Circuit Breaker-Type, Application, Rating, Working Principle and Operation-Part 1

Circuit Breaker is a device which converts a current carrying part into a voltage insulating part. (This definition was given by my college's departmental H.O.D Mr. P. K. P)

Circuit breaker is an essential element of electrical power system. Circuit Breaker is the device by which a faulty circuit can be isolated automatically. It can also be used for manual intentional operation.


In this article (Part 1) we shall discuss about basic working principle and types of circuit breaker.

BASIC WORKING PRINCIPLE OF CIRCUIT BREAKER:

Circuit Breaker is a device which can break or isolate a circuit on load condition, i.e. when current flows through the breaker it can isolate the circuit. Not only that, circuit breaker can also isolate a circuit under fault condition i.e. when very high current say 50 kA is flowing through it.

Now interesting fact is that when a current carrying conductor is suddenly broken an arc strike between the open contacts, which eventually ionized the surrounding, resulting converting the area between the open contact into conductor, through which the voltage continues the current to flow causing a sustained arc. So sudden break in the conductor does not isolate the circuit, as the current used to flow through the arc.

So for isolating the circuit completely we need to quench or eliminate the arc completely so that sufficient insulation is made-up between the contacts which can resist the voltage across the open to contact to re-strike.

Hence for successful circuit breaking we need an arc quenching medium, which eliminates the arc.

TYPES OF CIRCUIT BREAKER:

Circuit Breaker can be classified based on several properties. But the most important classification is done based on its arc quenching medium.

Based on the arc quenching medium Circuit breakers are classified as follows.

a. Air Circuit Breaker (ACB)- Air Circuit Breaker has no additional arc quenching medium, air is used for arc quenching.

This type of circuit breaker is used only for Low voltage application i.e. 415 V application.

Because the gap created between the contact can withstand voltage up to 415 V with a convenient size. If ACB for higher voltage application is to be made it can lead to an inconvenient breaker size.

So for voltage up to 415 V ACB is used in practice.

b. Oil Circuit Breaker (OCB)-This type of circuit breakers uses oil as the arc quenching medium.

It has two different types.

1. Minimum Oil Circuit Breaker(MOCB)

2. Bulk Oil Circuit Breaker.

The classificati between minimum oil circuit breaker and bulk oil circuit breaker is based on their oil capacity.

MOCB has less amount of oil for arc quenching, where as bulk oil circuit breaker has huge quantity of oil. 

MOCB was used for voltage of 3.3 kV, 11kV etc. i.e. for Medium Voltage.

And Bulk oil circuit breaker was used for High Voltage i.e. 132 kV, 220 kV, 440 kV etc.

But due to its frequent maintanance OCB has been obsolete now a days. Only a few industry constructed long time back still have the OCBs. But no modern industry uses Oil Circuit Breaker now.

c. Vacuum Circuit Breaker (VCB)- Vacuum Circuit breaker is the most common circuit breaker used now a days for all medium voltage system i.e 3.3 kV, 6.6 kV, 11 kV, and 33 kV.

In vacuum circuit breaker the contacts are housed in a vacuum bottle, in which the contact separation occurs. Arc is generated due to the ionization of the metal vapor of the contacts, which suddenly get concentrated to the vacuum bottle's surface, hence due to lac of ionised partcles the arc gets extinguished. In this type of Circuit Breaker the arc elemination happes very quickly. Hence it is the most commonly used circuit breaker for medium voltage system.

VCB in generally mounted in the switchgear panel. However for 33 kV, VCB is available for outdoor application too.

d. SF6 Circuit Breaker- In this type of circuit breaker SF6 is used as the arc quenching medium. SF6 stands for Sulphur Hexa-fluoride. It has an excellent dielectric strength. So it can build the di-electric strength to prevent re-strike for very high voltage. Hence this type of circuit breaker is used for high voltage and extra high voltage application. 

For voltage of 33 kV, 132 kV, 220 kV, 400 kV, 800 kV and above SF6 breaker is used. SF6 gas at different pressure is used for different voltage levels. For example for 132 kV circuit breaker SF6 gas pressure is generally maintained at 4 to 5 bar. For 220 kV it goes to 6 bar. However for MV system SF6 breaker could also be used, but it is not practiced due economic consideration.

In general upto 220 kV the circuit Breakers are single interrupter type, which means upto 220 kV the circuit breakers generally have only one breaking chamber per phase. For 400 kV 2 nos of interrupter chambers are used. Each are rated for 220 kV. So for 400 kV breakers the SF6 pressure is also kept at 6 bar in general. However 220 kV breaker is also available with two interrupters.

SF6 Circuit Breakers are generally outdoor type suitable for mounted on structure. SF6 breaker with switchgear mounting arrangement is rare.

SF6 breakers are also used as Generator Circuit Breaker due to its fast rate of rise of recovery voltage.

e. Air Bast Circuit Breaker- In this type of circuit breaker the arc is quenched by a huge flow of air. Hence known as air blast circuit breaker.

Extra high voltage (220 kV and above) breakers used to be air blast type. Also this type of circuit breaker has been used as the Generator Circuit Breaker.

Now a days this breaker is also obsolete.

So our focus will be on ACB, VCB and SF6 circuit breaker as these three breakers are the most used breaker.

So let us summarize , for which voltage what shall be the breaker choice.

Air Circuit Breaker- 415 V

Vacuum Circuit Breaker-3.3 kV, 6.6 kV, 11 kV, 33 kV

SF6 Circuit Breaker- 33 kV, 132 kV, 220 kV, 400 kV, 800 kV and above.

33 kV is such a voltage level where VCB and SF6 Circuit breaker both are used.

For 415 V there are another few types of circuit breaker which are commonly used.

Principally those are air circuit breaker, as they do not have any additional arc quenching medium.

These breakers are
a. Moulded Case Circuit Breaker (MCCB)

b. Motor Protection Circuit Breaker (MPCB)

c. Miniature Circuit Breaker (MCB)

d. Earth Leakage Circuit Breaker (ELCB) or Residual Current Circuit Breaker (RCCB)

e. Residual Current Circuit Breaker with overload (RCBO)

f. ACB with in built release.

All of the above mentioned circuit breakers have some similarities as mentioned below.

1. All of the breakers are suitable for 415 V or below.

2. All of the circuit breakers have inbuilt feature of detecting fault condition.

3. All the breakers trip automatically.

The major difference between the circuit breakers are-

1. Among all the breakers MCB, RCCB, and RCBO have limitation of fault current breaking. These breakers have maximum short circuit withstand capability of 10 kA for 1 sec, also symmetrical breaking capability  of 10 kA, whereas the other breakers have short circuit withstand rating of 50 kA for 1 sec or higher, so as the symmetrical braking capability.

2. Except the ACB the other breakers could not be swithed on by electrical signal. Those can only be switched on mechanically.

3. MCCB and MPCB could be tripped by other protection relay or logic (by applying electrical signal) by having external shunt trip coil. But closing shall be mechanical only.

4. ACB with release have both closing and tripping coil, so it can be closed or tripped by electrical signal.

5. MCB, RCCB, RCBO do not have any provision for operation with electrical signal.

Now let us see the functionality of each type of breakers.

1. MCCB-MCCB stands for Moulded Case Circuit Breaker. Its main feature is it has short circuit and earth fault detection system.
In general two types of MCCBs are available viz. Thermal magnetic type MCCB, and electronic MCCB.
In thermal magnetic type MCCB only pick up of the protection system can be set, but there is no control over the time setting for its operation. But for electronic MCCB both pick up and time setting can be adjusted.

MCCB is also available with protection against under voltage

2. MPCB-MPCB stands for motor protection circuit breaker. It has similar performance like MCCB, in addition MPCB gives protection against overload, i.e. a continuous low magnitude over current. Suppose if a motor draws 110% over current continuously, it will be detected by MPCB. Generally motor face the maximum overload situation, and hence it is used for motor protection.

However both MCCB and MPCB can be used for motor protection along with a contactor since these two breakers could not be closed by electrical pulse. But if MCCB is used then additional bimetallic thermal over load relay is required.

3. MCB-MCB stands for miniature Circuit Breaker. It is widely used for lighting circuit and control supply. It can give protection against the short circuit only. MCB has the rated symmetrical breaking capability of maximum 10 kA. Hence it is to be ensured that the system fault current shall be less than 10 kA where MCB to be used.

for the very same reason an unit ratio transformer of voltage ratio 415/433 V is used for lighting distribution to reduce the fault level from 50 kA to below 10 kA.

4. RCCB/ELCB- Residual Current Circuit Breaker or Earth leakage circuit breaker is similar as MCB but it is meant for only earth leakage detection. This type of breakrs are available for two range of earth leakage sensitivity viz. 30 mA and 100 mA. RCCB shall be used in series with fuse or MCB.

5. RCBO- RCBO stands for Residual circuit breaker with overload. Basically this circuit breaker is the combination of RCCB and MCB. It gives protection against short circuit and earth leakage. This breaker is also available with the earth leakage sensitivity of 30mA and 100mA.

6. ACB With Release- It has the same principle as Air Circuit breaker, but it has inter nal Short circuit, earth fault relay and release system. Unlike MCCB, MPCB it has both closing and tripping coil, so it can be closed and opened by electrical pulse.

Circuit Breaker is also classified based on the installation. Under this category circuit breaker can be classified into two parts.

a. Indoor Switchboard mounted circuit breaker and
b. Outdoor structure mounted circuit breaker.

Generally all 415 V Breakers and VCBs upto 33 kV are panel mounted breaker. These are housed in a switchgear panel.

33 kV VCB suitable for outdoor structure mounting is also available.

And SF6 circuit breakers for 132 kV and above are outdoor structure mounted type.

There are another breaker mounting arrangement i.e. Generator Circuit Breaker (GCB). It is installed between the generator terminal and Generator Transformer, Unit Transformer. It is connected with the generator and transformer with Isolated Phase Bus Duct.

This covers mostly the types of circuit breakers. In the upcoming articles we shall highlight rating, application with Single Line Diagram, construction, interlocks, control logic, operation etc.

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