A circuit breaker includes at least one coil for measuring electric current of an electrical conductor of an electrical circuit, connected to a control unit which compares the measured current or its equivalent with a first current limit value. Upon the first current limit value being exceeded, the control unit initiates an interruption of the electrical circuit. The two terminals of the coil are connected via a series circuit which has a first switch opened in its basic state, a voltage source and a voltage indicator. The control unit is designed such that, when a second current limit value is undercut and a first period of time is exceeded, the first switch is closed for a second period of time, the voltage indicator compares the voltage present with a first voltage value and, when the latter is exceeded, delivers information on the absence of faults of the coil.