Abstract: The system comprises a central control unit and a plurality of remote units connected to the control unit by a single, common, transmission link. This link acts simultaneously as a signalling link and as the electrical voltage supply line for the remote units. The control unit transmits electrical control signals to the remote unit by superimposing trains of pulses on the supply voltage that is fed via the transmission link to the remote units. Each remote unit is arranged to count the control signal pulses appearing on the line and to activate one or more controlled devices when the number of pulses counted corresponds to a unique number associated therewith. A first type of controlled device is constituted by sensor means arranged to monitor a physical quantity and to output, when activated, related electrical information signals indicating to the control unit the value of the monitored quantity; the information signals take the form of frequency modulation of the current flowing in the transmission link.