A control circuit (1) includes two input terminals (2, 3) for respectively electrically connecting with the active and neutral conductors (4, 5) of a mains power source (6). Two output terminals (7, 8) electrically connect with a load in the form of a two pin domestic electrical appliance (9). Conductors (4, 5) terminate at a domestic power outlet socket, and terminals (7, 8) are incorporated into a two pin plug for insertion into that socket. That is, terminals (7, 8) include elongate bundled conductors for extending between the outlet socket and the circuit. Appliance (9) has a conductive component, in this instance a housing (10) that, under normal operating conditions, is electrically insulated or otherwise electrically isolated from conductors (4, 5). The housing provides a reference point for circuit (1), and is electrically connected to that circuit by way of a conductor (11).