Abstract: A fault current circuit breaker has a summary current transformer whose secondary coil is connected to a voltage-independent energy storage circuit. A monostable or bistable transformer relay is actuated by an impulse by way of this energy storage circuit when a fault current flows to earth in the protected installation. When the relay contact is made, an electromagnetic lock trigger is actuated by the energizing current received from the mains supply, which triggers the switch lock of the FC switch. After this, the bistable transformer relay is returned to its initial position, either mechanically by way of an appropriate coupling with the switch lock, or electrically by way of a time delay or switch circuit, whereby the relay contact opens and the FC switch is again ready for switching on.