Circuit Breaker

Circuit Breaker

A circuit breaker is an automatically operated electrical switch device which is designed to protect an electrical circuit from damages caused by overload or short circuit.

A complex electrical power system needs some form of switchgear in order to control and regulate it safely and efficiently in both normal and abnormal condition. A circuit breaker serves the function of a switch with a fuse but has more complex features.

Broadly the application of a circuit breaker can be divided into two groups:

(A) Protective application In protective applications, the circuit breaker opens the contacts and interrupts the flow of current under abnormal condition. E.g. Overcurrent protection, Thermal overload protection etc.

(B) Automotive application Whereas in automotive applications, the circuit breaker makes and breaks the contacts when a small signal energizes the operating coil of C.B. under normal working conditions. E.g. - Motor control circuits

In both kinds of application, the basic function of a circuit breaker is thus separation of contacts in an insulating fluid which serves two functions:

1) It extinguishes the arc drawn between the contacts when the circuit breaker opens. 2) It provides adequate insulation between the contacts and from each contact to earth.

Many insulating fluids are used for arc extinction and the fluid chosen depends upon the rating and type of circuit breaker. Commonly used fluids are:

1) Air at atmospheric pressure 2) Compressed air 3) Oil which produces hydrogen for arc extinction 4) Ultra high vacuum 5) Sulphur hexafluoride (SF6)

The gases considered for C.B. must possess following properties:

(i) High dielectric strength (ii) Thermal and chemical stability (iii) Non- inflammability (iv) High thermal conductivity: - This assist cooling of current carrying conductor. (v) Arc extinguishing ability (vi) Commercial availability at moderate cost

Of all the gases mentioned above air is the cheapest and most widely used for circuit breaking.

In a Circuit Breaker, when the contacts are opened or closed, the interruption of current causes a very high voltage to be developed across the contacts and this high voltage consequently ionizes the medium between the contacts and arc is produced.

Composition of arc:

The arc produced inside the C.B. depends upon the material of contacts and the insulating medium.

ARC IN OIL:

In an oil C.B., the heat of the oil decomposes the oil which boils at 658K. The gases liberated are approx. (1) hydrogen 70%; (2) Acetylene 20%; (3) Methane 5%; and Ethylene 5%. The temperature of the arc is too high for the last three gases to exist and the arc itself runs into the mixture of hydrogen, carbon and copper vapor at temp. above 6000K. Hydrogen which itself is a diatomic gas

RATING OF CIRCUIT BREAKERS:

The applications of circuit breakers are in vastly differing circumstances. It is rated in terms of (i) no. of poles, (ii) rated voltage and current, (iii) rated frequency, (iv) rated making capacity, (v) rated symmetrical and asymmetrical breaking capacities, (vi) short term rating and (vii) operating duty.

(i) No. of poles: The no. of poles per phase of a breaker is a function of the operating

voltage.

(ii) Rated voltage: The voltage levels at various points in a system vary depending upon the

system condition and as a result the breaker has to operate under such variable

voltage conditions. The breaker is normally operated at a rated voltage (maximum

rms value of voltage) higher than normal operating voltage.

(iii) Rated current: The rated current of a circuit breaker is the maximum rms value of current

which it shall carry continuously without exceeding the temp. limit of various parts of

the breaker.

(iv) Rated frequency: The rated frequency is the frequency at which the beaker is designed

to operate.

(v) Making Current: The making current is the peak value of the maximum current loop,

including DC component, in any phase during the first cycle of current when the

circuit breaker is closed. The capacity of a breaker to make currents depends upon

its ability to withstand and to close successfully against the effect of electromagnetic

forces. The maximum force in any phase is a function of the square of the maximum

instantaneous current occurring in that phase on closing. It is therefore; practice to

specify making current in terms of peak value rather than in terms of rms value. The

making capacity is, therefore, specified by the product of the making current it can

make and carry instantaneously at the rated service voltage. In a particular phase,

the current is maximum right at the instant short circuit takes place. The current in

the first one or two cycles (depending upon the time constant of damper winding) is

known as sub transient current and in the next 8 to 10 cycles it is known as transient

current and finally when the effect of both damper and field winding dies down the

current is known as steady current.

In case the symmetrical breaking current is known, making current can be obtained

by multiplying this current by 2 to get to the peak value and again by 1.8 to include the doubling effect (i.e. D.C. component at first peak is almost equal to the a.c.

component).

The breaking current of a breaker depends upon the instant on the current wave

when contacts begin to separate.

The breaking capacity of a breaker is the product of breaking current and the

recovery voltage.

(vi) Short time rated current: It is the current that can be safely applied, with the C.B. in its

normal conditions, for 3 seconds, if the ratio of symmetrical breaking current to the

normal current is less than 40 or for 1 second otherwise.

Components of Circuit Breaker:

Frame:

Provides insulated space for mounting the components & holding them in place.

Must withstand mechanical stresses Provides insulation & isolation path for current

Contacts:

Provides a method for connecting the circuit with the system Forms a part of the isolation circuit.

Basic functions:

Opening Closing

Operation: Opening

Spring is generally used for the purpose. Opening process is faster. Spring gets recoiled during Closing Operation.

Operation: Closing

Solenoid is generally used for the purpose. Closing process is slower. Force of attraction increases with reduction in distance.

Trip unit:

Opens the circuit during Thermal overload Short circuit Ground Fault

Arc extinguisher:

Circuit Breakers differ in the type arc extinguishers used in them. Extinguishes an arc when contacts are opened.

Construction of circuit breaker:

Classification of Circuit Breaker: (1) Based on operational voltage

(i) Low voltage circuit breakers;.E.g. Airs break circuit breaker. (ii) Medium voltage circuit breakers; E.g. Vacuum circuit breaker. (iii) High voltage circuit breakers; E.g. SF6 circuit breaker.

(2) Based on arc quenching medium used (i) Air circuit breaker. (ii) Oil Circuit breaker. (iii) SF6 Circuit Breaker. (iv) Vacuum circuit breaker.

Tests on circuit breaker:

A Circuit Breaker should be capable of carrying, making, and breaking under normal and abnormal conditions. In any power system circuit breaker has to withstand power frequency over voltages and transient over voltages due to switching and lightning.

The performance of a circuit breaker under normal and abnormal conditions can be verified by performing different type of tests on circuit breakers. The main purpose of testing of circuit breakers is to confirm if circuit breaker is able to work on particular voltage and current ratings or not.

First an understanding of the various terms and definitions used in testing of circuit breakers are very important. A refresher of some of these terms is as listed below:

Timing of Breakers Definitions:

Opening Time O: Generally opening time of a circuit breaker is the time between the instant of energizing the opening release of a circuit breakers, which is I closed position to the instant when arcing contact of the all the poles have separated. The opening time shall include all such auxiliary equipment necessary to open the circuit breakers and forming an integral part of the circuit breaker. This is applicable to all circuit breakers, which are tripped & operated by any form of auxiliary power. However for self tripping circuit breakers the opening time is the time between the instant at which the current in the main circuit reaches the operating value of the over current release to the instant of separation of the arcing contacts.

Breaking Time:

Interval of time between the beginning of the opening time of the mechanical switching device and the end of the arcing time.

Closing Time C:

Interval of time between energizing the closing circuit of the circuit breaker, which is in open position to the instant when the contacts touch in all poles. The closing time shall include all such auxiliary equipment necessary to close the circuit breaker and forming an integral part of the circuit breaker.

Making Time:

Interval of time between energizing the closing circuit of the circuit breaker being in open position and the instant when the current begins to flow in the first pole.

Close Open time CO:

Interval of time between the instant when the contacts touch in the first pole during a closing operation and the instant when the arcing contacts have separated in all poles during the subsequent opening operation. It is assumed that the opening

releases incorporated in the circuit breaker is energized at the instants when the contacts touch the first pole during closing. This represents the minimum close-open time.

Make Break Time: Interval o time between the initiation of current flow in the first pole during a closing operation the opening release of the circuit breaker is energized one half cycle (in 50Hz system 10 millisecond) after current begins to flow in the main circuit during making. This time and end of the arcing time during the subsequent opening operation. It is assumed that mayvarydue to the variations in pre-arcing time.

Time spread between Poles:

Time discrepancy between the contacts, which is the interval, time characterizing the divergence fro coincidence of connection or disconnection of the breakers contacts between the poles (non-simultaneous switching). This shall be as low as possible probably less than one peak of a half cycle.

Different types of Tests on Circuit Breakers

There are mainly two tests classified:

1. Type Tests:

The purpose of type tests is to prove design features and the quality of circuit breaker. Type tests are not conducted on each circuit breaker. This is done to prove the capabilities and to confirm the rated characteristics of the circuit breakers.

Type Tests are listed below:

Mechanical endurance tests Thermal tests Dielectric tests Measurement of resistance of the main circuits Short Circuit tests

Dielectric Test includes

Power frequency voltage withstand tests Impulse voltage withstand test

Short Circuit Test includes

Breaking capacity test making capacity test Duty cycle test

short time current withstand test 2. Routine Tests:

Routine test is performed before circuit breaker dispatch to ensure the product. This gives result about defects in materials and construction of circuit breaker. We can check quality of material of circuit breaker by performing Routine Test.

Routine Test are listed below:

Mechanical Operational test One-Minute power frequency voltage dry withstand test Measurement of resistance of main circuits

Circuit Breaker Testing Procedure (how to test a circuit breaker?)

The preliminary preparations of circuit breaker testing include the following:

1. Connect the equipment.

2. Adjust the correct values of resistance and reactors to set the required magnitude of short circuit.

3. Connect and set the transformer to get desired test voltage.

4. The contacts on sequence switch are adjusted to get desired timings.

5. Oscillographs are adjusted and calibrated.

The automatic sequence of operations during a breaking capacity test is as follows:

1. Run the driving motor of the short circuit generator to a certain speed and then it is switched off.

2. Switch on the impulse excitations.

3. Close the master circuit breaker.

4. Switch on the oscillograph.

5. Close the MAKE SWITCH to pass the short circuit current.

6. Open the circuit breaker under test to interrupt the short circuit current at desired moment.

7. Switch ff the exciter circuit of the short circuit generator.

8. The breaking capacity is determined from the oscillograph.

Thus this was the whole procedure for "Circuit Breaker Testing". This takes in a very short time of the order of 0.2 seconds. Because of very short time this whole procedure follow automatically by "sequence switch".

Sequence Switch: Sequence Switch is drum switch with several pairs of contacts. The drum is rotated by a motor to close and open several circuits according to a certain sequence.

Tests on Breaker Operation:

Circuit breakers are an important and critical component in the electric power system. Because of their key role, circuit breakers are to be periodically tested and diagnosed. One of the most successful test methods is the timing test, which consists of measuring the mechanical operation time of the breakers contacts/ Timing tests are important to assess and prevent damages of circuit breaker. In correct operation of circuit breaker can have a disastrous consequences on the equipment connected downstream and safety of substation personnel.

Healthiness of the Contracts of the circuit breaker are also of prime importance. For diagnosing the healthiness of contacts Static & Dynamic contract resistance measurements are carried out. Timely intervention can not only extend the life of the contact but also avoid unscheduled breakdown of the circuit breaker.

Test on auxiliaries equipment forming an integral part for operating the circuit breaker are to be tested as any malfunctioning/defect on these component will affect the operation of the breakers. They include operating coils and recharging mechanisms. Diagnostic test like insulation resistance, coil resistance, current consumption and time of recharging are also out on these auxiliaries. Following are some tests:

Visual Observation: Visual observation shall include the following:

Air Pressure in case of AAABCB/Gas pressure in case of SF6 breakers. Check for contamination near operating releases Check for abnormal sound/vibration during operation, Corrosion on parts of operating mechanism, structures & supports. Any other visible abnormality

Diagnosis of Operating Characteristics of the Circuit Breakers: For regular field diagnosis the following operating characteristics are mentioned.

Closing Time Opening Time Close Open Time (O-0.3 sec-CO if applicable) Time spread between poles Recharging time of the operating device like motor charging, air releases etc. Consumption & recording of currents of the shunt closing releases and shunt

opening releases.

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Mechanical travel characteristics (if feasible) In case of Auto-enclosure OCO timing are also monitored to ensure proper functioning. These given an idea about the healthiness of the breaker operation and the breaker operating mechanism. These values are compared with the pre-commissioning values/guaranteed values. Any change in the timing or consumption will have to be thoroughly investigated and suitable rectification carried out.

It is preferably to carry out at least fifty no load operations of the breaker after any pre-commissioning or major maintenance in which the operating mechanism/replacements of units of one pole have been changed, after which the operating characteristics can be monitored. This is to ensure that all mechanical assemblies & sub-assemblies are in order, after which the operating characteristics are to be carried out which serve as a benchmark for future diagnosis measurements.

Tests on Contacts of Main Circuit:

a. Static Contact Resistance (CRM): Measurement of resistance shall be made on the main circuit. The measurement shall be carried out with the breakers in closed position individually across each breaking contact. The current during the measurement shall have any convenient value, from 50 Adc to the rated current. It shall b ensured that there are no loose connections during measurements, which will lead to higher contact resistance. The measured values shall be compared with the type test results giving consideration. Any increase in contact resistance indicates the unhealthiness of the contacts.

b. Dynamic Contact Resistance Measurement (DCRM): Dynamic contact resistance is an effective technique for assessing the condition of circuit breaker main contacts and arcing contacts. Static contact resistance can be measured with the breaker in closed position, but it does not give any indication of the conditions of the arcing contacts. Excessive wear & tear of the arcing contacts may lead to rupturing of the main contacts which decreases the Circuit Breakers Breaking capacity. To assess this only an internal inspection was done in earlier days. However DCRM up to rated speed were developed to access the condition of the arcing contacts. A current of 100A or above is applied across each of the separating contacts in a breaker pole undergoing Open or Close operation. A resistance plot obtained with the contacts in motion. By assessing these plots the healthiness of the contacts can be established.

Test on Shunt Operating Devices & Auxiliaries: a. Insulation Resistance:

To check the insulation of the operating coils an insulation resistance test is carried out at 500V between the shorted coil terminals and ground. These shall be comparable to the ore-commissioning values.

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b. Coil Resistance:

To check the healthiness of the operating coils conductor resistance test is carried out at 1A across the terminals. These shall be comparable to the pre-commissioning values.

c. Motor Charging Time:

Motors which are employed to charge springs shall operate satisfactorily between 85 to 110% of rated voltage & rated frequency. The consumption and time of recharging shall be measured.

Application of circuit breaker in the Circuit/Power system:

There are many different electrical bus system schemes available but selection of a particular scheme depends upon the system voltage, position of substation in electrical power system, flexibility needed in system and cost to be expensed.

The main criterions to be considered during selection of one particular Bus Bar Arrangement Scheme among others:

(i) Simplicity of system (ii) Easy maintenance of different equipment. (iii) Minimizing the outage during maintenance. (iv) Future provision of extension with growth of demand (v) Optimizing the selection of bus bar arrangement scheme so that it gives maximum return

from the system. Some very commonly used bus bar arrangements are discussed below:

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Single Bus Bar System:Single Bus System is simplest and cheapest one. In this scheme all the feeders and transformer bay are connected to only one single bus as shown.

Advantages of single bus system: (i) This is very simple in design

(ii) This is very cost effective scheme

(iii) This is very convenient to operate

Disadvantages of single bus system: One but major difficulty of these type of arrangement is that, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay. The indoor 11KV switchboards have quite often single bus bar arrangement.

Single Bus System with Bus Sectionalizer Some advantages are realized if a single bus bar is sectionalized with circuit breaker. If there are more than one incoming and the incoming sources and outgoing feeders are evenly distributed on the sections as shown in the figure, interruption of system can be reduced to a good extent.

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Advantages of single bus system with bus sectionalizer: If any of the sources is out of system, still all loads can be fed by switching on the sectional circuit breaker or bus coupler breaker. If one section of the bus bar system is under maintenance, part load of the substation can be fed by energizing the other section of bus bar.

Disadvantages of single bus system with bus sectionalizer: As in the case of single bus system, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay. The use of isolator for bus sectionalizing does not fulfill the purpose. The isolators have to be operated off circuit and which is not possible without total interruption of bus bar. So investment for bus-coupler breaker is required.

Double Bus Bar System In double bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus.Actually every feeder is connected to both of the buses in parallel through individual isolator as shown in the figure.

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By closing any of the isolators one can put the feeder to associated bus. Both of the buses are energized and total feeders are divided into two groups, one group is fed from one bus and other from other bus. But any feeder at any time can be transferred from one bus to other. There is one bus coupler breaker which should be kept close during bus transfer operation. For transfer operation, one should first close the bus coupler circuit breaker then close the isolator associated with the bus to where the feeder would be transferred and then open the isolator associated with the bus from where feeder is transferred. Lastly after this transfer operation he or she should open the bus coupler breaker.

Advantages of Double Bus System: Double Bus Bar Arrangement increases the flexibility of system.

Disadvantages of Double Bus System: The arrangement does not permit breaker maintenance without interruption.

Double Breaker Bus System In double breaker bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus similar to double bus bar system. Only difference is that here every feeder is connected to both of the buses in parallel through individual breaker instead only isolator as shown in the figure. By closing any of the breakers and its associated isolators, one can put the feeder to respective bus. Both of the buses are energized and total feeders are divided into two groups, one group is fed from one bus and other from other bus similar to previous case. But any feeder at any time can be transferred

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from one bus to other. There is no need of bus coupler as because the operation is done by breakers instead of isolator. For transfer operation, one should first close the isolators and then the breaker associated with the bus to where the feeder would be transferred and then he or she opens the breaker and then isolators associated with the bus from where feeder is transferred.

One and a half Breaker Bus System This is an improvement on the double breaker scheme to effect saving in the number of circuit breakers. For every two circuits only one spare breaker is provided. The protection is however complicated since it must associate the central breaker with the feeder whose own breaker is taken out for maintenance. For the reasons given under double breaker scheme and because of the prohibitory costs of equipment even this scheme is not much popular. As shown in the figure that it is a simple design, two feeders are fed from two different buses through their associated breakers and these two feeders are coupled by a third breaker which is called tie breaker. Normally all the three breakers are closed and power is fed to both the circuits from two buses which are operated in parallel. The tie breaker acts as coupler for the two feeder circuits.

During failure of any feeder breaker, the power is fed through the breaker of the second feeder and tie breaker, therefore each feeder breaker has to be rated to feed both the feeders, coupled by tie breaker.

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Advantages of One and a half Breaker Bus System During any fault on any one of the buses, that faulty bus will be cleared instantly without interrupting any feeders in the system since all feeders will continue to feed from other healthy bus.

Disadvantages of One and a half Breaker Bus System This scheme is much expensive due to investment for third breaker.

Main and Transfer Bus System

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This is an alternative of double bus system. The main conception of Main and Transfer Bus System is, here every feeder line is directly connected through an isolator to a second bus called transfer bus. The said isolator in between transfer bus and feeder line is generally called bypass isolator. The main bus is as usual connected to each feeder through a bay consists of circuit breaker and associated isolators at both side of the breaker. There is one bus coupler bay which couples transfer bus and main bus through a circuit breaker and associated isolators at both sides of the breaker. If necessary the transfer bus can be energized by main bus power by closing the transfer bus coupler isolators and then breaker. Then the power in transfer bus can directly be fed to the feeder line by closing the bypass isolator. If the main circuit breaker associated with feeder is switched off or isolated from system, the feeder can still be fed in this way by transferring it to transfer bus.

Switching operation for transferring a feeder to transfer bus from main bus without interruption of power (i) First close the isolators at both side of the bus coupler breaker.

(ii) Then close the bypass isolator of the feeder which is to be transferred to transfer bus.

(iii) Now energize the transfer bus by closing the bus coupler circuit breaker from remote.

(iv) After bus coupler breaker is closed, now the power from main bus flows to the feeder line through its main breaker as well as bus coupler breaker via transfer bus.

(v) Now if main breaker of the feeder is switched off, total power flow will instantaneously shift to the bus coupler breaker and hence this breaker will serve the purpose of protection for the feeder.

(vi) At last the operating personnel open the isolators at both sides of the main circuit breaker to make it isolated from rest of the live system.

So it can be concluded that in Main & Transfer Bus System the maintenance of circuit breaker is possible without any interruption of power. Because of this advantage the scheme is very popular for 33KV and 13KV system.

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Double Bus System with Bypass Isolators

This is combination of the double bus system and main and transfer bus system. In Double Bus System with Bypass Isolators either bus can act as main bus and second bus as transfer bus. It permits breaker maintenance without interruption of power which is not possible in double bus system but it provides all the advantages of double bus system. It however requires one additional isolator (bypass isolator) for each feeder circuit and introduces slight complication in system layout. Still this scheme is best for optimum economy of system and it is best optimum choice for 220KV system.

Ring Bus System The schematic diagram of the system is given in the figure. It provides a double feed to each feeder circuit, opening one breaker under maintenance or otherwise does not affect supply to any feeder. But this system has two major disadvantages. One as it is closed circuit system it is next to impossible to extend in future and hence it is unsuitable for developing system. Secondly, during maintenance or any other reason if any one of the circuit breaker in ring loop is switch of reliability of system becomes very poor as because closed loop becomes opened. Since, at that moment for any tripping of any breaker in the open loop causes interruption in all the feeders between tripped breaker and open end of the loop.

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Erection, Testing and Commissioning of Circuit Breakers: General Instructions: 1.1 The circuit breakers are to be erected, tested and commissioned as per the instructions

given in the Erection, Installation & Commissioning Manual of the Manufacturer.

1.2 The services of the Manufacturers Erection & Commissioning Engineer may be utilized

wherever required.

1.3 Bending of compressed air piping, if required, shall be done in a manner such that the inner

diameter of the pipe is not reduced. This should be done through cold bending using a

bending machine only.

1.4 Cutting of the pipes, wherever required, shall be done such that there is no flaring of the

ends of the pipe. Only a proper pipe cutting tool shall be used.

1.5 The SF6 gas should be filled in one pole at a time to ensure that gas is filled in all the three

poles.

The procedure / steps generally followed for erection of Circuit Breakers are given below: 2.0 Erection of Supporting Structures: 2.1 Assemble the supporting structure(s) if the members are received in loose condition.

2.2 Erect the supporting structure(s) on the foundation and carry out their levelling, centering

and grouting.

2.3 Level the top of the already erected supporting structure(s) and check their verticality.

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3.0 Preparation & Checking of the Circuit Breakers Poles: 3.1 Clean the insulators of the breaker poles and check for cracks in the insulators.

3.2 In case of SF6 Circuit Breakers, check that there is positive pressure of the SF6 gas in the

breaker poles, and the support columns if provided, by opening the cover of the pipe

connection and pressing the non return valve.

3.3 In case of Vacuum Circuit Breakers, check the vacuum in the individual poles as follows:

a) Remove the linkage of the individual pole from the operating lever.

b) Pull the moving contact rod of each interrupter manually towards OPEN position

during which operation appreciable force should be encountered. Release the moving

contact rod suddenly. It should return automatically to CLOSED position with a

loud metallic noise.

c) Reconnect the linkage of the individual pole to the operating lever.

4.0 Erection of Circuit Breaker Poles: 4.1 In case of CBs received with poles already fitted on a common base channel, erect the

base channel along with breaker poles on the supporting structures.

4.2 In case of CBs with common base channel but in which the poles are received separately,

first erect the base channel on the supporting structure(s) and carry out levelling.

Thereafter, erect the poles on the base channel. Erect the middle pole first followed by the

side poles.

4.3 In case of CBs in which the poles are to be erected on the operating mechanism, first erect

the operating mechanisms on the supporting structures and carry out levelling. Thereafter,

erect the CB poles on the operating mechanisms.

4.4 In case of other types of CBs, erect the CB poles on the supporting structures.

4.5.1In case of CBs with horizontal arc chambers, first erect the support columns on the

supporting structures.

4.5.2 Fit the pre insertion resistors and voltage grading capacitors, if provided, on the arc

chambers.

4.5.3 Thereafter erect the arc chamber assemblies on the support columns of individual phases.

4.6 Check the verticality of the erected poles of the CBs and correct the same wherever

required.

4.7 Erect the operating mechanism / operating drive on the supporting structure of the

designated phase, or on all the three individual phases / poles, or on the common structure,

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or on the common base channel, as applicable.

4.8 Fit the terminal connectors on the three poles of the breaker. 5.0 Erection of Operating Mechanism, Accessories & Associated Equipment: 5.1 In case of ground mounted control cubicle, erect it on its foundation and carry out its

centering, levelling and grouting. In other cases, erect the control cubicle on the supporting

structure / base channel as provided.

5.2 Place the air compressor, if provided separately, on its foundation & carry out its centering,

levelling and grouting.

5.3 In case of ground mounted operating mechanism, erect it on its foundation and carry out its

centering, levelling and grouting. In other cases, erect the operating mechanism on the

supporting structure / base channel as provided.

5.4 In case of circuit breakers up to 33 kV, the operating mechanism is generally received

mounted on the assembled structure. 6.0 For Gang Operated Circuit Breaker: 6.1 Connect the operating shaft / rod between the poles and between the first pole and the

mechanism, or as applicable.

6.2 IMPORTANT: DO NOT CHANGE OR ADJUST THE LENGTH OF THE OPERATING

SHAFT / ROD.

6.3 Fit the protective covers for operating shaft / rod, if provided. 7.0 Gas Filling in SF6 Gas Circuit Breakers: 7.1 CBs with Common SF6 Gas Pipeline:

7.1.1 Fit the SF6 gas density monitor on the support structure / base channel as provided and

connect to the SF6 gas pipeline.

7.1.2 Connect the SF6 gas pipeline to one pole & fill SF6 gas in the pole up to about 2 kg / cm2.

7.1.3 Check for leakage of SF6 gas in the pole and in the SF6 gas pipeline. Attend to the

leakages,if any. Fill SF6 gas in the pole up to the prescribed rated / filling pressure. The

filling pressure should correspond to the ambient temperature at the time of filling as per

the chart given in the manufacturers manual. The setting and operation of the lockout

contacts (closing) and the alarm contact (opening), provided in the density monitor, are

checked during SF6 gas filling.

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7.1.4 Connect the SF6 gas pipeline to the second pole. The gas pressure in the first pole will

fall.

7.1.5 Check for leakage of SF6 gas in the second pole. Attend to the leakages, if any.

7.1.6 Fill the SF6 gas in the two poles simultaneously up to the prescribed rated / filling

pressure.

7.1.7 Connect the SF6 gas pipeline to the third pole.

7.1.8 Check for leakage of SF6 gas in the third pole. Attend to the leakages, if any.

7.1.9 Finally fill SF6 gas in all the three interconnected poles up to prescribed rated / filling

pressure.

7.1.10 The procedure from para 7.1.4 to para 7.1.9 ensures that SF6 gas is filled in all the three

poles.

7.1.11 Ensure that the alarm and lockout contacts are in normal condition after SF6 gas filling.

7.2 CBs with SF6 Gas Density Monitors on Individual Poles:

7.2.1 Fit the SF6 gas density monitor on each pole alongwith pipeline if provided.

7.2.2 Fill SF6 gas in each pole one by one up to about 2 kg / cm2. Check for leakage of SF6

gas in the poles and in the SF6 gas pipeline. Attend to the leakages, if any.

7.2.3 Fill SF6 gas up to prescribed rated / filling pressure in each pole one by one. The filling

pressure should correspond to the ambient temperature at the time of filling as per the

chart given in the manufacturers manual. Check operation of the alarm and lockout

contacts provided in the density monitor during SF6 gas filling.

7.2.4 Check for leakage of SF6 gas from all the points (e.g., joints, couplings, cementing on

insulator & metallic joints, brazing, etc.) and attend if any leakage is detected.

7.2.5 Ensure that the alarm and lockout contacts are in normal condition after SF6 gas filling.

8.0 Circuit Breakers having Pneumatic Operated Mechanism:

8.1 Fabricate the compressed air pipeline as per pipe layout drawing & clean it. The pipeline is

first cleaned by passing a mulmul cloth through it with the help of a wire. Thereafter, one

end of the pipeline is connected to the air compressor and the other end is plugged. A small

air pressure is injected in the pipeline and then suddenly released by removing the plug so

that the pipeline is flushed.

8.2 Fit the pipeline as per drawing.

8.3 Check phase sequence of the A.C. supply to the air compressor and check direction of

rotation of air compressor motor. Check and top up oil in the air compressor. Also check

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that the V belt is at the correct tension.

8.4 Pressurize the compressed air pipeline and air storage tank(s) to about 5 kg / cm2 and

check for air leakage from all possible points (e.g., control blocks, joints, couplings, brazing,

etc.) and attending to the leakages if detected.

8.5 Pressurize the compressed air pipeline to the rated pressure. During the rising pressure,

check the operation of pressure switches and verify their settings, as per manufacturers

recommendations, for Low Air Pressure Lockout, Auto Reclose Blocking, Low Air

Pressure Alarm & Compressor Stop.

8.6 Switch off the A. C. supply to the compressor. Open the drain valve on the air storage tank

such that air is released slowly. During the falling pressure, check the operation of pressure

switches and verify their operating and differential settings, as per manufacturers

recommendations, for Compressor Start, Low Air Pressure Alarm, Auto Reclose

Blocking & Low Air Pressure Lockout. Adjust the pressure settings if required.

8.7 Check the auto / manual operation of air compressor.

8.8 Start the compressor in the manual mode and build up pressure. Check operation of safety

valve and verify its setting. If the safety valve does not operate even when the prescribed

pressure is exceeded, stop the air compressor and adjust the setting to the required value

and re verify this setting. Drain air so as to maintain normal pressure in the storage

tank(s).

9.0 Circuit Breakers having Hydraulic Oil Operated Mechanism:

9.1 Clean all the prefabricated hydraulic oil pipes by passing a mulmul cloth through them with

the help of a wire.

9.2 Fit the prefabricated hydraulic oil pipeline as per drawing.

9.3 Fill hydraulic oil, of the designated grade as supplied with the Circuit Breaker, in the

storage tank(s) up to the fill mark.

9.4 Check phase sequence of the A.C. supply to the oil pump motor and check its direction of

rotation.

9.5 Start the oil pump motor and release air from the venting screw(s) provided. Build up

pressure in the hydraulic oil pipeline. Top up oil, as and when required, to maintain oil level

in the storage tank.

9.6 During increasing pressure, check operation of pressure switches and verify their settings

for Low Oil Pressure Lockout, Auto Reclose Lock out, Low Oil Pressure Alarm &

Oil Pump Stop.

9.7 Check for oil leakage from all the points (e.g., joints, couplings, brazing, etc.) and attend to

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the leakages if detected.

9.8 Open bypass valve to reduce the oil pressure. During falling pressure, check operation of

pressure switches and verify their settings for Oil Pump Start, Low Oil Pressure Alarm,

Auto Reclose Lock out, and Low Oil Pressure Lockout.

9.9 Check the auto / manual operation of the oil pump.

9.10 Start the oil pump in the manual mode and build up pressure. Check operation of safety

valve and verify its setting. If the safety valve does not operate even when the prescribed

pressure is exceeded, stop the oil pump motor and adjust the setting to the required value

and re verify this setting. Open bypass valve to reduce the oil pressure to normal.

10.0 Circuit Breakers having Spring Operated Mechanism:

10.1 ENSURE THAT THE CLOSING SPRING IS FULLY DISCHARGED. If it is not fully

discharged, then discharge the spring as per instructions given in the manufacturers

manual.

10.2 Carry out slow mechanical operation (closing and tripping) of Circuit Breaker as per

procedure prescribed by the manufacturer. Take all the precautions mentioned in the

manufacturers manual.

10.3 Manually charge the closing spring and check electrical limit switch, mechanical latches

and stopper(s) as provided.

10.4 Discharge the spring as per directions in the manufacturers manual.

10.5 Charge the spring electrically and verify the operation of the limit switch. Adjust the setting

of the limit switch if required.

11.0 Cabling & Wiring:

11.1 Carry out laying of cables between the following:

i) Operating mechanisms of individual phases (R, Y & B) and control cubicle, if

applicable.

ii) Compressor and control cubicle.

iii) Density Monitor and control cubicle.

iv) Control cubicle and Control & Relay Panel.

v) Control cubicle and bay marshalling kiosk.

11.2 Fix the cables in cable glands and then fix the cable glands on cable gland plates in the

respective equipment.

11.3 Connect the cables as per schematic diagram of the circuit breaker. The following typical

connections are made at the circuit breaker end:

a) DC positive & DC negative for local operation.

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b) DC positive for remote closing.

c) DC positive for remote tripping.

d) Remote closing signal.

e) Remote tripping signal.

f) Protection trip signal.

g) Trip circuit supervision.

h) ON / OFF indications (Lamp & Semaphore).

i) Auto Trip / Spring Charged Lamp indication.

j) Air pressure / Oil pressure / Spring charging limit switch contacts for auto reclose

blocking.

k) Contacts of pressure switches for annunciations of low SF6 gas / air / oil pressure

alarms & lockout conditions and for loss of N2 pressure.

l) Contact for annunciation of pole discrepancy trip alarm.

m) Auxiliary contacts as required for various control circuits.

11.4 Dress and fix the cables in cable trays / trenches / supports / brackets.

12.0 PRE COMMISSIONING CHECKS:

12.1 Check the following in the Air Compressor:

i) Oil level is upto the mark.

ii) Oil colour is not black.

iii) Air filter is clean.

iii) V belt is properly tensioned.

12.2 Check SF6 gas / hydraulic oil / air leakages, as applicable, and attend if required.

12.3 Check the pressure of SF6 gas / hydraulic oil / air in the circuit breaker, as applicable.

12.4 Check clamping of the pipe line for SF6 gas / hydraulic oil / air, as applicable.

12.5 Check the oil level in the oil storage tank(s) of all the three poles of CBs with hydraulic oil

operated mechanism. The oil level should be between the maximum & minimum level

marks, otherwise, top up with oil.

12.6 Check the contact wear indication mark or the specified gap as given in the instruction

manual in case of Vacuum Circuit Breakers with the circuit breaker in the closed position.

12.7 Lubricate all the moving parts and the pins in the operating mechanism.

12.8 Check settings of air / oil pressure switches.

12.9 Check resistance of closing and tripping circuits.

12.10 Check and adjust the resistance in the closing / tripping coil circuits, if required.

12.11 Check alarm annunciations in the C&R Panel for low SF6 gas / hydraulic oil / air pressure

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alarm, SF6 gas / hydraulic oil / air pressure lockout and loss of N2, as applicable.

12.12 Check closing and opening operation of circuit breaker from local, remote and protection.

Check closing of Circuit Breaker through auto reclose scheme, if provided. Confirm that

the correct pole of the Circuit Breaker has operated.

12.13 Check operation of dash pot / damper. It should damp the speed of the Circuit Breaker at

the end of both closing and tripping operations.

12.14 Check functioning of operation counter.

12.15 Check operation of anti pumping / hunting relay by giving continuous closing and

tripping signals simultaneously. The breaker should close and then trip and should not

close again.

12.16 Check operation of pole discrepancy relay, if provided, and its alarm annunciation in the

C&R Panel.

12.17 Check lamp and semaphore indications, as applicable, in the C&R Panel for CB OPEN,

CLOSED, AUTO TRIP and SPRING CHARGED conditions.

12.18 Check the Trip Circuit Supervision circuits in both CB open and closed conditions by

removing the wires of the tripping circuit.

12.19 Check functioning of space heater and internal illumination circuits.

12.20 Check earthing of the poles, base channel, control cubicle, operating mechanism,

compressor and structure(s), as applicable.

13.0 PRE COMMISSIONING TESTS:

13.1 Measure insulation resistance with 5 kV megger of all the three phases between lower

terminal to earth and between upper and lower terminals with the breaker in the open

position.

13.2 Measure Closing (C), Opening (O) and Close Open (CO) operation timings of the

breaker with CB Timer or CB Analyser.

13.3 Test the operation of the CB with the emergency tripping arrangement, if provided.

13.4 Manufacturers of Vacuum Circuit Breakers recommend the following method for testing

the vacuum in the interrupters with the circuit breaker in OPEN condition:

Using high voltage testing equipment, apply the voltage as given below across the upper

& lower terminals of the VCB for 60 seconds.

(a) 33 kV CB - 70 kV

(b) 11 kV CB - 28 kV

The vacuum interrupters should withstand the applied voltage.

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Maintenance of Circuit Breaker Importance of adequate maintenance:

The maintenance of circuit breakers deserves special consideration because of their importance for routine switching and for protection of other equipment. Electric transmission system breakups and equipment destruction can occur if a circuit breaker fails to operate because of a lack of preventive maintenance. The need for maintenance of circuit breakers is often not obvious as circuit breakers may remain idle, either open or closed, for long periods of time. Breakers that remain idle for 6 months or more should be made to open and close several times in succession to verify proper operation and remove any accumulation of dust or foreign material on moving parts and contacts.

MAINTENANCE OF MOLDED CASE CIRCUIT BREAKERS

FREQUENCY OF MAINTENANCE.-Molded case circuit breakers are designed to require little or no routine maintenance throughout their normal lifetime. Therefore, the need for preventive maintenance will vary depending on operating conditions. As an accumulation of dust on the latch surfaces may affect the operation of the breaker, molded case circuit breakers should be exercised at least once per year. Routine trip testing should be performed every 3 to 5 years.

ROUTINE MAINTENANCE TESTS.-Routine maintenance tests enable personnel to determine if breakers are able to perform their basic circuit protective functions. The following tests may be performed during routine maintenance and are aimed at assuring that the breakers are functionally operable. The following tests are to be made only on breakers and equipment are deenergized.

Insulation resistance test.- A megohmmeter may be used to make tests between phases of opposite polarity and from current-carrying parts of the circuit breaker to ground. A test should also be made between the line and load terminals with the breaker in the open position. Load and line conductors should be disconnected from the breaker under insulation resistance tests to prevent test mesurements from also showing resistance of the attached circuit. Resistance values below 1 megohm are considered unsafe and the breaker should be inspected for possible contamination on its surfaces.

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Millivolt drop test.- A millivoltdrop test can disclose several abnormal conditions inside a breaker such as eroded contacts, contaminated contacts, or loose internal connections. The millivolt drop test should be made at a nominal direct-current voltage at 50 amperes or 100 amperes for large breakers, and at or below rating for smaller breakers. The millivolt drop is compared against manufacturer's data for the breaker being tested.

Connections test.- The con-nections to the circuit breaker should be inspected to determine that a good joint is present and that overheating is not occurring. If overheating is indicated by discoloration or signs of arcing, the connections should be removed and the connecting surfaces cleaned.

Overload tripping test.- The proper action of the overload tripping components of the circuit breaker can be verified by applying 300 percent of the breaker rated continuous current to each pole. The significant part of this test is the automatic opening of the circuit breaker and not tripping times as these can be greatly affected by ambient conditions and test conditions.

Mechanical operation.- The mechanical operation of the breaker should be checked by turning the breaker on and off several times.

MAINTENANCE OF LOW-VOLTAGE CIRCUIT BREAKERS FREQUENCY OF MAINTENANCE.-Low-voltage circuit breakers operating at 600 volts alternating current and below should be inspected and maintained very 1 to 3 years, depending on their service and operating conditions. Conditions that make frequency maintenance and inspection necessary are:

(a) High humidity and high ambient temperature. (b) Dusty or dirty atmosphere. (c) Corrosive atmosphere. (d) Frequent switching operations. (e) Frequent fault operations. (f) Older equipment. (g) A breaker should be inspected and maintained if necessary whenever it has interrupted

current at or near its rated capacity.

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MAINTENANCE PROCEDURES. -Manufacturer's instructions for each circuit breaker

should be carefully read and followed. The following are general procedures that should be

followed in the maintenance of low-voltage air circuit breakers:

(a) An initial check of the breaker should be made in the TEST position prior to withdrawing it from to enclosure.

(b) Insulating parts, including bush-ings, should be wiped clean of dust and smoke. (c) The alignment and condition of themovable and stationary contacts should be checked

and adjusted according to the manufacturer's instruction book.

(d) Check arc chutes and replacesany damaged parts. (e) Inspect breaker operating mecha-nism for loose hardware and missing or broken cotter

pins, etc. Examine cam, latch, and roller surfaces for damage or wear.

(f) Clean and relubricate operatingmechanism with a light machine oil (SAE-20 or 30) for pins and bearings and with a nonhardening grease for the wearing surfaces of cams,

rollers, etc.

(g) Set breaker operating mechanismadjustments as described in the manufacturer's instruction book. If these adjustments cannot be made within the specified tolerances, it

may indicate excessive wear and the need for a complete overhaul.

(h) Replace contacts if badly worn orburned and check control device for freedom of operation.

(i) Inspect wiring connections fortightness.

(j) Check after servicing circuit breakerto verify the contacts move to the fully opened and fully closed positions, that there is an absence of friction or binding, and that electrical

operation is functional.

MAINTENANCEOF MEDIUM-VOLTAGE CIRCUIT BREAKERS FREQUENCY OF MAINTENANCE.-Medium-voltage circuit breakers which operate in the range of 600 to 15,000 volts should be inspected and maintained annually or after every 2,000 operations, whichever comes first. The above maintenance schedule is recommended by the applicable standards to achieve required performance from the breakers.

SAFETY PRACTICES.- Maintenance procedures include the safety practices indicated in the ROMSS (Reclamation Operation & Maintenance Safety Standards) and following points that require special attention:

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(a) Be sure that breaker and its mechanism are disconnected from all electric power, both

high voltage and control voltage, before it is inspected or repaired.

(b) Exhaust the pressure from air receiver of any compressed air circuit breaker before it is

inspected or repaired.

(c) After the circuit breaker has been disconnected from the electrical power, attach the

grounding leads properly before touching any of the circuit breaker parts.

(d) Do no lay tools down on the equipment while working on it as they may be forgotten

when the equipment is placed back in service.

MAINTENANCE PROCEDURES FOR MEDIUM-VOLTAGE AIR CIRCUIT BREAKERS.-

The following suggestions are for use in conjunction with manufacturer's instruction books

for the maintenance of medium-voltage air circuit breakers:

(a) Clean the insulating parts including the bushings.

(b) Check the alignment and condition of movable and stationary contacts and adjust them

per the manufacturer's data.

(c) See that bolts, nuts, washers, cotter pins, and all terminal connections are in place and

tight.

(d) Check arc chutes for damage and replace damaged parts.

(e) Clean and lubricate the operating mechanism and adjust it as described in the

instruction book. If the operating mechanism cannot be brought into specified tolerances,

it will usually indicate excessive wear and the need for a complete overhaul.

(f) Check, after servicing, circuit breaker to verify that contacts move to the fully opened

and fully closed positions, that there is an absence of friction or binding, and that

electrical operation is functional.

MAINTENANCE PROCEDURES FOR MEDIUM-VOLTAGE OIL CIRCUIT BREAKERS.- The following suggestions are for use in conjunction with the manufacturer's instruction books for the maintenance of medium-voltage oil circuit breakers:

(a) Check the condition, alignment, and adjustment of the contacts.

(b) Thoroughly clean the tank and other parts which have been in contact with the oil.

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(c) Test the dielectric strength of the oil and filter or replace the oil if the dielectric strength is

less than 22 kV. The oil should be filtered or replaced whenever a visual inspection

shows an excessive amount of carbon, even if the dielectric strength is satisfactory.

(d) Check breaker and operating mechanisms for loose hardware and missing or broken

cotter pins, retaining rings, etc.

(e) Adjust breaker as indicated in instruction book.

(f) Clean and lubricate operating mechanism.

(g) Before replacing the tank, check to see there is no friction or binding that would hinder

the breaker's operation. Also check the electrical operation. Avoid operating the breaker

any more than necessary without oil in the tank as it is designed to operate in oil and

mechanical damage can result from excessive operation without it.

(h) When replacing the tank and refilling it with oil, be sure the gaskets are undamaged and

all nuts and valves are tightened properly to prevent leakage.

MAINTENANCE PROCEDURES FOR MEDIUM-VOLTAGE VACUUM CIRCUIT BREAKERS.- Direct inspection of the primary contacts is not possible as they are enclosed in vacuum containers. The operating mechanisms are similar to the breakers discussed earlier and may be maintained in the same manner. The following two maintenance checks are suggested for the primary contacts:

(a) Measuring the change in external shaft position after a period of use can indicate extent

of contact erosion. Consult the manufacturer's instruction book.

(b) Condition of the vacuum can be checked by a hi-pot test. Consult the manufacturer's

instruction book.

Circuit Breaker

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