elect. auth

Nuc lear Tra in ing Cent re

RAJASTHAN ATOMIC POWER STATION

Electrical Authorisation Training Manual

[Revision (0), July 1998]

COURSE NO. E-14

Prepared by :J.K.Goyal S.P. JoshiSr. Trg. Officer SO/E

Checked by :P.K. Dutta T.S. MarwahaSME(E) Head (F&IS)

Reviewed by :O.P. Goyal J.P. MoolaniMaint. Supdt. Trg. Supdt.

Approved by : K.P. OjhaStation Director

Electrical Authorisation

Contents

Chapter-1 : Electrical Safety

1.1 Introduction ................................................................................ 1

1.2 Current & Voltage ...................................................................... 1

1.3 Aiding Electrical shock victim .................................................. 3

1.4 Grounding ................................................................................... 3

1.5 Manually Operated Switches ................................................... 5

1.6 Transmission Lines ................................................................... 6

1.7 Inspection & Tests ..................................................................... 6

Chapter-2 : RAPS Electrical Output System

2.0 Introduction ................................................................................ 7

2.1 220 KV System .......................................................................... 7

2.2 220 KV ABCB ........................................................................... 10

2.3 Principle of Air blast circuit breaker operation .................... 10

2.4 ABCB Operation ...................................................................... 10


2.5 Pneumatic Pressure Switches ............................................... 12

2.6 ABCB Compressed Air System ............................................. 13

2.7 220 KV Switchyard Equipments ............................................ 15

2.8 Dedicated supply from RPS ................................................... 16

2.9 Usual OTOs carried out on 220 KV system ......................... 17

2.10 220 KV Breaker Protection .................................................... 18

2.11 Islanding ................................................................................... 19

2.12 Routines for Switchyard Compressed Air System .............. 19

2.13 Alarm & Indication ................................................................... 19

2.14 Pre-requisites for ABCB Operations ..................................... 19

2.15 Precautions & Hazards on ABCB .......................................... 20

Chapter-3 : 3.3 KV & 415V Circuit Breakers & SwitchgearPart-A : General Description

3.1 Introduction .............................................................................. 21

3.2 Types of Circuit Breakers ....................................................... 21

3.3 Switch Functions ..................................................................... 22

3.4 Circuit Breaker Operation : Protective Functions ............... 23

3.5 Common Circuit Problems ...................................................... 23

3.6 Thermal Element Trip Devices .............................................. 24

3.7 Electromagnetic Trip Devices ................................................ 25

3.8 Ground Protection ................................................................... 27

3.9 Breaker Ratings & System Co-ordination ............................ 28

3.10 Principles of Circuit Interruption ............................................ 29


3.11 Effect of DC on a Breaker's rating ........................................ 31

3.12 Switchgear ................................................................................ 31

3.13 Disconnecting a Breaker from Primary Power & ControlPower ........................................................................................ 33

Chapter-4 : 3.3 KV, 415V & LV SystemPart-B : System Description

4.1 3.3 KV System ......................................................................... 35

4.2 Auto Transfer Scheme (ATS) ................................................. 35

4.3 Rack in/Rack out operations .................................................. 40

4.4 415V and LV System .............................................................. 41

4.5 Breaker Operation ................................................................... 44

4.6 Protections ............................................................................... 45

4.7 Components of MCC ............................................................... 46

4.8 Types of MCC .......................................................................... 47

4.9 Isolation of MCC Cell .............................................................. 48

Chapter-5 : Preventive Maintenance of Breakers

5.0 Preventive Maintenance ......................................................... 49

5.1 Typical Rack-out Procedure for a Power breaker ............... 50

5.2 Inspection and Motor Cleaning .............................................. 51

5.3 Returning a Breaker to Service ............................................. 52

Chapter-6 : Overhauling of Breakers

6.0 Circuit Breaker Overhaul ........................................................ 49

6.1 Cleaning a Breaker with Compressed Air and Solvent ...... 50

6.2 Arc Chutes ................................................................................ 51


6.3 Operating Mechanism and other components .................... 55

6.4 Contacts .................................................................................... 55

Chapter-7 : Indian Electricity Rules

7.1 Extract of applicable Indian Electricity Rules as per IndianElectricity Act ........................................................................... 57

Chapter-8 : Glossary

Glossary .................................................................................... 59

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Electrical Safety Electrical Authorisation 1

1

ELECTRICAL SAFETY

1.1 INTRODUCTIONSafety is the most important aspect of any electrical job. Every one whoworks on or around electrical equipment must know what the dangersare and what steps can be taken to make sure that all personnel areprotected from these dangers.

1.2 CURRENT & VOLTAGEWhenever a person comes in contact with an energized conductor, thecurrent will pass through the person’s body. In general, the electricalcurrent will take the shortest possible path through the body. Forexample, if a person is touching energized conductors with both hands,or if he is touching an energized conductor with one hand and agrounded object with the other, the current will pass directly through theperson’s heart. However if only one hand is touching an energizedconductor, the current travels to ground along the path of leastresistance. In this case, the current moves up the arm, down the body,and out through the nearest leg. The current tends to stay on one side ofthe body, so it is less likely to pass through the heart.

The body is a conductor, which means that current will pass through itwhen given the chance. The severity of electrical shock depends on howmuch current passes through the body as well as the path that it takes.

Although the human body is a conductor, it does offer resistance to thepassage of current through it. Different people have different amount ofresistance, and different parts of the same body have differentresistance values.

The amount of resistance that an individual body offers to the flow ofcurrent can vary from one minute to the next. Resistance is affected by

Electrical Authorisation 2Electrical Safety

factors such as individual physiology, a persons emotional state, andthe moisture that may or may not be on a person’s skin.

The amount of current flow passing through a body depends on threefactors.

(1) The voltage of the source

(2) Body resistance along the current path and the

(3) Current capacity of the source.

The current level is greatest when voltage is high and resistance is low.The max. current value is limited by the third factor. The current capacityof the source. Touching an energized spark wire having voltage of30000 V will result in a shock but not a dangerous one because thecurrent capacity is not there. Whereas touching a 30V battery with ahigh current capacity can result in death if body resistance is lowenough. In general any current source at 30V or more must beconsidered dangerous.

The effect of various levels of current on the human body are asfollows :

Current value Effects

1. Less than 1mA No sensation

2. 1 to 20 mA Sensation of shock, possibly painful.May lose some muscular controlbetween 10 and 20 mA.

3. 20 to 50 mA Painful shock, severe muscularcontractions, breathing difficulties.

4. 50 to 200mA Same symptoms as above only moresevere, upto 100 mA. Between 100 &200 mA, ventricular fibrillation mayoccur- typically resulting in almostimmediate death unless specialmedical equipment and treatment isavailable.

5. Over 200 mA Severe burns and muscularcontractions. The chest musclescontract and stop the heart for theduration of shock.


From the above it is clear that 1 mA or less is not even noticeable to mostpeople. This very low level does not even cause a shock. Shock cangenerally be felt from current in the range of 1 to 20 mA. There may besome loss of muscular control, so that the victim may not be able to letgo off the source. A current of 20-50 mA will cause a painful shock, withmuscle contractions and breathing difficulties. As current continues toincrease, so do the pain and contractions. At approximately 100 mA, theheart goes into ventricular fibrillation can uncoordinated libration of theheart muscle and death will occur unless prompt medical treatment isgiven. Above 200 mA, flesh begins to burn, and muscles contractionsare severe that the chest muscles squeeze the heart, stopping it for theduration of the shock. These faults are more frightening when note istaken that the highest current value given is only one fifth of an amp.

Sometimes people survive a shock by sheer luck and sometimes theydon’t.

1.3 AIDING ELECTRICAL SHOCK VICTIMThe first thing to do is to cut the Electrical Power, if possible. If the powercannot be cut, an insulated pole or rope should be used to move theconductor away from the victim. The victim should not be touched whilehe is in contact with the conductor, otherwise the person touching couldalso become a shock victim. If the victim has stopped breathing or if hisheart has stopped breathing, artificial respiration or cardio-pulmonaryresuscitation (CPR) will be necessary to revive him.

1.3.1 It is the Current that KillsOff hand it would seem that a shock of 10,000 V would be more deadlythan 100V but this is not so. The real measure of shock intensity in theamount of current (amperes) forced through the body and not thevoltage.

The resistance of the victim’s contact decreases with time. The total 100to 200 mA level may be reached if action is delayed.

1.4 GROUNDINGGrounding practices have one paramount aim. It is to prevent anypotential difference from being applied across any part of a human body.The ground connections constitute a system from the stand point ofsafety of personnel and satisfactory operation of the internal andexternal power system.


The general requirements of grounding are as follows :

(i) The use of a conductor of sufficient size to withstand the heatgenerated during the most severe operating or fault conditions.

(ii) Corrosion must be prevented. It occurs principally at the junctionof dissimilar metals. It may be checked by making all joints ofsimilar metals and by periodically painting the joints.

(iii) All joints & connections in the grounding system must remaintight.

(iv) The resistance of the grounding system components must be aslow as possible.

The resistance of the complete ground connection is never quite zeroand large currents passing through this resistance may cause apotential difference (I x R) between grounded apparatus and earthcreating a hazard.

1.4.1 Purpose of Safety GroundingGrounding achieves safety by providing a low resistance path betweenthe appliance frame & the neutral of the supply system, which isgrounded. This means provide two distinct safety measures.

1. It provides a low resistance path from the normally non-currentcarrying parts of the device to the ground. This assures that anyelectrical failure that results in energizing the frame and hencethe handle or metal parts grasped by the hand, results in instantlyopening the circuit over current protective devices such as thefuses or circuit breakers.

2. Connecting the frame of the device to ground assures thatregardless of what kind of defect occurs, the voltage of the framewill be held sufficiently low so as not to become hazardous. Theground wire does not affect the normal operation of the deviceand its sole purpose is to provide safety in case a fault occurs.

Routine inspection of ground wires and connection to ground plates ismost essential so that grounding remains fully effective.

After every maintenance of any equipment, grounding connectionsmust be inspected to ensure that they are intact before returningequipment to service.


Proper grounding keeps electrical shock hazards to the absoluteminimum. People who work near high voltage apparatus canunconsciously fall into the habit of thinking that only the high voltagecircuits are really dangerous.

Shock from voltages well below 50 volts may produce the followingresults :

(a) Surprise a man so he may lose his balance and fall, with risk ofbodily injury.

(b) Cause an involuntary recoil so that he stumbles against movingmachinery or makes contact with live apparatus.

(c) Cause a muscular paralysis so that he cannot release his grip onthe apparatus causing the shock.

For the above reasons all possible steps are taken to eliminate even thesmallest shock hazards.

Metallic parts can become charged by electrostatic andelectromagnetic induction. Transformer tanks, breakers, motor framesand switching structures can also become charged by failure ofelectrical insulation, insulator flashover, arc caused by lightning orswitching surges.

Large power transformers, when isolated, are capable of retaining acharge. If work is to be done on isolated transformers it is first necessaryto ground the primary and the secondary windings. Other apparatusthat must be grounded for work, even though isolated are lightningarrestors, oil breaker bushings and bus sections.

1.4.2 CautionWhen applying portable grounds to isolated apparatus the connectionto the ground tap must be made first and broken last. The grounds mustremain applied throughout the duration of the work.

1.5 MANUALLY-OPERATED SWITCHESManually operated switches present a special hazard. There is thedanger that the arcing incidental to the switching will initiate an insulatorflashover. In some locations a switch may be particularly exposed tolightning or switching surges which can also cause insulator flashover.A flashover causes a very heavy fault current, a large part of which islikely to take the most direct path to earth down the steel operating rodto which the operating handle is attached.


Some switching locations are remote from the station. At such locationsa permanently installed ground gradient control mat may not be found.A portable grounding mat may be used to serve the same purpose.Ground rods are used at remote switching locations so that in case ofcurrent flow in the operating rod of the switch, the potential differencesat the earth’s surface tend to be larger than at the station where agrounding system exists.

1.6 TRANSMISSION LINESEven though a circuit may be physically isolated from all sources ofpower it is possible for the circuit to remain charged. A circuit canbecome charged by any of the following :

1. Electrostatic or electromagnetic induction from energizedcircuits paralleling or crossing over it.

2. Wind or fog blowing across the line.

3. An electrical storm in the vicinity of the line, a form of electrostaticinduction.

4. Induction from the earth’s magnetism.

Transmission lines have permanently installed grounding switcheswhich may be closed when the circuit is isolated. However, thesegrounds make the circuit safe to touch for only a short distance from theline terminals. This may be only a few hundred meters if a nearby line isheavily loaded.

It is necessary, for work on the circuit, to apply portable ground at bothsides of the point of work.

1.7 INSPECTION AND TESTSIt is important that the condition of the grounding at each installation beknown and the ground resistance values obtained at the time ofinstallation be maintained. This requires the keeping of records and thecarrying out of tests at intervals after installation.

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Electrical Output System Electrical Authorisation 7

2

RAPS ELECTRICAL OUTPUT SYSTEM

2.0 INTRODUCTIONInput/Output System :In RAPS power is generated at 21 KV and delivered at 220 KV to grid via 21/220 KV step up transformer (Main Transformer or Generator Transformer).

For station requirement power is taken from Two sources viz one from gridvia 220 KV/3.3 KV Transformer SSST (SUT) and other from our ownGenerator i.e. via 21 KV/3.3 KV Transformer (USST OR UT).

The power received at 3.3. KV is used at various potential levels i.e. 3.3. KVitself, 415V, 250V DC etc.

Thus, complete electrical system (input & output) of the station can becategorised as (broadly) :

i. 220 KV system

ii. 3.3 KV system

iii. 415V system

iv. 250V DC system

This manual deals with all the above systems.

2.1 220 KV SYSTEMIn 220 KV system following are the major equipments:

i. Circuit Breakers (CBs)

ii. Disconnect Switches (DS)

iii. Current Transformers (CTs)

iv. Potential Transformers (PTs)

v. Lightning Arresters (LA)

Electrical Authorisation 8Electrical Output System

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vi. Wave traps (WT)

vii. Capacitor Voltage transformers (CVTs)

viii. Power Transformers

All equipments are located in 220 KV outdoor switchyard. Main transformerand unit transformers are located outside turbine building.

220 KV System Layout is shown in fig. 2.1.

Buses A1 & A2 are called as main buses for unit #1&2 respectively whereas bus B1 & B2 are transfer buses for Unit 1&2 respectively. Buses A1 & A2are interconnected by tie breaker 513-CB-6 where as B1 & B2 by 513-CB-13.

Power is delivered from Gen.1 to main bus A1 via 513-CB-1. Whereas fromGen.-2 to main bus A2 via 513-CB-11.

Power transmitted to grid is via four feeders i.e. Kota-1, Kota-2, Kota-3 &Udaipur through CB-8, CB-7, CB-5 and CB-4 respectively. 513-CB-12 isused to feed plant load to Heavy Water Plant situated nearby.

For RAPP-3&4 Kota-2 line from RAPS end has been connected to RAPP-3&4 220 KV switchyard where as original Kota-2 line is being used as Kotafeeder by RAPP-3&4.

All the breakers in 220 KV Switch yard (Except Transfer Bus Tie CB-13 &circuit breakers in 50 MVA Transformers Ckt.) are ABCB (Air blast circuitbreakers). The CB-13 & circuit breakers in 50 MVA transformer are MOCB(Minimum Oil Circuit Breaker) type. Station power for RAPS-1 is taken fromSSST #1 (which is connected on RAPS-2 side 220 KV switch yard) via CB-9 where as SSST #2 gets supply via CB-3 for RAPS-2 (RAPS-1 side) CB-3 & CB-9 are called as transfer breakers. Supply to transfer bus B1 & B2 isvia CB-3 & CB-9 from main bus A1 & A2 respectively. The transfer bus B1can be connected to Heavy Water line, Main Gen. #1 SSST#2, Kota-3 line& Udaipur line Via Motorised DS MDS-35, MDS-3, MDS-4, MDS-15 & MDS-12 respectively. The interlock in motorised disconnect switch is that onlyone of these MDS can be closed at a time. In other words we can say thatCB-3 (transfer breaker) can serve any one of five above at a time. NormallyCB-3 supplies SSST#2. When maintenance on either of CB-12, CB-1, CB-4 & CB-5 is to be done CB-3 can replace any one of these.

In the similar fashion CB-9 can serve any one of Kota-1, Kota-2 line, Maingenerator-2 SSST #1 Via MDS-21, MDS-18, MDS-27 and MDS-30respectively, normally CB-9 supplies SSST#1.

When CB-3 and CB-9 are not serving for their own SSST, SSST protectionis taken care by bus protection.


A dedicated supply from 132 KV RPS/GS Hydel Power Station is connectedto transfer bus B2 via 132/220 KV 50 MVA transformer.

2.2 220 KV ABCB :The 220 KV ABCBs in our station are “Hindustan Brown Boveri” (nowrenamed as Asia Brown Boveri) make. Air blast, type DCF 24S qw 1250A.

Maximum interrupting capacity of breaker is 5000 MVA.

Before going in further detail, let us look in brief the principle of a circuitbreaker operation.

2.3 PRINCIPLE OF AIR BLAST CIRCUITBREAKER OPERATION.When two current carrying contacts are separated out, an arc is formed, asthe medium between them gets ionised. To interrupt the circuit completely;the arc has to be extinguished. This is done by blowing the Medium (air, oiletc.) through arc so as arc is lengthened & finally extinguished. The ABCBsare spring to close and air to open type breakers.

In ABCB pressurised air (at 15 kg/cm2) is used to separate out the contactsas well as it is blown through arc to extinguish it.

In ABCB three pairs of contacts per phase are there to open the circuit.

Three nos. of air storage tank (one per phase) are provided in which air isstored at 15 kg/cm2.

Working Air pressure in ABCB is 15 kg/cm2.

To open the contacts only 3-4 kg/cm2 pressure is enough but to quench thearc air blast at 15 kg/cm2 is required.

The air in the receiver is sufficient for two consecutive opening/closingoperations.

Input air supply to ABCB air receiver is through a separate “switch yardcompressed air system”.

2.4 ABCB OPERATION :Three methods of ABCB operation are there

(i) Remote electrical,

(ii) Local electrical &

(iii) Local pneumatic.


ABCB Control cabinet in field (near CB) has following HS :

(i) Normal/Test selection HS for Remote (C/R) & local operationrespectively.

(ii) Close/Trip HS for local electrical operation.

(iii) Pneumatic close and trip push button.

(iv) Normal/Bypass toggle switch for Generator Brk CB-1 & CB-11 only.

2.4.1 Remote electrical operation (C/Roperation)For this operation, Normal/Test HS should be in Normal position. Breakercan be closed or opened from C/R by 4 position disagreement switch. Thisswitch has 4 positions viz., C, CC, TT & T.

If breaker is in tripped condition, to close it bring the HS in ‘CC’ position &then give a closing signal by bringing it to ‘C’ position. The breaker will close.

It is spring to return in CC position. Similarly if breaker is closed and it is tobe tripped, bring switch in TT position & give tripping signal by bringing it to‘T’ position, the breaker will trip. HS will spring return to TT position.

Disagreement : If Breaker is in close position and HS is in ‘CC’ steady lightwill glow indicating an agreement. Whereas for TT position a flashing light willglow, which will indicate a disagreement. Similarly if breaker is in trippedcondition & HS is in ‘TT’ position, steady light, and for ‘CC’ position flashinglight will glow. The steady light indicates agreement and flushing lightindicates disagreement.

When ABCB is selected for Normal operation i.e. from C/R following featuresare incorporated automatically in breaker.

(i) All protections which trip the breaker.

(ii) Close & trip ckt. supervision coils.

(iii) Control ckt. failure alarm.

(iv) Low air pressure alarm.

2.4.2 Local Electrical :“Test” position is normally required for ‘Breaker test close’ & “normalisation”OTO (order to operate). Normal/Test HS to be selected in ‘Test’ positions.Breaker will close or trip as per close / trip HS is selected in field.

When “Test” position is selected, “C/R four position CB HS” light (steady &flashing both) disappears. Hence in “Test” position, C/R has no informationsabout breaker position.

When “Test” position is selected following features are bypassed


automatically in breaker.

(i) All protections which trip the breaker.

(ii) Close & Trip circuit supervision coils

(iii) Control circuit failure alarm

(iv) Low air pressure alarm

2.4.3 Local Pneumatic :Two nos. of Hand push buttons are provided in each breaker cubicle in field.One push button is for “close” & other for “trip”.

If breaker is in already “closed position” it will trip if “Trip push button” ispressed. If brk. is in “trip” position, it will “close” if close push button ispressed. But in this case after “closing” breaker will trip again if (1) “Normal/Test” HS selection in “Normal” (2) Any one of breaker “trip” signal exists.

“Normal/Bypass” Toggle switch for Generator Breaker CB-1 & CB-11 “ Asone of the trip signals to Generator breaker is “Field Breaker Open” Hencethis toggle switch is provided to bypass this parameter. When generator istripped, and “generator brk” “test close” OTO to be carried out, this ‘HS’ isput in “Bypass”. So as when “Normal/Test” HS is put in “Normal” andgenerator breaker is closed from C/R it will not trip again on “field breakeropen” parameter.

2.5 PNEUMATIC PRESSURE SWITCHES :2.5.1 Block trip pressure switch (63da)

This switch opens at < 156 psig (11 kg/cm2) Blocks all tripping of the breaker.

2.5.2 Block close pressure switch (63db)This switch opens at < 176 psig (12.4 kg/cm2) and blocks all auto or manualclosing of breaker.

2.5.3 Block reclosing pressure switch (63 dc)Opens at < 213 psig (15 kg/cm2) and again

Closes at > 219 psig (15.4 kg/cm2)

All tripping transfers to 3 pole and blocks reclosing feature

2.5.4 Low pressure alarm switch (63 dx)Opens at <190 psig, This setting is below the value to (13.3 kg/cm2) whichthe pressure falls after one normal three pole tripping of the breaker, startingfrom normal pressure of 230 psig (16 kg/cm2).

This pressure switch causes annunciation in C/R to call operator’s attention.


2.5.5 Phase disagreement pressure switchIf breaker has one pole open with two close or two open with one close andremains in that position for more than a preset time, all poles arepneumatically tripped. Time setting is variable up to 2 seconds by means ofan air escapement screw.

This pressure switch is a pneumatic comparator.

2.6 ABCB COMPRESSED AIR SYSTEM :This is also known as “switchyard compressed air system”. The blockdiagram is shown in fig.2.2.

Description : System consist of 3 nos. of air compressors each rated 20 HP,3 stages. Pressures developed after each stage are 2.53 kg/cm2, 11.95 kg/cm2 & 30 kg/cm2 respectively. Compressed air after III stage is stored in 2banks of 5 receivers, (3 x 700 litres + 2 x 400 litres) at 30 kg/cm2.

Each bank of receivers supplies air to one separate line through a PRV setat 15 kg/cm2. These two lines of 15 kg/cm2 forms a network of pipelines &isolating valves to supply air at 15 kg/cm2 to each of 220 KV breaker air tanks(3 tanks per breaker)

A line joining two banks of air receivers is provided with 3 no. of pressureswitches which actuates Auto/Manual & lead/lag operation of compressors.

Each compressor HS is having 3 positions viz Hand/OFF/AutoCompressor#1 & 2 are having another HS for lead/lag operation.Compressor #3 alone having one more HS having positions 1 and 2.Operations of compressors are as follow.

(1) HS on “Hand” (All three compr.) compressor simply run. (NOPROCESS LOGIC).

(2) HS on “OFF” (All three compr.) compressor simply put off.

(3) HS on “Auto”.

(a) For compressor#1 & 2 if HS is on “Auto” compressor will cutin and cut out on Auto as per lead/lag selection.

For Lead Compressor Cut in at 26.5 kg/cm2

Cut out at 29.0 kg/cm2

For Lag (Standby) Compressor Cut in at 26 kg/cm2

Cut out at 29 kg/cm2

(b) For compressor#3 if HS is on “Auto” compressor will run asa standby for either compressor#1 or compressor#2 as perHS selection (position 1 or 2).


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2.7 220 KV SWITCHYARD EQUIPMENTS.2.7.1 Current Transformer & Potential

TransformerCTs are used to measure large currents and PTs are used to measure highvoltages in switchyard.

In current transformer, primary consists of one or few turns whereassecondary has large number of turns. Hence current in secondary isreduced compared to primary to such a value that it can be measured bynormal ammeters. The important thing in a CT to take care is that it’ssecondary is never left open circuited. Either an ammeter is connected insecondary or it is short circuited. This is so because open secondary maylead to high voltage shock. (Personal Safety) and second reason is that itmay also saturate the transformer core. (Equipment safety)

In potential transformer primary consists of large number of turns whereassecondary is having one or few turns. Here voltage induced in the secondaryis a fraction (equal to turns ratio) of primary voltage. Secondary of a PT isnever short circuited.

CTs are always put in series with the line where current is to be measuredand PTs are put in parallel.

2.7.2 CVT Capacitor Voltage transformer.It is cascaded capacitors (capacitors in series) mounted on eachtransmission line. For RAPS switchyard 4 nos. of CVTs are there. CVThelps in PLCC communication as well as potential measurement andprotection. Following are the functions of CVT.

(1) For getting line potential at the time of synchronization.

(2) Carrier communication 300 Hz to 2000 Hz.

(3) Inter tripping frequency at 1330 Hz, 1500 Hz, 1700 Hz.

(4) For line protection & metering.

(5) Telemetering

2.7.3 LA (Lightning Arresters).Function of an LA is to save switchyard equipments, GT and Main Generatorfrom Lightning surges (High Voltages). LA are mounted on each phase atentrance of (a) transmission lines in switchyard, (b) on SSST and (c) on GT.Principle of operation of LA is that at normal system voltage it is perfectinsulator to ground. But for surges (voltages > 1.5 times of system voltage220 KV) it is good conductor to ground.


When a lightning surge comes, LA becomes conductor and surge passes toground alongwith power cycle. When surge ceases, to next power cycleZero, LA becomes insulator. Thus a loss of 1 or 2 power cycles occurs.

2.7.4 Wave TrapThis is a coupled circuit tuned for a particular frequency (Power frequency).The function of wave trap is to block the carrier frequencies to enter intostation. This improves the efficiency of PLCC transmission through powerline, and also enables the same carrier frequency to be repeated in otherselected sections of power system. Wave trap is installed in between linebreaker & CVT.

2.7.5 Manual and Motorised disconnectswitches (DS or Isolators).On either side of ABCB, DS are provided to facilitate maintenance of ABCB.Motorised DS are provided to connect any load with transfer Bus.

2.7.6 Grounding Switch.ABCB of all transmission lines are provided with GS on line side DS (Lineside Arm). Its purpose is to ground the transmission line during maintenance.

2.7.7 SUT (Start-up Transformer) or SSST(System Station Service Transformer).This is a 220 KV/3.3 KV transformer situated in 220 KV switchyard. It givesstation power from GRID. SUT of Unit-1 is situated in Unit-2 switchyard sideand vice versa.

2.8 DEDICATED SUPPLY FROM RPS.A dedicated supply from RPS Hydel plant and Gandhi Sagar system(combined) is provided via 132 KV/220 KV, 50 MVA transformer. This supplyjoins RAPS system at RAPS-2 220 KV transfer bus. As RAPS stationalways requires some power in all the states i.e. in operation, in Shutdownand while starting up. Normal source of station power is station generator itself (when unit is operating) and Northern Grid. RAPS system is a part ofNorthern Grid.

A dedicated supply has been provided from the point of view that if stationis under start up (or even shutdown) and “grid” has failed, then ̀ on demand’,supply will be provided to RAPS by RPS Hydel Station/Western grid. Alsoif station has shutdown and GRID has failed, supply can be asked from RPS/Western GRID.


2.9 USUAL O TOs TO BE CARRIED OUT ON 220KV SYSTEM.1. Generator breaker test close.

Let Unit #1 has tripped. 513-CB-1 has to be test closed. OTO is asfollows :

(1) Check 513-CB-1 is in open condition (all 3?? ) - Field

(2) 513-CB-1 Normal/Test HS to put on “Test” - Field

(3) Open 513-DS-2 (check all 3?? ?open) - Field

(4) Open 513-DS-2 (check all 3??? open) - Field

(5) Put Normal/Bypass toggle switch on “Bypass” - Field

(6) Put “Normal/Test” HS to “Normal” - Field

(7) Inform C/R. Close 513-CB-1 - C/R

(8) Check all 3?? of CB-1 (indication) - Field

2. Kota-1 line Breaker 513-CB-8 is to be replaced by Transfer Breaker513-CB-9.

Let Kota-1 line is being fed by normal route i.e. 513-CB-8. Maintenancehas to be carried out on CB-8. It is to be replaced by CB-9. CB-9 ispresently supplying to SSST of Unit #1. OTO is as follows :

(1) 513-DS-25 to be closed -C/RCheck all 3 ? ?closed - Field

(2) 513-CB-9 to be opened - C/R

(3) Check all 3?? ?opened - Field

(4) 513-CB-27 to be opened - C/R(Check all 3?? ?opened) - Field

(5) Put out power & control fuses for DS-27 - Field

(6) Put in power & control fuses for 513-DS-21 - Field

(7) 513 DS-21 to be closed - C/R(check all 3?? ?closed) - Field

(8) 513 CB-9 to be closed - C/R(check all 3?? ?closed) - Field

(9) 513 CB-8 to be opened - C/R(check all 3 ???opened)

Now the OTO for 513-CB-8 to “test close” will be similar to that for 513-CB-


1 as explained above, Except for “Normal/Bypass” toggle switch operationwhich is not there in CB-8. It is only with generator breaker CB-1 & CB-11for “Field Breaker open” Bypass.

It should be noted that above OTOs are just guidelines. Actual OTOproforma is, as issued by C/R.

2.10 220 KV BREAKER PROTECTION.220 KV breaker protection depends on which system it serves. Maingenerator breaker CB-1 (CB-11) protects main generator as well as maintransformer. The Main Generator Protections are

(i) Differential protection (87),

(ii) Stator ground protection (64),

(iii) Loss of excitation (40),

(iv) Phase current balance (46),

(v) Field breaker accidental opening (62),

(vi) Under frequency,

(vii) Phase back up, (51/50)

(viii) Supplementary start up protection,

(ix) Excitation protection,

(x) Over flux

(xi) Low forward power.

The main transformer protections are 87, 64, 63 and for USST, the protectionis 87.

Protections for SSST are Differential, phase back up, ground back up andgas relay. Protections (63).

For Bus A1/B1 & A2/B2 the protections are differential and impedance relay.

Protections for transmission lines are mainly Zone protections (distancerelays).

2.11 ISLANDINGAll the transmission line breakers automatically trips at under frequency.(47.1 Hz) An under frequency. alarming occurs at 47.5 Hz. Now any one orall of lines breakers can be selected for not to trip at under frequency. withthe help of a blocking switch in CER. With under frequency. Blocking aparticular line (say Udaipur line) can be blocked from tripping. All other lineswill trip at under frequency. where as Udaipur line will not. In that caseUdaipur load will be supplied by station only.


2.12 ROUTINES FOR SWITCHYARDCOMPRESSED AIR SYSTEM(a) All receivers and lines to be drained for free of moisture & oil once in

a shift.

(b) Once in a shift, drain condensate in each intercooler and after cooler,after switching off compressor motor.

(c) Check crank case oil level of each compressor once a day.

(d) Once in a month, clean or replace the intake air filters. (maintenance.unit)

(e) Once in a week interchange lead/lag duty of compressor.

(f) Once in 4 hrs, note down the pressure after PRV and in individualbreaker cabinet.

(g) Observe frequency of start of compr. It indicates air leaks.

2.13 ALARM & INDICATIONIf air receiver pressure falls below 24 kg/cm2 or increases more than 30 kg/cm2 or individual breaker. Cabinet pressure falls below 13 kg/cm2 a commonalarm “Switchyard Compressor trouble” appears in C/R. The individual flagappears in CER (Control Equipment Room).

2.14 PRE-REQUISITES FOR ABCB OPERATIONSAll ABCBs can be operated from C/R provided following conditions aresatisfied :

(1) Normal/Test HS in brk. cubicle is selected for “Normal” position.

(2) 250V DC control power supply is available.

(3) All protective lockout relays should be in reset position.

(4) All electrical fuses checked for healthiness.

Also see that air connections are checked to be air tight.

Concerned transfer breaker (viz. 513-CB-3 OR 513-CB-9) should be in openposition.

To open/close motorised DS (from C/R) respective GS should be confirmedto be in open position.

While operating DS, check from field, whether all poles are closing/openingsimultaneously and their closing/opening is uniform.

Closing will be done in clockwise direction whereas opening in anti-


clockwise direction.

2.15 PRECAUTIONS & HAZARDS ON ABCB.1. While operating the DS/GS do it from the marked area only.

2. Never attempt to close the ground switch for checking mechanicalinterlock of isolator with ground switch.

3. Never work on a CB above receiver level when receiver ispressurised.

4. Never work on a circuit breaker while it is alive.

5. Do not carry long poles/grounding sticks etc in vertical position whilemoving in the switchyard.

6. Do not close ground switches unless the isolators are open at bothRAPS and Kota/Udaipur/HWPK ends and line clear messageobtained.

7. While closing the isolators, close slow, and when arcing starts, do theoperation fast.

8. While opening the isolators, initially open them fast to ensure thatarcing does not persist for long time and then open slow.

9. While providing temporary earthing using earthing cables, firstconnect the cables to ground and then to the line to be earthed/discharged and not vice versa.

10. Additional grounding should be first removed before any operation onDS.

???

3.3 KV/415V & LV System (Part-A) Electrical Authorisation 21

3

3.3 KV & 415 V CIRCUITBREAKERS & SWITCHGEAR

PART-A : General Description

3.1 INTRODUCTIONCircuit breakers are devices that open and close a set of electricalcontacts to interrupt or complete an electrical circuit. Switchgear is aself-contained, enclosed assembly of circuit breakers and relatedcomponents. Both circuit breakers and switchgear serve to protect plantcircuits from various electrical problems. They can be used to switchpower on and off, and they can isolate circuits on which work is beingperformed.

All circuit breakers have two main function : (1) switch functions, whichcontrol the opening and closing of electrical circuits and (2) protectivefunctions, which sense electrical circuit problem and open electricalcircuits automatically. Breakers that are close to the main power sourcefor a circuit are larger than breakers near the loads, because they carrymore power. For instance, a distribution breaker is responsible forcarrying power to a larger part of the circuit than a load center breaker;therefore, the distribution center breaker is larger and able to handle alarger amount of current than a load center breaker.

3.2 TYPES OF CIRCUIT BREAKERSLow voltage circuit breakers are generally classified into two types:moulded-case breakers and power breakers.

Electrical Authorisation 223.3 KV/415 V & LV System (Part-A)

3.2.1 Moulded-Case BreakersMoulded-case circuit breakers are small, self-contained breakers. Theirinternal working parts are usually sealed in an insulated plastic case.The case protects the breaker’s internal parts and prevents anyone fromcoming into contact with the energized parts inside it.

3.2.2 Power BreakersPower breakers are large circuit breakers in which the parts are usuallycontained within a metal framework. Power breakers are sometimesreferred to as frame breakers because of this feature of their design.Power breakers are usually built to carry larger amounts of voltage andcurrent than molded-case breakers.

Power breakers are sometimes called switchgear. The term“switchgear” is normally used to mean an entire assembly of breakers,control devices, and other components that are all contained in a singleenclosure.

3.3 SWITCH FUNCTIONS3.3.1 Opening

As stated earlier, a circuit breaker’s switch functions control the closingand opening of a set of electrical contacts to energize or de-energizeelectrical circuits. The contacts close or open because of the action ofvarious mechanical and electrical components. While thesecomponents are often mechanically complicated, and may vary indifferent breakers, their operating principles are basically the same. Themajor parts of the circuit breaker are a stationary contact, a movablecontact, a spring, and a latching mechanism. The latching mechanismholds the contacts closed.

A number of devices can be used to activate the spring and open thebreaker contacts. These devices can be grouped into those that operatemanually and those that operate electrically. Any action that causes thecontacts to open is known as “tripping” the breaker.

The latching mechanism in most of the breakers is operated electricallyby means of a device called a shunt trip. The shunt trip consists of a coiland a movable metal plunger. Wires connect the coil to a separate plantpower source, known as control power. (The control power circuit isusually an independent, low voltage power source that controls breakeroperation. Often, this control is actuated from a remote location). Whenthe coil is energized, the shunt trip is activated. Magnetic attraction


draws the plunger up and extends the rod that moves the latchmechanism, tripping the breaker.

3.3.2 Switch Functions : ClosingWhen a breaker trips, it opens its contacts to open a circuit. The breakercontacts must be closed before the circuit can receive power again.Breaker contacts can be closed manually, electrically, or mechanically.

The contacts of the circuit breaker can be closed manually, by movingan insulated handle. Manual closing is common for small breakers.Circuit breaker contacts can be closed electrically through the use of asolenoid. When control energizes the solenoid coil, the coil attracts aplunger. The plunger is quickly drawn into the coil, and it drives againstthe breaker’s mechanical linkage. This snap action causes the contactsto close and latch.

Another method combines electrical and mechanical operation andelectric motor is used to charge the spring. when the motor operates,the spring stretches, and energy is stored. The spring’s energy isreleased by pressing a button, and the released energy causes thecontacts to close.

3.4 CIRCUIT BREAKER OPERATION : PROTECTIVEFUNCTIONSThe protective functions of circuit breakers are associated withcomponents called trip devices. A trip device is a mechanism thatsenses circuit problems and causes breaker contacts to open whennecessary to interrupt current flow. Trip devices thus allow for theautomatic isolation of faulted circuits.

3.5 COMMON CIRCUIT PROBLEMSThe protective functions of circuit breakers are activated by threecommon problems. Short circuits, overloads, and grounds. The shortcircuit is an accidental complete circuit that has minimum resistanceand maximum current flow. Short circuits involve uncontrolled amountsof current flow, which can generate tremendous amounts of heat -enough to damage insulation, melt wires, or destroy components.

The term “overload” is frequently used in reference to many circuit


problems, In a simplified term an overload is a controlled AMOUNT ofexcessive current flow. Overloads can occur in three basic ways: (1) asa result of adding equipment to a circuit so that the circuit draws morecurrent than it is designed to handle; (2) as a result of the initial surge ofenergy required to start motors or other equipment; and (3) as a resultof the load that a particular component draws too much current.

A ground is an unwanted path for current flow through or over insulationto ground. A ground may or may not involve an excessive amount ofcurrent.

Trip devices are built to automatically open circuit breaker contracts.They enable a breaker’s contacts to open and interrupt a circuitimmediately when potentially damaging excess current is detected.Temporary variations in current may also be permitted in a circuit. Whena circuit is designed to handle surges that occur when motors start, atrip device can include a mechanism that provides for a delay in breakertripping.

Ground protection is provided by yet other devices. Some are designedto signal a breaker to trip if there is an absence of current in one phaseof a three phase circuit as current is diverted to ground. Others aredesigned to sense a current flow to ground where there should notnormally by any.

3.6 THERMAL ELEMENT TRIP DEVICESThermal element trip devices are commonly used in molded-case circuitbreakers. The thermal element is a bimetallic strip, which is composedof two different metals that expand at different rates when heated.

The primary power flows through the contacts and the thermal element.If a circuit problem occurs so that current flow exceeds a predeterminedvalue, the bimetallic strip heats and bends. (The direction of bendingand the amount of the bend are determined by the construction of thebimetallic strip). As the strip bends, the latching mechanism is trippedand the contacts open.

The use of thermal element trip devices is generally limited to smallmolded-case circuit breakers. They are not used on larger breakersbecause the thermal elements would have to be very large to handlehigher currents. They would produce too much heat and react slowly tocircuit problems.


3.7 ELECTROMAGNETIC TRIP DEVICESFor circuits that carry large current loads, large power breakerscommonly use electromagnetic trip devices. The trip device consists ofa coil, connected in series with the primary power circuit, and a movablemetal core. The core is held in place by a spring, which acts as arestraining mechanism. The latching mechanism consists of two parts:an insulated latch and a metal trip bar.

As primary power through the circuit, it flows through the breaker andcreates a magnetic field within the coil. When circuit current is withinnormal limits, the spring prevents the core from moving up into the coil.The magnetic field is not strong enough to attract the latchingmechanism’s metal trip bar.

If an excessive amount of current flows through the circuit, the magneticfield intensifies. The increased magnetic attraction draws the core intothe coil, overcoming the resistance of the spring. The metal trip bar isattracted to the core and it moves, forcing the latch to move, to trips thebreaker.

To allow for temporary overloads, it is sometimes desirable for a breakerto delay tripping until current exceeds a certain current value for aspecific time period. While time delay features vary with breaker design,most features are associated with a restraining mechanism in anelectromagnetic trip device.

One common device that is used to delay the movement of a breaker’strip device is a dashpot. The dashpot consists of a fluid chamber and apiston with passages for fluid flow. The dashpot is connected to the tripbar of the electromagnetic trip device.

Under normal conditions, the piston rests above the fluid level. When anexcessive amount of current flows through the electromagnetic tripdevice, the electromagnetic device attracts the trip bar. Since thedashpot is connected to the trip bar, the dashpot piston also moves;however, its movement is restricted, because fluid is forced to flowthrough the narrow passages in the piston. Movement of the piston andthe latching mechanism are both slowed, and a time delay in tripping isthus achieved. If an overload persists, the piston is eventually drawn farenough into the chamber to move the latch, tripping the breaker.

Some dashpots can be adjusted to alter the amount of time delay beforea breaker trips, with the amount of delay depending on the amount ofcurrent in the circuit. The delay settings are often made by means oflevers on the front of the dashpot. Usually, a lever is moved to increase


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or decrease the area through which fluid flows in the dashpot; thisincreases or decreases the breaker’s trip time.

3.8 GROUND PROTECTIONAs stated earlier, a ground exists when current is diverted to anunintended path. Since a ground may not involve excessive current,thermal or electromagnetic trip devices may not provide adequateprotection against grounds. More importantly, even low current leaksmay create a safety hazard to plant personnel coming into contact withgrounded equipment. Thus, a separate system of circuit protection,capable of detecting small amounts of current flow to ground, is neededin most breakers. Unlike thermal and electromagnetic trip devices,which are an integral part of a circuit breaker, ground protection is oftenaccomplished by devices that are separate from the breaker.

One common system of three-phase ground protection is shown infigure 3.1(a) . It includes a power source; three phases of current,labeled R,Y and B; three current transformers, one for each currentphases; a load; and a comparator, which is connected to the currenttransformers. A control circuit connects the comparator to the breaker’strip mechanism. For the purposes of this explanation, an accidentalground has been drawn in phase B of the circuit.

The comparator is a device that senses any differences between phasecurrents. Since the ground is in phase B of Figure 3.1(a), thecomparator would sense the lack of current flow in that phase and signalthe breaker to trip.

Another common system of ground protection relies on detecting thepresence of current flow to ground where there should not be any.Figure 3.1(b) is a typical example of such a system. It includes a powersource; a breaker; a load; a small current transformer attached to thesecondary of the star-connected transformer; and a relay and controlcircuit, which runs from the current transformer to a shunt trip inside thebreaker. Again, a ground has been drawn in the figure, in phase B of thecircuit.

Since a ground generally takes the path of least resistance, current inthe figure would travel through the transformer secondary’s ground. Thecurrent transformer would detect this current flow and send a signalthrough the relay and control circuit to trip the breaker.


3.9 BREAKER RATINGS AND SYSTEM COORDINATIONCircuit values vary in magnitude. So, to provide a way of telling what abreaker is capable of doing in a particular circuit, all breakers havevoltage and current limits, usually listed on the breaker nameplate.These limits are expressed in the form of ratings.

The nominal voltage of a breaker is the maximum voltage at which thebreaker is designed to be used under normal circuit conditions. Thenominal voltage can be represented on the nameplate as V., volt, volts,etc., and it may be expressed in either AC or DC volts.

Another set of ratings are a breaker’s frame size and continuous rating.These two ratings are a lot alike, but there is one important difference.Frame size is the maximum current that the breaker is designed to carrycontinuously at its nominal voltage without damaging the contacts orinternal parts. The frame size is usually represented on the nameplateas a., amp, or amps, etc. Low voltage circuit breakers can have a framesize of between 10 amps and 4,000 amps. Of course, the breaker mayactually trip at some value less than the frame size. The current at whichthe breaker will trip is known as the continuous rating. Tripping dependson the settings of the trip devices associated with the breaker.

A breaker’s interrupting capacity is the maximum current that thebreaker is designed to interrupt at its nominal voltage. Interruptingcapacity is usually abbreviated as “interr. capac” or “interrupt. cur.” Abreaker with a nominal voltage of 240 volts AC, and a continuous ratingof 10 amps can have an interrupting capacity as high as 5,000 amps; abreaker with a nominal voltage of 240 volts AC and a continuous ratingof 800 amps can have an interrupting capacity as high as 50,000 amps.

Most nameplates contain additional information such as the breakertype, the ambient temperature in which the breaker best performs, themanufacturer’s name and instruction book reference book number, andthe frequency.

The importance of breaker ratings becomes apparent when consideringthe needs for circuit protection throughout an entire plant. Circuitbreakers are arranged in plant systems so that the breaker closest to acircuit problem trips first to isolate the problem.

If there is a problem in a lighting fixture away from the plant’s main powersource, the breaker that is nearest to the light should trip first.

If circuit problems persist, larger breakers closer to the power source actas protective devices for the circuit.


3.10 PRINCIPLES OF CIRCUIT INTERRUPTIONFrom the time that electrical power was first produced, circuit breakershave been needed to extinguish arcs. an arc is electrical current thatjumps through the air between two terminals of different potential. Incircuit breakers, arcing occurs as the breaker contacts open.

Arcs must be controlled and extinguished before circuits areinterrupted. If arcs are not controlled and extinguished quickly, the heatthat results from them can seriously damage cables and circuitcomponents and injure plant personnel.

3.10.1 How an Arc FormsAs stated earlier, circuit breakers operate by pulling a set of contactsapart. As the contacts separate, a gap forms between them. The currentflowing through the circuit tries to maintain the circuit by jumping the gapbetween the contacts.

At normal room temperature, air is a very poor conductor of electricity.However, when air becomes hot enough, it becomes a very goodconductor. If the arc is not extinguished, it will eventually vaporize thecontacts and other circuit components in its path. Even an arc that lastsonly a fraction of a second may be able to cause significant circuit andplant damage.

3.10.2 How an Arc is ControlledMost larger breakers have both main contacts and arc contacts on eachmovable and stationary contact.

The main contacts are the breaker’s main current-carrying conductorswhen a circuit operates at normal load. The arc contacts are connectedin parallel with the main contacts. They are designed to handle arcs thatoccur as the breaker opens. (Some breakers may also haveintermediate contacts located between the main contacts and the arccontacts). The contacts associated with any one phase of a breaker areoften referred to as poles.

When breaker trips the main contacts open first. No arc occurs, becausethe primary power circuit is still complete through the arc contacts. Thearc is thus controlled by confining its occurrence to the gap between the


movable arc contact and the stationary arc contact. The arc contacts aremore rugged and are made to withstand the heat of the arc. As themovable arc contact pulls away from the stationary contact, the arcforms. At this point, the arc can be extinguished.

Most larger breakers carry three-phase AC current. These breakers arealso referred to as three-pole breakers, because they have three sets ofmovable and stationary contacts, separated by insulated barriers. Thebarriers insure that arcing occurring in one phase does not spread toother phases or to other breaker components.

3.10.3 How an Arc is ExtinguishedFactors in Extinguishing Arcs

To extinguish arcs, breakers utilize three physical factors: speed,distance, and cooling. In terms of speed, the rate at which the movablearc contacts separate from the stationary arc contacts has a directbearing on whether an arc is extinguished. The faster the arc contactsseparate, the less chance there is of arc formation, because the air hasless time to get hot enough to maintain current flow between thecontacts.

As the distance increases when the movable arc contacts separate fromthe stationary contacts, an arc stretches, or elongates as a result, thearc has elongated in an attempt to sustain current flow between the twocontacts. Elongating the arc increases the chance of extinguishing it,because the greater the distance an arc must travel, the greater thevoltage needed to sustain it.

Since cool air is a good insulator, mixing cool air with the hot air thatoccurs when an arc forms will help cool and extinguish the arc. An arcchute is a device that is similar in operation to a simple chimney. thechute confines and directs the arc and the air that surrounds it. Hot airrises within the chute, and cool air is drawn in at the base of the chuteand directed at the arc. The cool air will help cool and extinguish the arc.

Most breakers use a combination of speed, distance, and cooling toextinguish arcs. The combinations that are used with a particularbreaker vary with the manufacturer of the breaker and the breaker’sinterrupting capacity rating.


3.11 EFFECT OF DC ON A BREAKER'S RATINGCurrent ZeroIn a typical AC current since wave, there are points in each cycle atwhich the polarity of the power source changes. There is no current flowat these points, which are commonly called current zero.

Circuit breakers take advantage of current zero when extinguishing arcsin AC circuits. As the arc’s AC cycle repeats, the current zero situationsoccur in every cycle. This momentary absence of current flow is anotherfactor that combines with other methods to diminish and extinguish thearc. For DC there is no current zero feature so breakers are derated forDC use.

3.12 SWITCHGEARThe term “switchgear” can be used to refer to more than one thing.Normally switchgear is defined as an assembly of devices that controlelectric power and protect machines and circuits.

Switchgear is typically large, free-standing, fully enclosed, and self-contained. In a switchgear assembly, when circuit problems occur in onecircuit, that circuit can be interrupted and isolated while otherunaffected circuits continue to function normally.

The switchgear assembly resembles a file cabinet; its “drawers” arecalled cubicles.

The enclosure protects personnel from contact with the energized partsinside by isolating the components of the switchgear in separatesections. Most switchgear assemblies can be divided into threesections: a front section, which contains circuit breakers and relatedinstrumentation; a bus section, which contains bus work that distributespower throughout the assembly; and a cable section, which containscables that supply power to the assembly and distribute power to load.

One important feature of the switchgear sections is that the sections arephysically separated from one another by partitions within the metalenclosure. This separation confines any damage to one section andprevents it from spreading to other parts of the switchgear.

3.12.1 Front SectionAt the front of the switchgear assembly are the cubicles. Some of thecubicles contain power breakers; others contain instrumentation that is


used in monitoring the condition of the assembly or the control ofindividual breakers inside the assembly. This instrumentation mayconsist of meters, relays, and controls.

In this example, the instrumentation consists of three protective relays,an AC ammeter, an AC voltmeter, control switches for meters andbreakers.

The front section of a typical switchgear assembly also includescubicles where breakers are connected to primary power and controlpower. There are many way in which a breaker can be connected toprimary power. Usually, a breaker has primary disconnect fingers thatattach the breaker to metal projections at the back of the cubicle calledbus stabs. The primary disconnect fingers are spring loaded fingers thatclamp on the stabs, insuring a tight connection to primary power. Thesefingers carry line-side power, or power entering the breaker. There is arow of three disconnect fingers located underneath the line-side fingers.This row of fingers carries load-side power away from the breaker.Depending on the construction of the breaker, the line-side and load-side fingers may be arranged horizontally or vertically.

The bus stabs are, flat electrical conducting bars that protrude from thebus work inside the switchgear assembly. The stabs serve as the linkbetween the front section and the bus section of the assembly.Whenever the switchgear is energized, the line side stabs are energizedas well. They should be tested according to the proper procedure usingan approved voltage detector before any work is performed on or nearthe stabs.

Some breakers have ring-type current transformers located around thebus stabs. These transformers sense the amount of current entering andexiting the breaker. If current exceeds a predetermined value, thesemeasuring devices activate a relay, which triggers a shunt trip deviceinside the breaker, which releases the trip mechanism and opens thecircuit breaker contacts.

The method of connecting a breaker to control power also varies fromone breaker to another. The secondary fingers are connected inside thecubicle to a set of contacts that are connected to the control circuit.Usually, the control circuit is a separate low voltage power source that isused for breaker functions such as remote operation and shunt tripping.

3.12.2 Bus SectionThe bus section in a typical switchgear assembly distributes primarypower from the main breaker to all of the other breakers in the assembly.The bus section also connects the front section of the assembly to the


primary power cables at the back of the assembly.

Power is distributed through a network of horizontal and vertical busbars that run the entire length and height of the assembly. The bus barsare heavy-duty metal conductors, usually made of copper or aluminum.

3.12.3 Cable SectionIn the cable section, primary power is brought into the assembly bylarge, heavy cables. Smaller cables carry load power out of theassembly. Other cables may also be used to supply control power toindividual breakers or sets of breakers within the enclosure.

3.13 DISCONNECTING A BREAKER FROM PRIMARYPOWER AND CONTROL POWERBefore maintenance is performed on a breaker from one of theswitchgear cubicles, the breaker must be disconnected from primarypower and control power. The disconnect procedure is commonlyknown as racking a breaker out.

Since the switchgear assembly contains a number of current carryingcircuits, there is a safety hazard involved in removing a breaker from acubicle. The breaker must be tagged out in accordance with plantprocedures, and the breaker’s contacts must be open to prevent arcingat the bus stabs inside the switchgear as the breaker is disconnectedfrom the primary power circuit.

De-energizing the control circuits will prevent the breaker from beingoperated from a remote location while the breaker is being racked out.It would be dangerous for a breaker to reclose while being disconnectedfrom the bus. As a further precaution, it is also important to make surethat the springs in the breaker’s operating mechanism are fullydischarged. This helps prevent injuries to the hands from a breakeroperating unexpectedly.

Often, it is impossible to rack a breaker out with its contacts closed. Abreaker may be equipped with a device known as a cubicle interlock,which insures that the breaker’s contacts are open before the breakercan be racked in or out. The interlock is usually connected to amechanical linkage that triggers the latch and opens the contacts.Often, a key must be turned or a hand crank inserted before the rackingmechanism can be engaged. As this occurs, the interlock is activated.

Breakers pass through various stages as they are racked out. Thesestages are called draw-out positions, and they correspond to the points


at which a breaker is connected to primary power or control power.

In normal operating provision of a breaker, the breaker is connected tocontrol power, to primary power, and to ground.

The other position is the test position. In this position the breaker isconnected to control power, through the secondary disconnect fingers,and to ground; it is disconnected from primary power.

The third position is the disconnected position. The breaker is no longerconnected to control power and the ground connection is also broken. Inthis position, the breaker is de-energized, and it can be removed fromthe cubicle.

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3.3 V, 415 V & LV System (Part-B) Electrical Authorisation 35

4

3.3 KV, 415 V & LV SYSTEM

PART-B : SYSTEM DESCRIPTION

4.1 3.3 KV SYSTEM.In RAPS, 3.3 KV electrical system comes under class-IV. All the classesof power supply viz class IV, Class III, Class II 415 V and Class I 250 Vare basically drawn from Class IV 3.3. KV in normal condition. 3.3 KVelectrical Input/Output system is shown in Fig. 4.1.

3.3 KV namely BU-C, BU-D, BU-E, BU-F. BU-C and BU-F are suppliedby 2 nos. of USST secondaries where as BU-D & BU-E are supplied by2 secondaries of SSST. BU-C and BU-D are having Tie breaker 5241-CB-2 whereas BU-E & BU-F are also having a tie breaker 5241-CB-5.Output supply from all 4 buses either goes to direct 3.3 KV load via 3.3KV breaker or to the primary of Auxiliary transformers via a 3.3 KVbreaker. Auxiliary transformers secondary in turn supply to 415 V ClassIV and 415 V Class III buses.

There are 6 nos. of Aux. Tr. in Unit #1 whereas Unit-2 has got 9 nos. ofAuxiliary Transformers.

4.2 AUTO TRANSFER SCHEME (ATS) :As mentioned, BU-C and BU-D plus BU-E & BU-F are having a tiebreaker. When all the 4 buses are having their own normal supply, tiebreakers are open. If supply to BU-C or BU-D fails (one at a time), theirrespective Incoming breaker will open. At the same time tie breaker ofBU-C & BU-D will close. Thus Bus, of which normal supply has gone, willresume it is supply. In fact, closing signal to tie brk. actuatessimultaneously with the opening signal to the incoming brk. The trippingtime of the brk. is 80 msec and the closing time is to 120 msec. That’swhy for 1 to 2 cycles supply to the affected bus is not there. No outgoingbreaker on the affected bus will trip during this dead period of 1 to 2cycles. Similar scheme exists for BU-E & BU-F.

Electrical Authorisation 363.3 V, 415 V & LV System (Part-B)

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This is called as “automatic transfer” Examples of 3.3 KV Class IV loadsare PHT circulating pump, BFP pump, CEP pump, PW pump, CCWpump, HWP(K) raw water pump etc.

All the 3.3 KV breakers are of 2 types viz spring operated and solenoidoperated. All the supply breakers to BU-C, BU-D, BU-E and BU-F and C/D and E/F tie breakers are spring operated (stored energy) and allfeeder breakers are solenoid operated.

3.3 KV Breaker, Instrument transformer, relay, meters and instrumentsall together are provided in a common cabinet nominated as metal cladswitch gear.

3.3 KV circuit breakers are of draw out air magnetic ruptair type. Forbreaker maintenance, complete breaker truck is taken out. Arcinterruption in 3.3 KV breaker, during the time of tripping takes place inair at atmospheric pressure, with the help of magnetic of blow out field& air draft. The blow out coils are connected in each phase in series withthe arc runner to help in arc interruption, by forcing the arc into thebarrier stack in the arc chute. At the time of circuit interruption, thecurrent in blow out coil produces a magnetic flux which interacts the arcand forces the arc into barrier stack.

In a spring operated CB, A 250 V DC motor charges a spring. When aclose signal is given, a 250 V DC coil actuates and a latch is released.CB contacts close by release of spring energy. During closing of CB,another spring is charged for tripping. When a tripping signal is givenafter a closing, a 250 V DC coil, releases a latch and CB is tripped. Again for next closing, 250 V DC motor charges the closing spring.

In the solenoid operated breaker, closing is by direct mechanicalmovement of a 250 V DC solenoid plunger. Rest mechanism is similar tothat of spring stored energy type breaker.

Operating time of (closing time) a spring operated breaker is much lessthan that of a solenoid type. Hence all the bus supply breaker and bustie breaker of 3.3 KV system are spring operated from auto transferscheme point of view. Total 6 nos. of spring operated breakers are therein one unit. All other breakers are (feeders) solenoid operated.

All 3.3 KV breakers are operated from C/R. All bus supply breakers (4)tie breakers (2) and all auxiliary transformer supply breakers areoperated through 4 position CC/TT disagreement switch. Rest of thebreakers are operated by a toggle switch (2 positions). C/R operation iswith 48V DC (via 3c & 3T relay). The control circuit is shown in fig.-4.2.

3.3 KV breaker truck is having 3 positions viz, rack in / Test &disconnected (rack out). In “rack in” position, all auxiliary contacts &power contacts are made. Breaker can be operated from C/R. In “test”


position, power contacts are disengaged, only auxiliary contacts aremade. Breaker can be tripped or closed from field only with a 2 position“HS” provided on breaker cabinet. In disconnected position (or rack out)position, all the contacts are disengaged. Breaker truck can be justtaken out for maintenance.

On breaker cabinet door, lamp indication is provided and on breakerconsole (or truck) flag indication is provided regarding “open” and“close” position of breaker.

Aa “leg push” lever is provided on breaker truck to “trip” only. In casebreaker tripping is not operable from C/R ‘HS’ or local ‘HS’, to “trip” thebreaker only, this spush lever can be used. This is also a safety feature.

Safety Features : To bring the breaker from “Rack in” position to “Test”position, and from ‘Test’ position to “Rack out” position and vice versa,(1) a “safety latch” levels is provided, which is to be pushed by leg. (2)Breaker should be in “open” (Trip) position. When breaker is taken from“rack in” position to ‘Test’ position, A shutter falls automatically in front ofpower contacts (fringes) coming from incoming & outgoing buses, tocover it from human safety point of view.

When breaker cabinet door is opened, a safety lever mechanism isengaged which prevents door from closing on it’s own.

250V DC close fuses rating is 15 A whereas trip fuses rating is BOA.

When breaker cabinet door is opened, 250 V DC close & trip fuses & 4PST appear in front.

Protection : A composite “P&B Gold” type relay is provided with eachbreaker which covers 5 protections namely - Reverse power (presentlyremoved), stalling, instantaneous overcurrent, time over current andunder voltage. Whereas only “Alarms” have been provided for “phaseunbalance”.

4.3 RACK IN/RACK OUT OPERATIONS

(1) RACK IN TO TEST POSITION & VICE VERSA

(a) Check that breaker is in open position.

(b) Take clearance from C/R.

(c) Open the cabinet door. See that door safety lever has engaged.

(d) Remove 250 V DC trip & close fuses & 4 PST

(e) Engage the “Hand lever” in proper position and push the “safetylatch” by leg.


(f) Just shift breaker from “In” position towards test position with thehelp of Hand lever. Safety latch will remain in “pushed position”.

(g) By “Hand efforts” bring the breaker truck to test position.

(h) Achievement of “test” position will be indicated by release of“safety latch”.

(i) Again put 4 PST & 250 V DC fuses.

Breaker is ready for operation in “TEST” position.

For “test” position to “rack in position” OTD, almost similar operationswill be close in reverse order.

(2) “TEST” POSITION TO “Rack out position” & vice versa.

(a) Check breaker is in Test position & in open position.

(b) Take clearance from C/R.

(c) Remove all 250 V DC fuses & 4 PST.

(d) Engage the “Hand lever” in proper position and push the safetylatch by leg.

(e) Just shift breaker from “Test position” towards “Rack out” positionwith the help of hand lever. Safety latch will remain in “pushed”position.

(f) By “hand efforts” Bring the breaker truck to “rack out” position.

(g) Achievement of rackout position will be indicated by release of“safety latch”.

(h) Breaker is in “rack out” position. Breaker truck can be taken outany where for maintenance.

For “vice versa” OTO a similar & reverse order of operations will bedone.

4.4 415V AND LV SYSTEM415 V AC electrical system of RAPS supplies power to 415 V class Iv,Class III, Class II and indirectly to 250 V DC Class I. (VIA ACVR orrectifier). A simple layout of this system is shown in fig.-4.3. 415 V ClassIV and Class III get it’s power from output of 6 nos. of 1.250 MVAauxiliary transformers ( 9 in unit #2) fed by 3.3 KV system. Class II getsit’s power from Class III tie (Through MG set in Unit #2) in normal


Fig

.-4.

3 M

CC

Sch

emat

ic


condition. Where as Class I 250 V DC is supplied by rectifier (ACVR inUnit #2) fed by Class III in normal condition.

So we can say that in normal condition all class of 415 V AC Class IV, III& II and 250 V DC Class I is fed by 3.3 KV Class IV.

When Class IV 3.3 KV trips, (1) Class IV 415 V also trips (2) 415 V ClassIII gets it’s power from 1 MW DG after a while (3) 415 V Class II gets it’spower from MG set without interruption fed by battery bank. (4) Class I250 V DC gets it’s power from battery bank, without interruption.

Class IV 415 V has got 4 buses namely G,H,J & K (Unit #1) Class III hasgot two, L&M. Class II also has got two, N & P and Class I 250 V DC hasgot 4 buses namely A, B, E & F. Bus E & F in 250V DC are for batterybanks.

Bus ‘K’ in the 415 V Class IV is called as standby bus. It does not supplyany direct load, but it serves the loads of BU-G,H,J, L & M (one at a time)when maintenance on Input auxiliary transformers of these buses is tobe taken or it is out of service otherwise.

All the 415 V buses Input breakers are 415 V air circuit breakers. 415 Vbuses (all classes IV, III and II) supplies to (1) Motors of 60 HP to 300 HP(2) Lighting loads (3) MCC

Note :Lighting loads are not on Class III. All these above supplies from415 V buses are through 415 V air circuit breakers.

Classification in 415V Air Circuit breaker.

(A) On the basis of Make : Unit #1 415 V brk. are I.T.E. make. Unit #2has got L&T type Both types of breakers are air circuit brk. drawout type.

(B) On the basis of current capacity : They are in the range of 2000 A(Bus supply brk.) 1600 A, 1200A, 800A, 600A, 400A & 200A. Thisdepends on the load being supplied by brk.

(C) On the basis of type of operation.

All the Class IV, III, II 415 V bus supply breakers, bus tie breakers, ClassIII MCC L and MCC K supply breakers can be operated from C/Rthrough 4 position disagreement switch. Some other important loadslike moderator pump, Shutdown pumps etc. can also be operated fromC/R with toggle switch.

Other breakers like lighting panel supply breaker, Class IV and Class IIMCC supply breaker etc. can be operated from field only. Here breaker“Closing spring” charging is by 250 V DC motor. Where as a third


category of breakers are there which can be operated from field only &that too manually. “Closing spring” charging is through a manual lever.Example of 3rd category is LP compressor supply breaker.

In all the 415 V breaker, closing is through a stored energy spring.

Now spring is either charged through 250V DC motor or manuallythrough hand lever. In case of breakers having spring charging motor, atoggle switch is provided on brk. front in field to switch off the motorsupply. (Unit #2 L&T make brk. this facility is not there).

In the field, each breaker front has got following (i) Test close/Test openpush button. Breaker can be closed or opened in test position. (ii)Emergency Trip push button - Brk. can be tripped in any case with thispush button. (iii) A toggle switch for charging spring motor ON/OFF. (iv)“Over load” Reset push button. (iv) Spool mechanism to engage Handlever with it so as breaker can be taken from one position to other. (Incase of Unit #1 I.T.E. breaker, a shutter closes this spool as long asbreaker is in closed condition. This prevents breaker position to changefrom Rack in / Test / Rack out to each other when it is in closed condition.4 PST and 250 V DC close and trip fuses are mounted on the back of brk.cabinet.

4.5 BREAKER OPERATIONBreaker closing and tripping is by a spring. Closing spring chargesthrough a motor or manually. When close signal goes to Brk., a 250 V DCCoil energises and a latch releases spring & brk. is closed. Whileclosing on other spring is charged for tripping. When tripping signalcomes, a latch releases tripping spring through 250 V DC ckt. & finaltripping occurs. 415 V breaker has got 3 position viz Rack in / Test / Rackout.

When breaker is in “rack in” position both power & control contacts aremade.

When breaker is in “Test” position, power contacts are cut off, controlcontracts are still there. Breaker can be test close / or test trip from fieldonly.

When breaker is in rack out or disconnected position, power & controlboth contacts are disconnected. Breaker rack can be taken out formaintenance.

Test and Rack out positions are marked on breaker rack.


4.5.1 Rack in to test position :(i) Take clearance from C/R

(ii) Check breaker is in OPEN position.(If it is a manual breaker, trip it)

(iii) Remove 4PST & close & trip fuses.

(iv) Engage “Hand lever” & move it (Rotate it by hand) till ‘test’position is achieved.In RAPS-1 in I.T.E. type breakers when any of In/Test/Outposition is exactly achieved, safety shutter falls down).

(v) Again put 4PST & fuses ON.

4.5.2 Test position to Rack out position(i) Take clearance from C/R.

(ii) Check breaker is in ‘OPEN’ position.(If not then trip it)

(iii) Remove 4PST & 250 V DC fuses.

(iv) Engage “Hand lever” & rotate it till “Out” or “disconnect” positionis achieved.

Now breaker rack can be taken out for maintenance.

4.6 PROTECTIONSAll the 415 V breakers are provided with dual type over current relay.Apart from this breakers of some imp loads like mod. pump etc. are alsoprovided with ground fault relay.

Overload Reset is by a push button provided on breaker front.

MCC : Full form of MCC is motor control centre. As stated above 415 Vbuses supply comparatively bigger loads directly through breaker.Motor of 60HP to 300 HP are directly supplied by 415 V buses. Where assmaller loads (up to 60 HP) are supplied through MCC. MCC in turn issupplied by 415 V buses through 415 V breaker.

Small loads on all cl-IV, III & II are supplied by MCC. One MCC has gotinput supply from any one of class of power. Obviously all loads suppliedby one MCC are on same class of power.

MCC is nominated by English letters A,B,C,D,........... One MCC consistsof many small cells arranged in vertical columns & horizontal rows. Eachcell supplies a unique load. Columns are nominated by english lettersA,B,C, ......, where as rows are by numbers 1,2,3,......... Thus one MCC


cell address consist of 2 english letters & one number. For exampleMCC-CD-5 means MCC is ‘C’ in which ‘D’ column, 5 row cell, is there.Each MCC cell has got a metallic tag on it, addressing on it to which loadit is supplying to.

Following are some merits of MCC -

(i) To facilitate the motor supply from one place.

(ii) To have motor power supply as per their priority i.e. cl-IV, cl- III,cl-II etc.

(iii) To isolate the faulty circuit only.

(iv) This makes fault finding quick & easy.

(v) Hence down time of m/c is less.

(vi) Safety aspects of operator are observed.

(vii) Draw out type MCC reduce the down time and are practically freefrom human error.

4.7 COMPONENTS OF MCCEach MCC cell has got following components : (Refer fig.-4.3)

(a) Isolator

(b) HRC fuses (Power)

(c) Contactor

(d) Control transformer

(e) Control fuses

(f) Over load element

(a) Isolator :

This is just mechanical “make & break” of contacts. It has no automationnor any protection contained in it’s own.

Purpose of isolator is to cut off the supply to MCC cell whenmaintenance. is to be done. Isolator has mechanical interlock with MCCcell door. When isolator is open then only MCC cell door can be closedor opened.


(b) HRC Fuses :

These are used for short circuit protection. Since isolator OR overloadrelay both can not protect against short circuit, HRC fuses are used.Once the fuse is blown, it is to be thrown, & new one has to be installed.

(c) Contractor :

These are contacts which makes & breaks actual supply to load.Contacts are operated by a 115 V solenoid. It is termed as M contactalso. Types of contactor depends on current rating of load.

(d) Control Transformer :

This is a 415 V/ 115 V step down transformer. Primary is connected toany of two phases out of R,Y,B of main supply. Secondary is connectedto contactor solenoid (115 V).

(e) Control fuse :

This is again HRC fuse put in transformer secondary to save against anyshort ckt. in transformer.

(f) Overload :

This is a bimetallic element. Due to over current or overload, it makes/breaks contact in contactor solenoid circuit.

4.8 TYPES OF MCCApart from the above six basic components of an MCC it may have otherfeatures also. For example :

(i) Load/motor can be operated from C/R.

(ii) Load/motor can be operated in both forward & reverse direction.

(iii) Load/motor may require one or many operation logic to besatisfied before start.

Hence type of MCC depends on “Features it incorporate”.

Type of MCC is designated by english letter A,B,.......

For example type A MCC has got only isolators & HRC fuses.

Type E MCC has got 3C relay. (Operation from C/R)


4.9 ISOLATION OF MCC CELL(i) Get clearance from C/R.

(ii) In field recognize required MCC cell.

(iii) Check MCC cell is off. (From C/R load is off)

(iv) “Open” the Isolator switch.

(v) Open the MCC Cell door.

(vi) Take power fuses out.

(vii) Take control fuse out.

4.9.1 Safety precautions in MCC1. Before starting the work on the MCC cell obtain work permit.

2. Before complete withdrawal of any MCC cell it should be realisedthat part of the isolator switch is still energised. Therefore allsafety instructions for working on live electrical equipments mustbe followed strictly.

3. Since isolator switch will not give visual isolation, with avoltmeter, ensure power supply is off and also observe controlckt. side, no voltage is existing due to other sources.

4. Never try to switch off an equipment by means of the isolator. Theisolator contacts may get damaged.

5. Before switching on the isolator make sure that the control switchat the remote end is kept in the off position. You may start theequipment inadvertently if the switch is already ON.

6. While disconnecting & connecting wires ensure the identificationnumbers are matching on either side.

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Preventive Maintenance of Breakers Electrical Authorisation 49

5

Preventive Maintenance ofBreakers

5.0 PREVENTIVE MAINTENANCEPreventive maintenance is planned, scheduled maintenance. Whenperformed regularly and satisfactorily, preventive maintenance helps toassure smooth and continuous operation of plant machinery andcircuits protected by circuit breakers and switchgear.

The frequency of preventive maintenance procedures depends on theequipment’s service and its operating conditions. For instance, if abreaker is used repeatedly for routine switching operations, or if it sitsidle for months at a time, it should be inspected more often than one thatis occasionally used. Also, a breaker that is exposed to conditions suchas high humidity, high temperature, or damaging atmospheres shouldreceive more more frequent attention than a breaker located in cleanerand cooler area of the plant.

Occasional operation of circuit breakers and switchgear is an importantpart of preventive maintenance. With occasional operation, theoperating mechanisms of breakers can be exercised; dirt can be keptfrom settling on internal breaker parts; and problems can be discoveredthat may indicate the need for an overhaul of the breaker.

An important consideration with any preventive maintenance task issafety. Since switchgear assemblies contain a number of current-carrying conductors, the use of the tools and equipment that couldconduct electricity should be avoided. Metal ladders or step stools arepotentially dangerous, as are metal nozzles on air and vacuum hoses.All rings, wrist watches, and metal jewelry should be removed.

When performing maintenance on a circuit breaker, it is important tokeep hands away from internal moving parts. Springs should be fullydischarged before any work is performed on the operating mechanismof a circuit breaker.

Electrical Authorisation 50Preventive Maintenance of Breakers

5.1 TYPICAL RACK-OUT PROCEDURE FOR A POWERBREAKERAfter taking proper permits the first step in racking out a typical powerbreaker is to make sure that the breaker contacts are open. This isusually done by pushing the trip button on the breaker face plate.

To make sure that the breaker contacts stay open during the rack-outprocedure, the control circuit fuse block and the fuse block for the springcharging motor and pulled. Pulling the fuse blocks de-energizes thecontrol circuit and the spring charging motor; then the breaker contactscannot be accidentally closed, or the springs accidentally charged, by asignal from a remote plant location.

The maintainer is using a hand crank to rack a breakerout of its cubiclein the switchgear assembly.

As the crank is turned, the breaker moves forward out of its cubicle.During this procedure, it is good practice for the operator to stand to oneside. If the breaker were inadvertently connected to a faulted circuit, itcould be blown out of the cubicle with enough force to cause seriousinjury to anyone standing in front of it. The same precaution is advisablewhenever the manual “close” and “trip” buttons, usually located on theface of the breaker, are used.

The breaker is first racked out to the test position.

With the breaker in the test position, the control circuit fuse blocks canbe reinstalled. This energizes the control circuit so that the breaker canbe cycled. Cycling a breaker means checking a breaker’s mechanicaloperation by pushing the close and trip buttons on the breaker face plateseveral times to close and open the breaker contacts. Cycling a breakercan sometimes reveal a problem with the operating mechanism - if thereis any hesitation or delay in the contacts closing or opening, the breakermay need to be overhauled.

After it has been determined that the breaker contacts are closing andopening normally, the control circuit fuse blocks can be pulled again tode-energise the control circuit. Then the breaker is racked out to thedisconnected position. In the disconnected position, the breaker can beremoved from the cubicle.

Care must be taken when removing any power breaker from a cubicle.Breakers are often too heavy to be lifted out; special regging may beneeded.

Inside the empty cubicle, the line bus stabs are still energized withprimary power unless the entire switchgear assembly is de-energized.To insure that no one touches the interior of the cubicle, a protectivecover is usually bolted in place over the opening .

Preventive Maintenance of Breakers Electrical Authorisation 51

5.2 INSPECTION AND MOTOR CLEANINGThere are many parts of a typical power breaker that can be inspectedand cleaned as part of a preventive maintenance procedure. Forexample, a rag can be used to dust off the breaker. Compressed air isnot generally used to clean a breaker during a routine inspection,because it can blow dirt and dust into the operating mechanism andcause moving parts to bind.

Another general preventive maintenance procedure is checking thebreaker’s wiring connections. All wiring connections should be tight -loose connections could result in the breaker not operating when it issupposed to do so.

5.2.1 Arc Chutes and ContactsAfter the breaker is dusted off with a rag, the breaker’s arc chutes can beremoved. Each arc chute is wiped off with a rag to remove any dirt anddust that may have collected on the chute and would impair its ability toextinguish an arc.

The interior of each arc chute is inspected for signs of heat damage,such as pitting, discoloration, or cracks.

All three sets of stationary and movable contacts are checked forcracking, burning, putting, or any traces of metal that may havespattered on them. Main contacts should show no signs of damage,because no arcing should occur between them. If damage is evident,the main contacts and the arc contacts should be checked for propersequence (main contacts open first and close last), proper gap (maincontacts open to a specific distance before arc contacts begin to open),and sufficient contact pressure (poor pressure results in highresistance/high heat connections).

Arc contacts are usually made of different materials than main contactsmaterials that are more resistant to the intense heat of the arc.Consequently, the standards for inspecting the appearance of arccontacts are different from those for main contacts. Since an arc formsbetween the arc contacts, the arc contacts usually show some signs ofwear. Arc contacts are designed to withstand a certain amount of wearcontinue to operate normally.

5.2.2 Other ComponentsShunt Trip CoilsShunt trip coils are inspected for any signs of overheating, such asdiscolored insulation, indicating that the coil has been subjected to

Electrical Authorisation 52Preventive Maintenance of Breakers

excessive current or has been energized too long. Control wiring ischecked for good connections. If the coil fails to operate, it will not beable to trip the breaker when the need arises, and a breaker that fails totrip will not protect a circuit.

Primary & secondary disconnect fingers. The primary disconnectfingers are usually checked for discoloration or wear on their metalsurfaces that may occur as a result of excessive current flow. Also, if theprimary disconnect fingers are spring-loaded, the spring tension istested. A weak spring could result in overheating, because it creates ahigh-resistance connection to the bus stabs.

5.3 RETURNING A BREAKER TO SERVICEAfter the inspection is completed and it is determined that the breakercan be returned to service, the breaker is reassembled and returned toits cubicle. The breaker is then racked in to the test position, and thecontrol circuit and spring charging motor fuse blocks are inserted. Atthis point, the breaker should be cycled several times to make sure thatits operating mechanism is working properly. After the breaker is cycled,its contacts should be left open.

The control circuit and spring charging motor fuse blocks should then bepulled again before the breaker is racked in from the test position to theconnected position. De-energizing the control and spring chargingcircuits insures that the springs will not charge and the breaker contactswill not close as the breaker is connected to the bus; this would causearcing between the primary disconnect fingers and the bus stabs. Afterthe breaker is in the connected position, control and spring chargingfuses can be reinstalled, and the breaker can be returned to normalservice.

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Overhauling of Breakers Electrical Authorisation 53

6

OVERHAULING OFB R E A K E R S

6.0 CIRCUIT BREAKER OVERHAULCircuit breakers may be overhauled periodically, as part of a regularschedule, or they may be overhauled when certain conditions signal theneed for an overhaul. These conditions include damaged arc chutes,binding operating mechanisms, and faulty electrical components insidea breaker.

Overhauls also provide an opportunity to test, adjust, and if necessary,correct certain problems with circuit breakers.

6.1 CLEANING A BREAKER WITH COMPRESSED AIRAND SOLVENTWhen a breaker is due for an overhaul, it is racked out, as describedearlier and then usually taken to the shop to be overhauled. A typicalway of beginning the overhaul is by removing the arc chutes from thebreaker and setting them aside. The next step is to give the breaker athorough cleaning with dry compressed air. Since this proceduregenerates airborne dust, it is important to wear the appropriateprotective equipment. Such equipment may include face shields, dustmasks, and approved safety glasses. When solvent is used, gloves andsleeves are also recommended.

Cleaning the breaker with compressed air loosens dirt and dust from allof the insulating surfaces and linkages inside the breaker. Anyaccumulation of dirt and dust could cause insulating surfaces to losetheir effectiveness and linkages to bind.

Then the breaker is sprayed with a solvent. Cleaning a breaker withsolvent removes dirt that may be lodged inside the moving parts of abreaker and helps to restore the quality of the breaker’s insulatingsurfaces. However, cleaning a breaker with solvent also washes awaylubricants from the moving parts, so these parts are usually lubricated

Electrical Authorisation 54 Overhauling of Breakers

after the solvent cleaning. After the breaker is cleaned with solvent, it isdried off with compressed air.

6.2 ARC CHUTESDry compressed air is also used to clean the insides of the arc chutes.This cleaning removes dirt and dust that could interfere with the chute’sability to extinguish an arc.

The arc chutes are then disassembled. The outside shell of the chute isinspected for pitting or cracks that could lead to an arc escaping from thechute and damaging other circuits or other parts of the breaker.

Each of the metal fins is examined for deposits or damage that may haveoccurred as arcs were extinguished in the chute. To improve the abilityof the fins to split arcs, the fins are usually cleaned with sandpaper toremove built-up deposits that may have formed on the fins during theoperation of the chute.

The arc chutes can then be reassembled. For the metal fin arc chutesthe shell halves are assembled first, and then the metal fins are wedgedinto slots inside the shell. After the rest of the chute’s internal parts arereinstalled, the chutes can be sprayed with solvent, and dried withcompressed air. After this is done, the chutes can be reinstalled in thebreaker.

6.3 OPERATING MECHANISM & OTHER COMPONENTSAs stated earlier, when a breaker is cleaned with solvent, lubrication isalso washed away from the moving parts inside the breaker. Among theparts that may need to be lubricated are the operating mechanism, themovable contacts pivot points, the primary disconnect fingers, and thesecondary disconnect fingers.

Each of the moving parts of the operating mechanism should be wipedoff with a rag to eliminate or dirt that could hamper the movement of themechanism. As the parts are wiped, they are checked for signs ofdamage and excessive wear.

The parts of the mechanism are then lubricated with an approvedlubricant. Only a little bit of lubricant is needed to do an effective job; anyadditional grease or oil can actually attract dirt, dust, and lint that cancause the mechanism to bind. After the mechanism is lubricated, it isreassembled and reinstalled in the breaker.

Two other components are typically checked and lubricated: the primaryand secondary disconnect fingers. Since the primary disconnect fingers

Overhauling of Breakers Electrical Authorisation 55

are connected to primary power, they should be checked for signs ofoverheating and metal wear. A small amount of conductive lubricantmay be added to the connecting surfaces of the fingers to insure properconnection to the bus stabs. Any excess should be removed, because itcould melt and form an unwanted current path inside the switchgear. Ifthe construction of the secondary disconnect fingers is similar to that ofthe primary disconnect fingers is similar to that of the primarydisconnect fingers, the inspection and lubrication procedures will besimilar.

6.4 CONTACTSIn a three phases breaker, each of the movable arc contacts shouldtouch its stationary arc contact at the same time when the breaker isclosing. Likewise, the three contacts should separate simultaneouslywhen the breaker opens. If all three phases do not make or break at thesame time, equipment damage could result. Another importantsequence is that of the arc contacts and the main contacts. Arc contactsshould be the last to open and the first to close.

Three common inspections can be made on the contacts when using aslow close handle: gap, wipe, and pressure.

Contact gap may refer to two different measurements : (1) any gap thatremains between arc contacts if the three phases are not closingsimultaneously, and (2) the gap that occurs between the movable maincontacts and the stationary main contacts when the arc contacts firsttouch or first break. If the gap in any phase is found to vary from thespecified distance, it can usually be corrected by adjusting part of themovable contact linkage.

Contact pressure is the spring force of the contacts pushing against oneanother. If the contacts do not press firmly against each other, theresulting loose connection could cause arcing or overheating. Thesprings holding the contacts together should be checked, adjusted, andreplaced, if necessary, according to the breaker manufacturer’sinstructions.

After overhauling of the breaker megaohmmeter tests are usuallyperformed on the wiring and conductors inside the breaker to insure thatinsulation is per-forming properly. On a three phase breaker, testsshould be made line to load within each phase with the breaker contactsopen; from phase to phase; and from phase to ground. The readingsshould be recorded and compared with other maintenance records fortrends of deteriorating insulation.

Electrical Authorisation 56 Overhauling of Breakers

An instantaneous trip test is typically performed to see if the breaker willtrip at or below its rated interrupting capacity.

One phase of the breaker is tested at a time, because each phaseoperates independently and each phase usually has its own trip device.The leads of a test set, which supplies the current, are placed on the lineand load primary disconnect fingers. The current on the test set is thenset for the value that the breaker is designed to trip at instantaneously.when the test set is energized, the breaker should trip. The interruptingcapacity of a breaker and the settings of the trip devices are speciallydetermined to protect specific circuits in the plant, and are coordinatedwith the protection of other circuits as well. Breakers and trip devicesmay not be interchangeable, even if the parts seem to fit properly. Theratings and settings should remain the same or the circuits may not beadequately protected.

A time-delayed trip test is performed to see if the breaker will trip after anoverload current is carried for a certain amount of time. The test leadsare again connected across the primary disconnect fingers one phaseat a time and the test button is pushed. An indicator on the test set mayshow the number of seconds or cycles that elapse before the breakertrips. Changes to trip settings should not be made.

After a breaker overhaul is completed, the breaker should be closed andopened as final test of its operation. During this test, it is ensured thatthe spring charging motor is operating and that the breaker contacts areclosing and opening instantaneously. If they are, the breaker can bereturned to service.

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Indian Electricity Rules Electrical Authorisation 57

7

INDIAN ELECTRICITYR U L E S

7.1 EXTRACT OF APPLICABLE INDIAN ELECTRICITYRULES AS PER INDIAN ELECTRICITY ACT

Rule-3 : Authorisation :

1. A supplier or consumer, or the owner, agent a manager ofa mine, or the agent of any company operating in oil field orthe owner of drilled well in an oil field or a contractor for thetime being under contract with a supplier or a consumer tocarry out duties incidental to the generation,transformation, transmission, conversion, distribution oruse of energy may authorise any person for the purpose ofany or all of the following , namely, sub-rule (2) of rule-36,cl.(a) of sub-rule (1) of rule-51, cl.(a) of sub-rule (1) and(Cls. (e) and (f) of sub-rule(2) of rule-64), sub-rule (2) ofrule-110, sub-rules (1) and (4) of rule-121, sub-rule (4) ofrule-123, rule-124 & sub-rule (8) of rule-125.

2. No person shall be authorised under sub-rule(1) unless heis competent to perform the duties specified in the rules forthe purpose for which he is authorised.

(2-A) (a) No person shall be authorised to operate or undertakemaintenance or any part (of whole) of a generating stationof capacity 100 MW or above together with the associatedsub-station unless he is adequately qualified & hassuccessfully undergone the type of training specified inAnnexure-XIV.

Electrical Authorisation 58Indian Electricity Rules

Provided that the provisions contained in this sub-rule shallhave effect in respect of the persons already authorised tooperate or undertake maintenance of any part or whole ofa generating station as aforsaid from the date to bespecified by the appropriate Government., but such a dateshall not be later than a period of (6 years 2 months) fromthe date this rule comes into force.

(2-A) (b) The appropriate Government may, on the recommend-ations of the owner of such generating station, relax theconditions stipulated in Cl.(a) of this sub-rule for anyengineer and such other persons who have alreadysufficient experience in the operation & maintenance of agenerating station.

Rule-44: Instruction for restoration of persons suffering fromelectric shock :

In everyman, high voltage or extra-high voltage station,sub-station or switch station an artificial respirator shall beprovided and kept in good working condition.

Rule-44A : Intimation of accident :

If any accident occurs, resulting in loss of human or animallife, in any injury to human or animal, a telegraphic reportwithin 24 hours of the knowledge of the occurence of thefatal accident and a written report in the prescribedperforma within 48 hours of the knowledge of occurence offatal and all other accidents should be sent to theinspectors.

Rule-51 : Provisions applicable to medium, high or extra highvoltage installation.

1(a) All conductors (other than those on O. H. line) shallbe completely closed.

(b) All metal work enclosure shall be earthed.

(c) Every switch board shall be compiled with thefollowing :


i) a clear space not less than 0.914 Mtrs. (3 feet),in and width shall be provided in front of theswitch board.

ii) The space behind the back of the switch boardshall be either less than 0.229 Mtrs. (9 inches)or more than 0.762 Mtrs. (30 inches) in width,measured from the farthest outstanding partsof any attachment or conductors.

iii) If space behind the switch board exceeds0.762 Mtrs.(30 inches) in width there shall be apassage way from either end of the switchboard clear to a height of 1.829 Mtrs. (6 feets).

Clause (a) of Sub-Rule (1) of Rule 64 :

The inspector shall not authorise a supplier to connect a supply ofenergy at high or extra high voltage to any consumer unless :

(a) All conductors and apparatus intended for use at high or extrahigh voltage and situated on the premises of the consumer areinaccessible except to an authorised person and all operations inconnection with said conductors and apparatus are carried outonly by an authorised person.

Sub-Rule (4) of Rule-121 :

Every motor shall be controlled by switchgear which shall be soarranged as to disconnect the supply from the motor and from allapparatus connected there to. Such switchgear shall be so placed as tobe easily operated by the person authorised to operate the motor.

Sub-Rule (4) of Rule 123 :

Every flexible cable attached to a portable or transportable machineshall be examined periodically by the person authorised to operate themachine, and if such cable is used underground, it shall be examined atleast once in each shift by such person. If such cable is found to bedamaged or defective, it shall forthwith be replaced by a cable in goodcondition.

Rule 124 : Portable and Transportable Machines :

The person authorised to operate an electrically driven coal cutter, orother portable or transportable machine, shall not leave the machine


while it is in operation and shall, before leaving the area in which suchmachine is operating, ensure that the supply is disconnected from theflexible cable which supplies the machine. When any such machine is inoperation, steps shall be taken to ensure that the flexible cable is notdragged along by the machine.

Sub-Rule (8) of Rule - 125 :

All apparatus, including portable and transportable apparatus, shall beoperated only by those persons who are authorised for the purpose.

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G L O S S A R Y

Arc - Electric current that jumps through theair between two terminals of differentpotential; usually, between a set ofcontacts.

Arc chute - The part of a circuit breaker in which anarc is confined and extinguished.

Arc contacts - The set of contacts on a circuit breakerwhere arc formation takes place. Arccontacts are designed to carry the arcuntil it is extinguished.

Bus section - The section of a switchgear assemblycontaining conductive bars thatdistribute power throughout theassembly.

Bus stabs - A set of conductive bars that connect acircuit breaker to the bus and cablesections of a switchgear assembly.

Cable section - The section of a switchgear assemblycontaining cables that connect theassembly to primary power circuits,control power circuits, and loadcircuits.

Charged spring - A spring that is stretched orcompressed so that it has sufficientenergy to close or open a set of circuitbreaker contacts.

Circuit breaker - A device that opens and closes a set ofcontacts to make or break an electricalcircuit.


Comparator - A device used in a common system ofground protection in circuit breakers;senses differences in phase currentsand provides a signal that is ultimatelyused to trip a breaker.

Connected draw-out position - The position of a circuit breaker inwhich primary power, control power,and ground are all connected; abreaker’s normal operating position.

Continuous rating - The maximum current load in ampsthat a circuit breaker is designed tocarry continuously at its nominalvoltage.

Control Power - Typically, a low voltage power supplyused for the remote operation of circuitbreakers and instrumentation.

Cubicle - A drawer-like compartment in aswitchgear assembly; houses circuitbreakers or instrumentation.

Cubicle interlock - A device that causes breaker contactsto open before a breaker can be rackedin or out of a switchgear cubicle.Usually, the device is connected to thecontacts through a mechanical linkageand latch mechanism.

Current Transformer - A transformer used to step downprimary circuit current levels forinstrumentation and for various tripdevices.

Current Zero - Points in an AC current sine wavewhere there is no current flow.

Cycling - The closing and opening of circuitbreaker contacts performed as a checkon the breaker’s operating mechanism.

Dashpot - A device, consisting of a piston in afluid chamber, used to provide a timedelay in circuit breaker tripping.

Delayed trip test - A test in which a breaker is checked to


see if it will carry a predeterminedoverload current for a specified periodof time before tripping.

Derating - The reducing of breaker ratings for usein DC circuits.

Disconnected down-out - The position of a circuit breaker inwhich position primary power, controlpower, and ground are alldisconnected; the position in which abreaker can be removed from itscubicle.

Draw-out positions - The positions through which a circuitbreaker passes as it is being rackedout of a cubicle.

Electromagnetic trip device - A trip device that is activated byexcessive current flow through a coilinstalled in series with the primarypower circuit.

Frame breaker - See power circuit breaker.

Front section - Typically, the section of a switchgearassembly that contains power circuitbreakers, instrumentation, andconnections to primary power, controlcircuits, and ground.

Gap - The distance between the movableand stationary main contacts as the arccontacts touch; also, any distance thatremains between the arc contacts if allpoles of the breaker fail to makecontact simultaneously.

Ground - A current path to ground where thereshould not be one; usually caused by abreakdown of insulation; also used torefer to intentional paths to groundinstalled to protect equipment andpersonnel.

Instantaneous trip test - A test in which a breaker is checked tosee if it will at or below its ratedinterrupting capacity.


Insulated fin arc chute - A device, usually made of a ceramicmaterial, which extinguishes an arc byforcing current to travel a longerdistance around a set of insulated fins.

Interrupting capacity (current) - The maximum current value that acircuit breaker is designed to interruptat its nominal voltage.

Latching mechanism - A mechanism that holds a set ofcontacts in a circuit breaker closed oropened.

Line-side (components) - Components associated with theincoming primary power of aswitchgear assembly.

Load-side (components) - Components associated with primarypower leaving a switchgear assembly.

Main contacts - The contacts that carry primary powerthrough a circuit breaker when thecircuit is operating normally.

Metal fin arc chute - A device, containing a set of metal finsmounted in an insulated shell, that canextinguish an arc by splitting it intosmaller pieces and creating amagnetic induction to oppose thevoltage of the arc.

Moulded-case circuit - A small, self-contained circuit breakerwith breaker the working partsenclosed in an insulated case.

Nominal voltage - The maximum voltage at which acircuit breaker is designed to be usedunder normal circuit conditions.

Operating mechanism - A device in a circuit breaker that opensor closes the breaker contacts.

Overload - A controlled amount of excessivecurrent flow through an electricalcircuit.

Poles - Another name for the contactsassociated with any one phase of acircuit breaker.


Power circuit breaker - A large circuit breaker that is usuallyenclosed in a metal frame.

Primary disconnect fingers - A set of spring-loaded fingers thatconnect a power circuit breaker to thebus section of a switchgear assembly.

Primary power - The main power circuit in a plant.

Racking a breaker out - The action of removing a power circuitbreaker from a switchgear cubicle.

Secondary disconnect fingers - A set of spring-loaded fingers thatconnect a power circuit breaker to asource of control power.

Short - An accidental complete circuit ofminimum resistance and maximumcurrent flow.

Shunt trip - An electrical device that, uponreceiving a signal from a control powersource, automatically trips a circuitbreaker.

Slow-close handle - A special maintenance handle that canbe used to slowly operate a powercircuit breaker so that the breakercontacts can be observed as theyclose.

Splitter - See metal fin arc chute.

Switchgear - An assembly of circuit breakers andrelated components that are housed insingle enclosure.

Test draw-out position - The position of a circuit breaker inwhich control power and ground areconnected, but primary power is not.

Thermal element trip device - A trip device, typically consisting of abimetallic strip, that acts to trip abreaker when the heat generated bycurrent flow through the breakerbecomes excessive.

Trip - Any action that causes the contacts ina circuit breaker to open.

Trip device - A device that senses electrical circuitproblems and activates the latchingmechanism to open a circuit breaker’scontacts.


Wipe - The amount of surface area actuallymaking electrical contact as themovable and stationary arc contactstouch one another during breakerclosing : also refers to the rubbing orpolishing effect as the contacts presstogether.

Earth Mat - The earth mat is a mesh of steel pipesor rods laid at a depth of 0.5 mtr. in theentire sub-station area (excludingfoundations).

Touch Potential - Touch potential is defined as thepotential between the fingers of araised hand (2 mtrs. from ground)touching a structure and the feet.

During an earth fault, the fault currentflows from the fault structure to earththrough the earthing. The line toground voltage gets distributed no-uniformly from fault point to the ground.The voltage between touch point (2Mtr. height) and the ground should bewithin safe limits (100 V).

Step Potential - The step potential is defined as thepotential difference between two stepsof a person standing on the groundwith feet apart during the flow of theearth fault current. The step potentialshould be within safe limits (say 100V).The earth mesh should be uniform inthe entire switch board & should havesufficient low earth resistance.

Resistance of Earthing System -

Rated voltage Max. earth resistance

Below 1kv 6 ohms

upto 36 kv 2 ohms

above 36 kv 0.5 ohms

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elect. auth

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