more than one amplitude level and or by providing a much higher clock frequency for example 1368 khz thus the signal which reaches catalogue store 17 is a spread spectrum signal substantially restricted to the frequency range of 40 to 90 khz and formed of a particular arrangement of six bit words each representing a data bit either 34 0 34 or 34 1 34 for one communication channel produced by ascending frequency selection thus in due course the catalogue store 17 holds a record of all the data bits for the channels so that the output from this store 17 can be used to programme an eprom 30 in a interface unit 31 which is connected between a domestic electrical appliance 32 and the electrical mains circuit 33 of the dwelling see fig2 in this way the eprom 30 acts as a source of spread spectrum signals for a communication channel for use in mains signalling the interface unit 31 has a central control 34 which regulates all the necessary operations for example the transmission of any signals e g relating the appliance status or instruction completion into the system or the reception of any signals e g relating to status interrogation or instruction issuing from the mains signalling system in order to transmit a message to the system.