purposes and thus prevent the plc apparatus associated with the controller means of which that plc recognition means is a part from attempting to transmit and or receive plc signals until plc usage by the other apparatus has terminated as the various loads may be at widely scattered locations the power line is the preferred physical connection between the various controller means associated with those loads accordingly it is preferred that the plc recognition means 40 at each controller means be utilized to check for the presence of a transmitted plc carrier at the transformer secondary winding to ascertain whether the plc apparatus associated with another controller means load combination is actually transmitting at a time when a first plc apparatus associated with another load controller means combination desires to transmit recognition means 40 may therefore be a plc carrier receiver tuned to the one or more carrier frequencies which may be used in any particular system or may be a receiver tuned to a seperate carrier frequency upon which an auxiliary carrier signal is transmitted by that one of plc apparatus 16 then transmitting one or more plc carriers as plc apparatus 16 only transmits within the zero crossing time intervals t p a data transmission may require more than one zero crossing time interval for completion e g the time interval t m commencing at time t.