A device for detecting a fault in an AC supply comprises a circuit (CT, 100) for detecting a particular type of fault in an AC supply to a load (LD) and providing a corresponding output (10). A relay (RLA) is responsive to said output (10) to open a set of load contacts (SW1) in the AC supply to disconnect the load from the supply. Test means (TS, Rt, W2) are provided for simulating a supply fault of the said type, and means (C3, R5, SCR2, SOL, SW2) are provided for causing the load contacts (SW1) to open if they do not open in response to the simulated fault within a certain period of time.