Abstract: Electrical appliances each include an identity module which carries a code. Whenever mains electrical power is reconnected to one of the appliances, FSK signals are sent via the main electricity circuit to a security mains-telephone transfer point module which then causes a call to be set up via a telephone network to a remote security unit. The remote security unit uses customer line identity (CLI) signals from a telephone network interface to identify a code which it returns via the telephone network to the security mains-telephone transfer point module associated with the appliance site. The security mains-telephone transfer point module forwards the received code using FSK signals to the requesting appliance by way of the mains electrical circuit. If an incorrect code (or no code) is received, the identity module in the appliance inhabits the supply of power to its operational circuits.