Abstract: A security system has a locality station sensing security risks such as fires, electrical faults and break-ins, being powered by a single line from a central station. The loading on the power supply is determined by logic circuits to give direct remote indications of the security state at the locality station. The power supply is a square wave which is altered in waveform to give alternative commands to the locality station, e.g. to sound a warning or to operate a fire sprinkler, while continuing to supply power. To enable maximum use of the single line and single square wave, the latter is treated as two separate signals in that loading by the locality station is selective as regards the polarities. Similarly the alternative commands available are doubled by using separate control of the durations of the positive and negative square wave portions, while continuing to supply power.