Data communications system using carrier currents, in particular for a vehicle

The communication system is for use in a vehicle having controls or sensors (6) distributed throughout the vehicle, and its central network management unit (7), all connected by two supply wires (2,3) to the battery (4) and negative earth (5). Each incorporates a communication interface (10) for the associated equipment (12) which is either energised or de-energised, depending on the satisfaction or otherwise of a predetermined criterion which is checked by test equipment at the interface. The latter also incorporates a protocol management device and a switch for its supply.