Abstract: The detection of events such as illegal entry detection, or the occurrence of predetermined hazards are detectable by a system including a circuit loop 2 which incorporates event actuatable switches 1, 35, 37, 38. Each switch has associated therewith means (9, 36, 37a, 39) for both maintaining electrical continuity in the loop, following operation of the associated switch and for producing an electrical condition in the loop which is indicative of switch operation. A control circuit 7 is arranged to respond according to the condition produced and produce an alarm characteristic of the condition.