Monitoring equipment for monitoring equipment in an office or similar place using the mains wiring, the equipment comprising a main-power outlet casing (20) having at least one power socket (31); at least one connector for a monitoring wire pair (45) and electromagnetic transmitter (36) adapted to transmit a signal differing in accordance with differing electrical impedance between the said pair (45), the transmitter being housed in the casing (20). The monitoring wire pairs are provided either in association with the power socket (45) or independently (55) therefrom. In use a plurality of equipments (20) will be arranged as remote stations associated with another equipment (80) incorporating a receiver (86). On installion the electrical impedance between each monitoring wire pair (45) is registered on a computer (88) at the base station (80). Each transmitter (36) is adapted to relay a signal to the base station (80) if the electrical impedance across any of its monitoring pairs (45, 55) changes. The base station (80) initiates an alarm (89).
