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CIRCUIT BREAKER is an electric device used to interrupt an electric current in a circuit when the current becomes too high. The special feature that separates circuit breaker from fuse is its ability to resume normal operation without any replacement. In other words, unlike a fuse which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.
Working
Basically, Circuit breakers are often operated with a solenoid (electromagnet) whose strength increases as the current increases and accordingly, trips the circuit breaker. A bimetallic strip may be used instead of solenoid as it exhibits the required property of bending due to heating in case of increased current. However, Some circuit breakers are designed to incorporate both techniques, enabling them to react at required moment. This feature provides the flexibility to suit the application, with the electromagnet responding in case of abrupt short and large surges in current (short circuit) and the bimetallic strip reacting to overcurrent conditions, (which hangs back for a long time). Circuit breakers for larger currents are usually arranged with pilot devices to detect a fault current and to operate the trip opening mechanism. To rephrase it, Circuit breakers incorporate features to divide and extinquish the arc.
Usually, in case of short-circuit conditions, excessive current may flow through the circuit causing heavy damages. When a circuit breaker tries to interrupt this unrequired current, an arc gets generated; thus allowing the flow of current to continue even though the contact of the circuit breaker is not closed. In air-insulated and miniature breakers, an arc chute structure made (often) of ceramic ridges or metal plates cools the arc, and blowout coils deflect the arc into the arc chute. The Larger circuit breakers used in electrical power distribution makes use of vacuum and an inert gas such as sulfur hexafluoride, or have contacts immersed in oil to suppress the arc. The maximum short-circuit current that a breaker can withstand is decided by testing; moreover, it is necessary to note that application of a breaker in a circuit with a higher prospective short-circuit current may lead in disruption of circuit breaker to safely interrupt a fault.
Types of circuit breaker
Thermal Circuit Breakers: The working of Thermal circuit breaker depends on current flowing through bi-metallic strip (bonded to each other), which is made of two different types of metal having different coefficients of expansion.
Working
As the current flowing through the strip increases, the strip gets heated up. Hence, one type of metal expands more than the other strip resulting in bending of one of the strips upward and thus, causes disconnection of the contacts. In simple words, when the temperature reaches a given point, the piece will snap into the open position and the current flow will stop. The bi-metal strip is stamped into a special shape which causes the `snap` action. This will assure that there is EITHER a solid connection OR a complete disconnect. When the bi-metal strip cools in the breaker, it comes back to its original posistion.
Magnetic Circuit Breakers:
This type of circuit breaker uses a magnetic actuator to stop the flow of current in the circuit.
Working
In this type of breaker, the current flow through the electrical device passes through an electromagnetic actuator. When the current flow reaches a threshold level, the magnetic field in the electromagnet reacts immediately by tripping the breaker and opening the contacts. This type of breaker generally has to be manually reset. A well-designed `magnetically` actuated circuit breaker can operate very quickly.
Thermal/Magnetic Breakers:
This type of breaker uses the function of both thermal and magnetic trip.
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