Abstract: A socket (10) for connecting a plug (15) to installed mains lines (17) has its base (12) provided with a casing (18) in which an electronic bus coupler (BA) is arranged. The bus coupler (BA) is connected to a bus line (19) also installed. Signals generated by the bus coupler (BA) are converted into optical signals by a photoelectric converter (21) and transmitted to the plug (15) The plug (15) comprises the end of an optical wave-guide (23) which extends through the connected cable (24) besides the electric supply lines. Thus it is achieved that the signals are present as optical signals in the cable (24) and cannot be influenced by electric disturbances.