How does Circuit Breaker Work ?

What is Circuit Breaker ?

Circuit breaker (CB) is an electrical switch which is used to protect electrical circuit from damage caused by excess current. The flow of excess current in power system is mostly due to overload or short circuit. Basic function of circuit breaker is to interrupt the flow of excess current during faulty conditions. It is designed in such a way that it can be operated manually under normal conditions (i.e. during maintenance ) and automatically during faulty conditions.

How does a Circuit Breaker work ?

Circuit breaker is generally connected in series with the equipment which is to be protected from overcurrent or short circuit faults. Two common methods used by circuit breakers to detect the strength of electric current are thermal and magnetic. Thermal sensors depends on temperature and gets heated in case of excessive current (faulty condition). Magnetic breakers on the other hand uses a solenoid or an electromagnet in order to sense the magnitude of current.

Operating mechanism of circuit breakers are designed to act either fast or slow during the faults. Thermal breakers responds little slowly and doesn’t open the contacts of circuit breaker immediately after the fault. Magnetic breakers are designed to respond quickly for large excessive current as in case of short circuit fault. Each of the circuit breaker has predefined current rating that it will allow to flow. These two breakers can be combine together called as thermal-magnetic circuit breaker used in some applications. Now, lets discuss the working of circuit breaker.

Thermal Circuit Breaker :

Thermal circuit breaker consists of thermal bimetallic strips and spring loaded operating mechanism which holds two metallic contacts together. During normal conditions, bimetallic element doesn’t bend and electric current will continue to flow through the circuit. When flowing current exceeds the maximum allowable current rating of circuit breaker, thermal bimetallic strip gets heated up. Heated bimetallic strip due to over current condition change its position and pulled the spring mechanism which in turn open the contacts of circuit breaker (faulty condition). This whole procedure during faulty condition takes milliseconds. When bimetallic strip cools, it return back to its original condition and contacts of circuit breaker gets closed for reuse.

Thermal Circuit Breaker


Magnetic Breaker :

Magnetic circuit breaker uses solenoid or electromagnetic coil in order to trip the breaker during the faults. When current passes through the breaker, magnetic field is develop around the solenoid. During normal conditions, the develop magnetic field is not strong enough to pull the bimetallic element. As the amount of current flowing through the circuit increases, magnetic field around the electromagnet also gets stronger. Once the electric current exceeds the breaker’s limit, magnetic field becomes so strong that it pulls the bimetallic strip which in turn open the contacts of circuit breaker. Hence current will stops to flow through electric circuit until the breaker is reset.

Magnetic Circuit Breaker


Arc Phenomenon in Circuit Breaker :

When current carrying contacts of circuit breaker begin to separate during faulty condition, the contact area decreases rapidly which results in the increase of current density in that region. Hence large fault current flow between the opening contacts of circuit breaker which results in the rise of temperature. Heat produce due to the large faulty current ionizes the medium due to which arc (conducting path ) is formed between the contacts of CB. In this way, interrupting current gets low resistive path and it continues to flow between the contacts as long as the arc persists.

Current flowing through the circuit even when the contacts are separated may damage the protected equipment or the breaker itself. Hence it is necessary to extinguish the electric arc as soon as possible. For that purpose, different methods are used for extinguishing the arc in order to stop the flow of current. It is to be noted that the amount of current flowing between the contacts depends on the arc resistance. If greater is the arc resistance, smaller will be the current flow and vice versa.

Conclusion :

Circuit Breakers are generally used in high voltage power circuits. Magnetic circuit breakers are better to use when we have to deal with sudden power surges since circuit breakers have to respond immediately in that scenario. Advantage of magnetic circuit breaker is that it can be reset instantly after the fault since they do not have to wait for the cooling of bimetallic strip like thermal breakers. Although magnetic circuit breakers are faster, but they are larger and more expensive as compare to thermal breaker. Thermal breakers on the other hand detects the overload faults with time delay. Hence they can be used in motors and transformers ↗. It is to be noted that the type of circuit breaker chosen depend on the characteristics of load.


That’s all. Hope this will helps you.

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