How Circuit Breakers Work

By Scott Dreger

A circuit breaker is one of the most important safety devices in a home or building. Without breakers, using electricity would be unsafe. Electricity is always looking for the easiest return path—the path with least resistance. As resistance decreases, current increases, as in a short circuit—where current travels along an unintended path, often with little or no resistance. If current becomes too high for the gauge of wiring, it can overheat and start a fire. This could have a variety of causes, such as an overload caused by plugging in too many appliances, a short circuit caused by accidently driving a nail through an electrical wire when hanging a picture, or simply a faulty appliance.

Circuit breakers trip whenever current exceeds a specified amount, cutting power to the circuit. To do this quickly at high current, breakers use electromagnets—which increase in force as current increases—to mechanically throw a switch when current exceeds an acceptable amount (see diagram below). These simple devices greatly reduce the risk of electrical fires and make use of electricity in homes and buildings practical.