A circuit breaker cuts off an electrical circuit in building wiring any time that electrical current goes above a safe level. Its function is similar to a fuse, although circuit breakers can be used more than once.
A basic circuit breaker includes a simple switch; a "hot wire" in the switch connects to both ends of the switch. If the switch is flipped to "on," current flows, and the electricity flows to an electromagnet, the magnetic force of which can be boosted or reduced by sending through more or less current. When the current goes to an unsafe level, the electromagnet activates a lever connected to the switch, which tilts the moving contact away from the stationary contact to break the circuit, turning off the electricity.