Protection of pilot wires from induced potentials

The use of pilot-wire relays for protection of power transmission circuits usually requires a reliable pilot channel which is free of any induced potentials which may cause transient voltages to appear between wires even though they may be of relatively low magnitude. This is particularly true in the case of pilot relays using a sequence network in which a single-phasealternating voltage is applied to the pilot wires. Conventional methods of protection, such as those used on telephone circuits, are not suitable. As a result it frequently has been necessary to install special pilot circuits which are insulated for voltages far in excess of their normal operating voltage to insure reliable operation. This paper describes a method of protection which provides reliable and satisfactory operation of a-c pilot channels through low-voltage telephone-type cable, even though the cable may be subject to induced potentials high enough to operate discharge gaps.