The function of the Circuit Breaker is to ensure electrical safety. Ever spent a night without electricity? One of the reasons behind this situation could be short circuit which can be avoided through the implantation of Circuit Breaker. In this post, we will discuss in detail about what is Circuit Breaker, its various types, how it works, its applications, advantages and disadvantages.
1. What is Circuit Breaker (C/B)
Circuit Breaker is a mechanical device that protects electrical circuit from damage due to the flow of high magnitude current or short circuit. It is an automatic operated switch which is basically designed for the safety of electrical system by interrupting the abnormal or fault current. A C/B also interrupts other currents at system voltage such as capacitive currents, small inductive currents and load currents.
Fig. 1 – Introduction to Circuit Breaker (C/B)
A C/B can be reset once the purpose is done as it operates via switch, while a fuse needs replacement to resume normal operation. There are various types of C/B’s available in the market from small devices to large switch-gear to protect both low and high voltage currents. Fig. 2 shows High Voltage C/B which are installed in power stations.
2. Types of Circuit Breaker
Circuit Breakers are broadly classified in to two types:
- DC Circuit Breaker
- AC Circuit Breaker
Fig. 2 – Image of High Voltage Circuit Breaker
2.1 DC Circuit Breaker
Electrical devices that operate with Direct Current are protected using DC Circuit Breaker. The voltage is always constant in DC (Direct Current). Battery powered electric circuits, UPS make use of DC Circuit Breakers.
2.2 AC Circuit Breaker
AC C/B’s are used in power transmission and operation of high power motor devices as Alternating Current (AC) cycles between positive and negative voltage, multiple times per second.
They are further classified into two types:
- Low Voltage Circuit Breaker
- High Voltage Circuit Breaker
2.2.1 Low Voltage C/B
A C/B which is preferred for circuits rated below 1000V. In this kind of Breaker, voltage output cycles several times per second.
2.2.2 High Voltage C/B
High Voltage C/B is used for circuits rated above 1000V. It is further classified into two types –
- Oil C/B
- Oil less C/B
2.2.2.1 Oil Circuit Breakers
In such kind of breakers, oil is used, preferably mineral oil for an arc extinction. It acts as a good insulator as compared to air. There are two types of Oil C/B’s –
- Bulk Oil C/B
- Minimum or Low oil C/B
2.2.2.1.1 Bulk Oil C/B
Bulk Oil C/B uses transformer oil for an arc extinction. The oil acts as an insulator between the two conducting parts of the C/B. The range lies from 25MV at 2.5KV to 5000 MVA at 230KV.
2.2.2.1.2 Minimum or low Oil C/B
In Minimum or low oil C/Bs, oil is used for arc extinction by blast action, but it does not act as an insulator for live parts of the earth.
Fig. 3 – Classification of Circuit Breaker
2.2.2.2 Oil less Circuit Breakers
Oil Less C/B use very small quantity of insulating oil in arc extinguishing chamber.
Oil Less C/B are further classified into: –
- Air Break C/B
- Air Blast C/B
- SF6 C/B
- Vacuum C/B
2.2.2.2.1 Air Break Circuit Breaker
In Air Break C/B, the arc is initiated and extinct in the air and it is used in the range of low voltage up to 15 KV. There are three types of Air Circuit Breakers which is shown below:
- Plain Air Break C/B
- Arc-Chute C/B
- Magnetic Blow-out type C/B
2.2.2.2.2 Air blast Circuit Breaker
In Air Blast C/B, compressed air is stored and released through nozzles order to extinguish the arc. It is used for system voltage of 245 KV, 420 KV, and even more. Air Blast Circuit Breaker can be classified into two:
- Axial Blast C/B
- Cross Blast Air C/B
2.2.2.2.3 SF6 C/B
SF6 is also one of the types of C/B wherein the current carrying contracts normally operates in Sulphur Hexa Fluoride gas to extinguish the arc. It is considered as superior as compared to oil and air.
2.2.2.2.4 Vacuum C/B
It is kind of circuit in which vacuum is used to extinct the arc. It is more reliable and has higher dielectric strength. It is very compact and requires negligible maintenance.
3. How does C/B Work
In the Fig. 4 (a) an AC motor is connected through a C/B to a voltage source. A complete path for current exists between the voltage source and the motor when the C/B is closed. Similarly opening the C/B breaks the path of current flow and the motor stops as shown in the Fig. 4 (b).
Fig. 4 – (a) C/B in Closed Condition (b) C/B in Open Condition
When there is any fault, the C/B automatically opens and once the fault has been discovered and fixed, the C/B can be closed, allowing the motor to operate. Whenever the load current increases the contacts of C/B open and there is an Arc which is established between the separating contacts in C/B.
Arc helps in bridging the contact gap through which the current gets a low resistive path to flow and hence there is no sudden interruption of current which prevents abnormal switching of the voltage. Arc provides a conductive path of electricity and the Arc is quenched as quickly as possible.
Switching of Voltage when the circuit is open can be expressed in the form of equation:
V = L.(di/dt) where;
di/dt = rate of change of current with respect to time during opening of the contacts.
I = Current
V = Voltage
L = Inductance
t = Time
4. Applications of Circuit Breaker
Applications include:
- They are used for switching of loads in Industries, Buildings, Commercial complexes, Hotels etc.
- Air C/B is used for the protection of plants, electrical machines, transformers, capacitors and generators.
- Air Blast C/B is used in Indian Railways for electrification.
5. Advantages of Circuit Breaker
Advantages include:
- It protects from damage caused by flow of excessive current.
- It is are more reliable.
- It can be reset as it operates via a switch.
- It is very sensitive in operation.
6. Disadvantages of Circuit Breaker
Disadvantages include:
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