Definition of a Siemens Shunt Trip Breaker

A Siemens shunt trip breaker is a required safety feature in many industrial and commercial establishments. The breakers act much in the manner of regular circuit breakers in that they prevent sudden surges in electrical current caused by a decrease in electrical resistance (short circuits, power surges, damage to electrical wiring, etc.) by disconnecting the power source in the building from the main power supply. In addition, Siemens breakers have a feature that allows them to disconnect power from the source prior to a power surge rather than during a power surge.


The Siemens Shunt Trip Breaker has three contact points. One connects to the power source leading into a building and the other passes that power into the building. The third connects to a safety system, such as a fire alarm or smoke detector housed in the surrounding building. The first and second contacts have separate wires; a metallic strip at the end of a spring-driven switch connects them. Beneath the switch is an electromagnet, which connects to the power supply leading into the building as well as the third contact point.


Normally, the Siemens Shunt Trip Breaker lets power pass in through the first contact point and out through the third without obstruction, but should there be a sudden surge in current beyond a given threshold, the extra current will give the magnet enough power to throw the switch. With the switch thrown, the connection between power coming in and power going out is broken. Also, if an electrical signal is sent in through the third contact point from the safety system (alarm, sprinkler, etc.), it will also cause the magnet to throw the switch. In order to turn it back on, the switch must be pushed back into place by hand.

Siemens Breaker Example

When a sprinkler system in an office building is activated, water will spray over computers, phones, and other machinery, thus creating the hazard of short circuits and potential electrocution of occupants. When the fire alarm detects that water is passing through the sprinkler system it sends an electrical signal to the Siemens Shunt Trip Breaker via the breaker's third contact point. The breaker disconnects all power leading into the office building before a surge in current can take place.