The system includes an electrical supply unit, a number of temperature-responsive switches (11-20). The switches are connected at one terminal to a line connected with the supply unit and at another terminal to a location at a potential different from the supply line such that closing of a switch causes current flow through the switch. Each switch is associated with a respective resistor (25-36) such that closing each switch causes a different voltage to be applied to a monitor (45-54) allowing the monitor to determine the identity of the closed switch. The supply line can be formed in a first loop (22) where the resistors are connected in series. Each end of the loop can be connected to a respective source of constant current. The monitor device receives the voltage at opposite ends of the loop.
