Circuit breakers are essential safety devices that protect electrical circuits from overload and short circuits. There are many different types of circuit breakers, each with its own unique features and applications. In this article, we will take a comprehensive look at the different types of circuit breakers, including their applications, ratings, working principles, and operation.
What is Circuit Breaker
Circuit Breaker is a device that converts a current-carrying part into a voltage-insulating part.
A circuit breaker is an essential element of the 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.
We shall discuss basic working principles and types of circuit breakers.
Basic Working Principle Of Circuit Breaker
Circuit Breaker is a device that 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, but circuit breakers can also isolate a circuit under a fault condition i.e. when a very high current say 50 kA is flowing through it.
Now the interesting fact is that when a current-carrying conductor is suddenly broken an arc strikes between the open contacts, which eventually ionizes the surrounding, resulting in converting the area between the open contact into the 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 to isolate 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 contact to re-strike.
Hence for successful circuit breaking, we need an arc-quenching medium, which eliminates the arc.
Types Of Circuit Breakers
Circuit Breakers can be classified based on several properties. But the most important classification is 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 applications i.e. 415 V applications.
- 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 breaker uses oil as the arc quenching medium.
It has two different types.
1. Minimum Oil Circuit Breaker (MOCB)
2. Bulk Oil Circuit Breaker.
Minimum Oil Circuit Breaker (MOCB)
- The classification between minimum oil circuit breakers and bulk oil circuit breakers is based on their oil capacity.
- MOCB has less amount of oil for arc quenching, whereas bulk oil circuit breaker has a huge quantity of oil.
- MOCB was used for voltages of 3.3 kV, 11kV, etc. i.e. for Medium Voltage.
Bulk Oil Circuit Breaker.
- A bulk oil circuit breaker was used for High Voltage i.e. 132 kV, 220 kV, 440 kV, etc.
- However, due to its frequent maintenance, OCB has been obsolete nowadays. Only a few industries constructed a 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 nowadays for all medium voltage systems i.e 3.3 kV, 6.6 kV, 11 kV, and 33 kV.
In a 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 gets concentrated on the vacuum bottle’s surface, hence due to lack of ionized particles the arc gets extinguished. In this type of Circuit Breaker, arc elimination happens very quickly. Hence it is the most commonly used circuit breaker for the medium-voltage system.
VCB is generally mounted in the switchgear panel. However, for 33 kV, VCB is available for outdoor applications 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 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 applications.
For voltages of 33 kV, 132 kV, 220 kV, 400 kV, 800 kV, and above SF6 breaker is used. SF6 gas at different pressures is used for different voltage levels. For example, for a 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 the MV system SF6 breaker could also be used, but it is not practiced due to economic considerations.
In general, up to 220 kV the circuit Breakers are single interrupter type, which means up to 220 kV the circuit breakers generally have only one breaking chamber per phase. For 400 kV 2 nos of interrupter chambers are used. Each is rated for 220 kV. So for 400 kV breakers, the SF6 pressure is also kept at 6 bar in general. However, a 220 kV breaker is also available with two interrupters.
SF6 Circuit Breakers are generally outdoor type suitable for mounted on the structure. SF6 breaker with switchgear mounting arrangement is rare.
SF6 breakers are also used as Generator Circuit breakers due to their 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 an 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.
Nowadays this breaker is also obsolete.
So our focus will be on ACB, VCB, and SF6 circuit breakers as these three breakers are the most used breakers.
So let us summarize, which voltage shall be the breaker choice?
33 kV is such a voltage level that VCB and SF6 Circuit breakers both are used.
For 415 V there are another few types of circuit breakers that are commonly used.
Principally those are air circuit breakers, 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 an inbuilt feature of detecting fault conditions.
3. All the breakers trip automatically.
The major difference between the circuit breakers is-
1. Among all the breakers MCB, RCCB, and RCBO have limitations of fault current breaking. These breakers have a maximum short circuit withstand capability of 10 kA for 1 sec, and also a symmetrical breaking capability of 10 kA, whereas the other breakers have a short circuit withstand rating of 50 kA for 1 sec or higher, so the symmetrical braking capability.
2. Except for the ACB the other breakers could not be switched on by an electrical signal. Those can only be switched on mechanically.
3. MCCB and MPCB could be tripped by other protection relays or logic (by applying an electrical signal) by having an external shunt trip coil. But closing shall be mechanical only.
4. ACB with release has both a closing and tripping coil, so it can be closed or tripped by an electrical signal.
5. MCB, RCCB, and RCBO do not have any provision for operation with an electrical signal.
Now let us see the functionality of each type of breaker.
1. MCCB-MCCB stands for Moulded Case Circuit Breaker. Its main feature is it has a 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 set for its operation. But for electronic MCCB both pickup and time settings can be adjusted.
MCCB is also available with protection against under-voltage
2. MPCB-MPCB stands for motor protection circuit breaker. It has a similar performance to MCCB, in addition, MPCB gives protection against overload, i.e. a continuous low magnitude over current. Suppose a motor draws 110% over current continuously, it will be detected by MPCB. Generally motor faces 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 cannot be closed by an electrical pulse. But if MCCB is used then an additional bimetallic thermal overload relay is required.
3. MCB-MCB stands for miniature Circuit Breaker. It is widely used for lighting circuits and control supplies. It can give protection against the short circuit only. MCB has the rated symmetrical breaking capability of a maximum of 10 kA. Hence it is to be ensured that the system fault current shall be less than 10 kA where MCB is to be used.
for the very same reason, a 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 to MCB but it is meant for only earth leakage detection. This type of breaker is available for two ranges 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 a combination of RCCB and MCB. It gives protection against short circuits and earth leakage. This breaker is also available with an earth leakage sensitivity of 30mA and 100mA.
6. ACB With Release– It has the same principle as an Air Circuit breaker, but it has an internal Short circuit, earth fault relay, and release system. Unlike MCCB, MPCB has both a closing and tripping coil, so it can be closed and opened by an electrical pulse.
Circuit Breaker is also classified based on the installation. Under this category, a 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 up to 33 kV are panel-mounted breakers. These are housed in a switchgear panel.
33 kV VCB suitable for outdoor structure mounting is also available.
SF6 circuit breakers for 132 kV and above are outdoor structure mounted type.
There are other breaker mounting arrangements i.e. Generator Circuit Breaker (GCB). It is installed between the generator terminal Generator Transformer and Unit Transformer. It is connected to 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.