A circuit breaker is a device that automatically breaks an electrical circuit whenever the circuit becomes overloaded or an unintentional short circuit occurs. Circuit breakers have a set electric current load capacity which when breached results to automatic circuit shutdown.

Overloading occurs whenever the wires comprising an electrical circuit are forced to carry and conduct an electric charge beyond their capacity. Whenever this happens, the wires heat up. This may result in insulation breakdown (in which a short circuit may occur) or an electrical fire.

An unintentional short circuit occurs when electricity traverses a path that is different from what has been intended. This usually happens when the insulating material or layer between two conductive materials breaks down or when a conductive material is directly introduced between these two conductive materials. This results in an unintended direct flow of charge from one node to another. This may result in overheating, circuit damage, explosion, and even fire.

Circuit breakers are therefore protection against the undesirable consequences of wire overloading and accidental short circuiting. In this way, they are similar to fuses. However, circuit breakers need only be reset after use whereas a fuse needs to be replaced with a new one.

Circuit-Breaking Mechanism

At its most basic, a circuit breaker has a switch and a moving, conductive contact plate that moves with the switch. When the switch is placed on an 'on' position, the circuit breaker's moving contact plate touches a stationary plate that is connected to the rest of the circuit so electric current can flow.

A circuit breaker has a mechanism for automatically breaking an electrical circuit. There are three main types of circuit breakers according to the mechanism used.

The first type uses electromagnetism. A magnetic circuit breaker has an electromagnet whose magnetism increases with the current load. The moment the load exceeds that which has been prescribed, the electromagnet's pull becomes powerful enough to force the circuit breaker's lever down and move the circuit breaker's contact plate away from the stationary contact plate. The circuit is therefore opened and electric current ceases to flow.

The second type of circuit breaker uses heat to break a circuit. A thermal circuit breaker has a bimetallic strip, which is a strip of material that has two types of metal welded together. The two metals expand with heat but each metal does so at a different rate from the other. Thus, the metal strip bends with the heat generated by the electric current flowing through the circuit. At dangerous heat levels, the strip would be bent at a certain angle that would be enough to pull the circuit breaker's lever down and sever the connection between the circuit breaker's contact plate and the circuit's stationary contact plate.

The third type of circuit breaker uses both heat and electromagnetism to protect electrical systems. An electromagnet pulls the lever in case of sudden jumps in electric load, and a bimetallic strip protects the system from prolonged cases of overcurrent that manifests in overheating.