A fault detecting device having a plurality of supply conductors, including a first current transformer having a primary winding comprising at least two supply conductors and a secondary winding into which a current is induced in response to a fault current in the primary winding. A fault detection circuit connects the secondary winding to an electromechanical actuator having mechanical contacts in the supply conductors, the fault detection circuit generating an output signal to the actuator when an induced current is detected in the secondary winding, the output signal causing the actuator to open the mechanical contacts. The device further includes a second current transformer having a primary winding connected to an oscillator and a secondary winding comprising at least one supply conductor, the second current transformer arranged to induce an oscillating current into the at least one supply conductor in the event of a fault condition.