Power and data are both transmitted over a single transmission line to which a power source and a number of stations are connected. A bipolar AC voltage is supplied to the line by the power source, the half cycle pulses of one polarity being used for supply of power only, while transmission of data is achieved by modulation exclusively of the half cycle pulses of the other polarity. High power is supplied to the stations by providing pulses of the one polarity at a low ohmic level. Only low power is required for modulation for the purpose of data transmission, and so the pulses of the other polarity are supplied at a relatively high ohmic level. Thus, only two conductors are required, one of which can be replaced by the system ground in the simplest case. If three conductors are provided, it is possible to achieve improved tolerance of defects in a conductor. At least parts of the system are maintained in operation, in the event of a short-circuit, by providing separation units which automatically disconnect different segments of a short-circuited line and automatically reconnect them after the short-circuit has been eliminated.