multiple conductors per phase. This is a common way of increasing the effective diameter of the conductor, which in turn results in less resistance, which in turn reduces losses.
    6. Elimination of sharp points: electric charges tend to form on sharp points; therefore when practicable we strive to eliminate sharp points on transmission line components.
    7. Using Corona rings: On certain new 345 kV structures, we are now installing corona rings. These rings have smooth round surfaces which are designed to distribute charge across a wider area, thereby reducing the electric field and the resulting corona discharges.
    8. Whether: Corona phenomena much worse in foul weather, high altitude
    9. New Conductor: New conductors can lead to poor corona performance for a while.
    10. By increasing the spacing between the conductors: Corona Discharge Effect can be reduced by increasing the clearance spacing between the phases of the transmission lines. However increase in the phase’s results in heavier metal supports. Cost and Space requirement increases.
    11. By increasing the diameter of the conductor: Diameter of the conductor can be increased to reduce the corona discharge effect. By using hollow conductors corona discharge effect can be improved.

Sources of Corona and Arcing in Polymer Insulators:

  •  Loose hardware
  • Contamination and surface tracking
  • Missing corona rings
  • Damaged or incorrectly installed corona ring
  • Damaged end fittings or end fitting seal
  • Exposed internal rod due to: Carbonized internal rod by internal discharges Split sheath due to weathering

 Electro Magnetic Inductions:

  •  EM1 field or radio noise field from high-voltage transmission lines are caused by corona, which is essentially due to the electrical breakdown of the air surrounding the conductors at higher voltage.
  •  When the conductor surface electric field exceeds the corona onset electric field, a partial electrical breakdown occurs in the surrounding air medium near the conductor surface and is called the corona discharge. The increase of conductor surface gradient takes place with increase of supply voltage. In addition, organic contamination or attachment -of water droplets also may contribute to localized field enhancement.
  • When organic particles or water droplets are attached to the conductor surface, the charge accumulation at that point increases which enhances the local electric field. The intensification of surface gradient locally leads to the corona discharge.
  • The streamer generated during corona discharge, transports electric charge into the surrounding air during the discharge cycle. These moving charges contribute directly to the noise fields. ‘They also cause currents to be induced on the transmission line conductors. Since the charge is moved by a time varying electric field, it is equivalent to a current pulse and this When a communication line passes near the corridor of a HV or EHV transmission line, if the frequency of the radiated EM signal due to corona matches with that of the transmitted signal on the communication line, then the communication signal may get distorted. To mitigate this effect, the communication line should pass at a safe distance away from the transmission line.
  • Hence there is a need to estimate the radiated EM1 signal in dB at a given distance from the HV or EHV transmission line. In this paper, radiated EM1 in dB is computed for a single conductor high voltage over headline. This theoretical result is compared with the published experimental results available in the literature. In the computational work, earth is considered as an infinitely conducting ground.

 Physical description of corona and Electro Magnetic Induction:

  •  When alternating supply voltage energizes the conductor, the conductor surface electric field exceeds the corona on set electric field of the conductor. The corona discharge occurs in both positive and negative half cycle. So the corona is divided into positive and negative corona depending upon the polarity of the supply voltage.
  • When the conductor is positive with respect to ground, an electron avalanche moves rapidly into the conductor leaving the heavy positive-ion charge cloud close to conductor, which drifts away.
  • The rapid movements of electrons and motion of positive ions gives the steep front of the pulse, while the further drift of positive ions will give slow tail of the corona pulse.
  • When conductor is negative with respect to the ground, an electron avalanche moves away from the energized conductor and the positive heavy ions move towards the conductor. Since the heavy positive ions are, moving towards the higher electric field, their motion is very rapid which gives rise to a much sharper pulse than the positive pulse. Due to rapid moment of the electrons from the conductor surface, the electric field regains its original value at conductor surface very quickly than in the case of positive polarity. Thus the negative corona pulses are lower in amplitude and lower in rise and fall times as compared to positive corona pulses. They have also higher repetition rates than the positive pulse

 Corona Detection:

  •  Light Ultraviolet radiation: Corona can be visible in the form of light, typically a purple glow, as corona generally consists of micro arcs. Darkening the environment can help to visualize the corona.
  • Sound (hissing, or cracking as caused by explosive gas expansions):  You can often hear corona hissing or cracking Sound.
  • In addition, you can sometimes smell the presence of ozone that was produced by the corona.
  • Salts, sometimes seen as white powder deposits on Conductor.
  • Mechanical erosion of surfaces by ion bombardment
  •  Heat (although generally very little, and primarily in the insulator)
  • Carbon deposits, thereby creating a path for severe arcing
  • The corona discharges in insulation systems result in voltage transients. These pulses are superimposed on the applied voltage and may be detected, which is precisely what corona detection equipment looks for. In its most basic form, the following diagram is a corona (or partial discharge) measuring system:
  • It is important that the voltage source and the coupling capacitor exhibit low noise so as not to obscure the corona. In its simplest form the pulse detection network is a resistor monitored by an oscilloscope. Don’t dismiss this simple technique as crude, as we once used this method to observe the presence of corona in an improperly terminated high voltage connector, even after a dedicated corona tester failed to find any. Commercially available corona detectors include electronic types (as above) as well as ultrasonic types.

 Corona Calculations

  •  The following corona calculations are from Dielectric Phenomena in High Voltage Engineering

1.     For Concentric Cylinders in Air:

  • Corona will not form when RO / RI < 2.718. (Arcing will occur instead when the voltage is too high.)

2.     For Parallel Wires in Air:

  • Corona will not form when X / r < 5.85. (Arcing will occur instead when the voltage is too high.)

3.     For Equal Spheres in Air:

  • Corona will not form when X / R < 2.04. (Arcing will occur instead when the voltage is too high.)
  • Arcing difficult to avoid when X / R < 8

Where

  • RO = Radius of outer concentric sphere
  • RI = Radius of inner concentric sphere
  • R = Sphere radius
  • r = wire radius
  • X = Distance between wires or between spheres

 Effects of Corona:

 (1) Audible Noise

  •  During corona activity, transmission lines (primarily those rated at 345 kV and above) can generate a small amount of sound energy. This audible noise can increase during foul weather conditions. Water drops may collect on the surface of the conductors and increase corona activity so that a crackling or humming sound may be heard near a transmission line. Transmission line audible noise is measured in decibels using a special weighting scale, the “A” scale that responds to different sound characteristics similar to the response of the human ear. Audible noise levels on typical 230 kV lines are very low and are usually not noticeable. For example, the calculated rainy weather audible noise for a 230 kV transmission line at the right-of-way edge is about 25 dBA, which is less than ambient levels in a library and much less than background noise for wind and rain.

 (2)Radios and Television Interference:

  •  Overhead transmission lines do not, as a general rule, interfere with radio or TV reception.
  • There are two potential sources for interference: corona and gap discharges. As described above, corona discharges can sometimes generate unwanted electrical signals.
  • Corona-generated electrical noise decreases with distance from a transmission line and also decreases with higher frequencies (when it is a problem, it is usually for AM radio and not the higher frequencies associated with TV signals).
  • Corona interference to radio and television reception is usually not a design problem for transmission lines rated at 230 kV and lower. Calculated radio and TV interference levels in fair weather and in rain are extremely low at the edge of the right-of-way for a 230 kV transmission line.
  • Gap discharges are different from corona. Gap discharges can develop on power lines at any voltage. They can take place at tiny electrical separations (gaps) that can develop between mechanically connected metal parts. A small electric spark discharges across the gap and can create unwanted electrical noise. The severity of gap discharge interference depends on the strength and quality of the transmitted radio or TV signal, the quality of the radio or TV set and antenna system, and the distance between the receiver and power line. (The large majority of interference complaints are found to be attributable to sources other than power lines: poor signal quality, poor antenna, door bells, and appliances such as heating pads, sewing machines, freezers, ignition systems, aquarium thermostats, fluorescent lights, etc.).
  • Gap discharges can occur on broken or poorly fitting line hardware, such as insulators, clamps, or brackets. In addition, tiny electrical arcs can develop on the surface of dirty or contaminated insulators, but this interference source is less significant than gap discharge.
  • Hardware is designed to be problem-free, but corrosion, wind motion, gunshot damage, and insufficient maintenance contribute to gap formation. Generally, interference due to gap discharges is less frequent for high-voltage transmission lines than lower-voltage lines. The reasons that transmission lines have fewer problems include: predominate use of steel structures, fewer structures, greater mechanical load on hardware, and different design and maintenance standards.
  • Gap discharge interference can be avoided or minimized by proper design of the transmission line hardware parts, use of electrical bonding where necessary, and by careful tightening of fastenings during construction. Individual sources of gap discharge noise can be readily located and corrected. Arcing on contaminated insulators can be prevented by increasing the insulation in high contamination areas and with periodic washing of insulator strings.

 (3) Gaseous Effluents

  •  Corona activity in the air can produce very tiny amounts of gaseous effluents: ozone and NOX. Ozone is a naturally occurring part of the air, with typical rural ambient levels ranging from about 10 to 30 parts per billion (ppb) at night and peaks at approximately 100 ppb. In urban areas, concentrations exceeding 100 ppb are common. After a thunderstorm, the air may contain 50 to 150 ppb of ozone, and levels of several hundred ppb have been recorded in large cities and in commercial airliners.
  • Ozone is also given off by welding equipment, copy machines, air fresheners, and many household appliances. The National Ambient Air Quality Standard for Oxidants (ozone is usually 90 to 95 percent of the oxidants in the air) is 120 ppb, not to be exceeded as a peak concentration on more than one day a year.
  •  In general, the most sensitive ozone measurement instrumentation can measure about 1 ppb. Typical calculated maximum concentrations of ozone at ground level for 230 kV transmission lines during heavy rain are far below levels that the most sensitive instruments can measure and thousands of times less than ambient levels. Therefore, the proposed transmission lines would not create any significant adverse effects in the ambient air quality of the project area.

 (4) Induced Currents

  •  Small electric currents can be induced by electric fields in metallic objects close to transmission lines. Metallic roofs, vehicles, vineyard trellises, and fences are examples of objects that can develop a small electric charge in proximity to high voltage transmission lines. Object characteristics, degree of grounding, and electric field strength affect the amount of induced charge.
  •  An electric current can flow when an object has an induced charge and a path to ground is presented. The amount of current flow is determined by the impedance of the object to ground and the voltage induced between the object and ground.
  • The amount of induced current that can flow is important to evaluate because of the potential for nuisance shocks to people and the possibility of other effects such as fuel ignition.
  • The amount of induced current can be used to evaluate the potential for harmful or other effects. As an example, when an average woman or man grips an energized conductor, the threshold for perception of an electric current is 0.73 milli ampere (mA) and 1.1 mA, respectively. If the current is gradually increased beyond a person’s perception threshold, it becomes bothersome and possibly startling.
  • However, before the current flows in a shock situation, contact must be made, and in the process of establishing contact a small arc occurs. This causes a withdrawal reaction that, in some cases, may be a hazard if the involuntary nature of the reaction causes a fall or other accident.
  • The proposed 230 kV transmission lines will have the highest electric field within the right-of-way, approximately 0.2 to 1.5 kV per meter (kV/m), and approximately 0.1 to 0.9 kV/m at the edge of the right-of-way. These fields are less than many other 230 kV transmission lines due to the use of cross-phasing on the double-circuit lines and higher clearance above ground. Induced currents have been calculated for common objects for a set of worst-case theoretical assumptions: the object is perfectly insulated from ground, located in the highest field, and touched by a perfectly grounded person. Even though the maximum electric field only occurs on a small portion of the right-of-way, and perfect insulation and grounding states are not always common, the calculated induced current values are very low therefore, in most situations, even in the highest field location, induced currents are below the threshold of perception and are far below hazardous levels.
  • Agricultural operations can occur on or near a transmission line right-of-way. Irrigation systems often incorporate long runs of metallic pipes that can be subject to magnetic field induction when located parallel and close to transmission lines. Because the irrigation pipes contact moist soil, electric field induction is generally negligible, but annoying currents could still be experienced from magnetic field coupling to the pipe. Pipe runs laid at right angles to the transmission line will minimize magnetically induced currents, although such a layout may not always be feasible. If there are induction problems, they can be mitigated by grounding and/or insulating the pipe runs. Operation of irrigation systems beneath transmission lines presents another safety concern. If the system uses a high-pressure nozzle to project a stream of water, the water may make contact with the energized transmission line conductor. Generally, the water stream consists of solid and broken portions. If the solid stream contacts an energized conductor, an electric current could flow down the water stream to someone contacting the high-pressure nozzle. Transmission line contact by the broken-up part of the water stream is unlikely to present any hazard.

 (5)  Fuel Ignition

  •  If a vehicle were to be refueled under a high-voltage transmission line, a possible safety concern could be the potential for accidental fuel ignition. The source of fuel ignition could be a spark discharge into fuel vapors collected in the filling tube near the top of the gas tank.
  • The spark discharge would be due to current induced in a vehicle (insulated from ground) by the electric field of the transmission line and discharged to ground through a metallic refueling container held by a well-grounded person. Theoretical calculations show that if a number of unlikely conditions exist simultaneously, a spark could release enough energy to ignite gasoline vapors. This could not occur if a vehicle were simply driven or parked under a transmission line. Rather, several specific conditions would need to be satisfied: A large gasoline-powered vehicle would have to be parked in an electric field of about 5 kV/m or greater. A person would have to be refueling the vehicle while standing on damp earth and while the vehicle is on dry asphalt or gravel. The fuel vapors and air would have to mix in an optimum proportion. Finally, the pouring spout must be metallic. The chances of having all the conditions necessary for fuel ignition present at the same time are extremely small.
  • Very large vehicles (necessary to collect larger amounts of electric charge) are often diesel-powered, and diesel fuel is less volatile and more difficult to ignite. The proposed 230 kV transmission line electric field levels are too low (about 0.2-1.5 kV/m on the right-of-way) for the minimum energy necessary for fuel ignition under any practical circumstances.

 (6) Cardiac Pacemakers

  •  One area of concern related to the electric and magnetic fields of transmission lines has been the possibility of interference with cardiac pacemakers. There are two general types of pacemakers: asynchronous and synchronous. The asynchronous pacemaker pulses at a predetermined rate. It is practically immune to interference because it has no sensing circuitry and is not exceptionally complex. The synchronous pacemaker, on the other hand, pulses only when its sensing circuitry determines that pacing is necessary.
  • Interference resulting from the transmission line electric or magnetic field can cause a spurious signal in the pacemaker’s sensing circuitry. However, when these pacemakers detect a spurious signal, such as a 60 hertz (Hz) signal, they are programmed to revert to an asynchronous or fixed pacing mode of operation and return to synchronous operation within a specified time after the signal is no longer detected. The potential for pacer interference depends on the manufacturer, model, and implantation method, among other factors.
  • Studies have determined thresholds for interference of the most sensitive units to be about 2,000 to 12,000 milli gauss (mG) for magnetic fields and about 1.5 to 2.0 kV/m for electric fields. The electric and magnetic fields at the right-of-way edge are below these values, and on the right-of-way, only the lower bound electric field value of 1.5 kV/m is reached. Therefore, the potential impact would not be significant.

 (7) Computer Interference

  •  Personal computer monitors can be susceptible to 60 Hz magnetic field interference. Magnetic field interference results in disturbances to the image displayed on the monitor, often described as screen distortion, “jitter,” or other visual defects. In most cases it is annoying, and at its worst, it can prevent use of the monitor. Magnetic fields occur in the normal operation of the electric power system.
  • This type of interference is a recognized problem by the video monitor industry. As a result, there are manufacturers who specialize in monitor interference solutions and shielding equipment. Possible solutions to this problem include: relocation of the monitor, use of magnetic shield enclosures, software programs, and replacement of cathode ray tube monitors with liquid crystal displays that are not susceptible to 60 Hz magnetic field interference. Because these solutions are widely available to computer users, potential impacts would be less than  significant

CORONA RING:                                                     

  • The ring, which surrounds the energized end of the transformer bushing, serves two functions.
  • It is a corona ring that is intended to electrically shield the bushing terminal and connections. It does so by reducing the voltage gradient to a level well below the ionizing gradient of the surrounding air at the maximum transformer output voltage.
  • It’s also a grading ring, which helps electrically grade the external voltage on the bushing from line to ground (at the bushing flange). The bushing is likely a condenser bushing, which contains a capacitance-graded core to grade the voltage radically from 100% at the central conductor to ground at the flange and, axially from ground to the top and bottom ends of the core.
  • Grounding the test transformer following a circuit breaker test is necessary for safety but you are grounding the entire test circuit; not just the corona ring. I suspect the corona ring just happens to be a convenient attachment point for the hook on your ground stick.
  • Die cast are usually 380, sand and permanent mold 356 or A356, and fabricated rings are usually made from 6061 thin wall tubing or pipe that is formed and welded; with appropriate brackets and other mounting provisions.
  • Corona grading ring should be designed to reduce the critical dielectric voltage gradient (typ. 20 to 30 kVrms/cm) to prevent corona effect, internal discharge and reduce E-field in live parts and fitting that cause radio/ TV interference (RIV), audio noise and losses. Corona ring could also help to smooth the voltage profile distributing the voltage more uniform along the insulator preventing concentration of over stresses.
  • For porcelain post insulators, some manufacturer recommends one corona ring and for 500 kV and above two rings. However, for composite insulator the corona ring is recommended for 220/230 kV. Most equipment manufacturer provide corona ring base on testing such surge arrester, switches, CT’s/PTs, etc.

Difference between Arcing Horn Gap and Corona Ring:

  • At transmission line voltage the arcing horns, when the breaker is closed normally have nothing except corona from the tips and arc marks, the instant the breaker begins to open an arc is established across the gap between the arc horns, when the gap is long enough the arc breaks. The plan is to keep the sliding contacts from getting arc metal removal so the contacts maintain low resistance, arcing horns are sacrificial.
  • At switchgear voltage, there are arc chutes and usually puffers to extinguish the arc during breaker opening, the arc chutes may be of a sand-crystal cast material (like space shuttle heat tiles), asbestos layers, and electrical insulating board to protect the works during an explosive event when temperatures get hotter than the sun. There is specific NFPA training for arc flash exposure.
  • Arcing horns are also commonly used to protect insulation from impulse and other overvoltages. The horn gap (distance between arcing horns) is set to ensure that flashover occurs across the gap rather than along the insulation surface thereby protecting the insulation surface and preventing arc termination and associated damage to the end terminals or line and ground end hardware. They may also be used to connect a surge arrester to protect transformers and other equipment from overvoltage surges (gapped arrester). A gapped connection is one method of preventing line lockout in the event of arrester failure
  • Corona rings are meant to distribute the electrical field and neither the hardware protected or the corona ring should have corona, the typical line voltage that corona rings are applied is 150KV and higher, altitude or high temperatures can reduce the voltage to 138KV lines. Properly designed corona rings do not have corona.
  • Corona can appear to start and stop at essentially the same voltage, there are other variables. Corona produces light (from UV thru visible and into the infrared), sound (thru all wavelengths), ozone, and nitric acid (in the presence of moisture).
  • Arcing arrestors were used long ago, some of the old-old transmission lines. They were opposing arcing fingers mounted in parallel with the insulators; the gap determined the flash-over voltage. The intent was to protect insulators from lightening surges. I don’t know if those old lines are energized anymore. You don’t see arcing fingers on modern (post WWII war) transmission lines.
  • To break an arc the voltage must be decreased below about 60% of the voltage an arc starts at, thus if a transmission line insulator arcing arrestor flashes over and maintains an arc the line is going to be shutdown. Thus arcing arrestors (without an arc extinguishing capability) decrease the reliability of a transmission line.

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