Alarm installation with a number of self powered indicating elements

In an alarm system comprising a plurality of indicating elements each including a warning device and an independent power supply source, the provision of

• (a) a first control circuit for each respective element connected between the source and the warning device to control the operation thereof;
• (b) coupling means comprising a pair of wires connected to the first control circuit of each indicating element to form a pair of common connections in parallel therebetween;
• (c) a second control circuit between the pair of wires forming said coupling means at a position remote from said first control circuits connected with each said first control circuit via said coupling means to form a common control for each said indicating element capable of maintaining a potential difference between the pair of wires of said coupling means, and
• (d) a switch mechanism for actuating said alarm connected between said common second control circuit and each said first control circuit via said coupling means, said mechanism being operable to effect at least one of the conditions of short circuit of said second control circuit, open-circuit between the first control circuit and the second control circuit, and both the aforesaid short circuit and open circuit conditions causing a change in circuit conditions normally dependent on said second control circuit, to effect operation of said alarm when an open circuit or short circuit condition occurs.