A shunt trip breaker is a normal overload circuit breaker with the addition of an external electromagnetic coil that also allows for remote tripping of the breaker. For example, shunt trip breakers are used in elevators.
In the case of elevators, a shunt trip breaker is installed so that the fire alarm can send a signal to the shunt trip breaker to interrupt the flow of electricity to the elevator. In this way, the risk of electrocution or control malfunctioning resulting from the operation of the overhead water sprinkler is eliminated.
In addition to its remote throw capability, a shunt trip breaker also functions as a standard circuit breaker. A circuit breaker is designed to protect electronics from overload conditions or a short circuit. Circuit breakers have largely replaced fuses, which serve the same function, because circuit breakers can be reset and used almost indefinitely, whereas fuses must be replaced after each episode. Another advantage of circuit breakers over fuses is that circuit breakers can be manually thrown to test or isolate individual electrical pathways.
A shunt breaker has three connections: to the power source, the output, and a third contact usually hooked into a safety system such as a smoke detector. The first two contact points of a shunt breaker are connected by a metallic strip on a switch, and an electromagnet placed underneath this switch. Electrical power flows through the strip under normal circumstances. When a surge occurs, the magnet has enough power to activate, throwing the switch and breaking the connection. In a shunt trip breaker, the magnet is also wired to an external system, which can send an electrical signal that will also activate the magnet and trip the switch. After the switch has been tripped, it must be manually reset.
Circuit breakers are tripped by current and not voltage, but they are usually specified with a maximum voltage. Specialty circuit breakers are designed for extremely high voltages, including up to 1.2 million volts in power lines that carry 800 million watts of electricity.