Multi-sensor alarm system with fault checking - has relay contacts wired in double circuit to provide alarm and checking facilities

摘要

An alarm system using a large number of sensors enables a simple check to be made to distinguish an alarm condition from a circuit fault. Each alarm sensor is coupled to a relay that has a pair of change over contacts. A number of the relays are coupled in series. The alarm circuit is made between terminals such that all relays adopt identical states with contacts and made. If one relay drops out an alarm warning is immediately triggered. A check on the alarm circuit function is made such that all relays are in the dropped-out condition such that a connection exists between other terminals. Any relay remaining switched is indicative of a fault. An alternative design utilises optically isolated coupling stages.