Electronic condition sensing system

In the electronic sensing system control means 12 are coupled via two wires 1, 2 to a sensing device 3 forming an electrical loop, the performance of which is altered by the sensing device sensing a particular condition. A current supplied to the loop is repeatedly modulated, using amplitude and frequency or phase variation one of which is non regular. An alarm condition occurs if one or more characteristics of the modification differ from the original. Apparatus operating this method has particular use in integrated security systems for large sites.