Circuit breaker

A three-phase power breaker

A circuit breaker (often used in the short form, breaker) is a switch device which, when wired into a circuit, acts in a manner similar to a fuse; i.e. it opens (breaking the circuit) when too much current flows through it. ('Too much' is defined by the rating of the breaker; a breaker of any type is usually available in a range of amperage ratings.)