Circuit Breaker

Circuit breaker is an automatic switch. It protects electric motors, household wiring, long-distance power lines, and other electric circuits against damage caused by too much electric current. Too much current may flow in a circuit as a result either of a fault in the circuit or of an outside event, such as lightning.

Every circuit breaker is designed to allow a specific maximum amount of electric current to pass. If the current exceeds this limit, an automatic mechanism inside the circuit breaker throws open a set of contacts (switch) and stops the current. Mechanisms used to open the switch include electromagnets and temperature-sensitive devices similar to a thermostat. As the switch opens, an electric arc leaps across the open contacts. Electricity continues to flow through this arc until it is extinguished. In an oil circuit breaker, the switch is immersed in an oil that extinguishes the electric arc. In an air-blast circuit breaker, a blast of compressed air blows out the arc. In a magnetic arc-suppression circuit breaker, a magnetic field deflects and breaks the arc.

A circuit breaker called a ground fault circuit interrupter (GFCI) helps prevent electric shocks. Most electric shocks occur when people use faulty appliances. Such appliances can produce slight variations in the electric current entering and leaving the circuit. A GFCI detects these changes, which are too small to activate an ordinary circuit breaker. It then automatically shuts off the electricity to the defective appliance.

Some circuit breakers are about as small as an ordinary light switch. But some are as large as a small two-story house. A large circuit breaker can interrupt currents up to 40,000 amperes at 345,000 volts. It can open a circuit in less than 1/30 of a second, and reclose it in less than 1/3 of a second.

Contributor: Donald W. Novotny, Ph.D., Prof. of Electrical and Computer Engineering, Univ. of Wisconsin.

Circuit Breaker

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