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.
If a power surge occurs in the electrical wiring, the breaker will trip. This means that a breaker that was in the "on" position will flip to the "off" position and shut down the electrical power leading from that breaker. Essentially, a circuit breaker is a safety device. When a circuit breaker is tripped, it may prevent a fire from starting on an overloaded circuit; it can also prevent the destruction of the device that is drawing the electricity.
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.