Scheme simplicity is achieved by careful design so that switching out of the pilot supervision equipment is not necessary. Starting relays are thus avoided and double-end tripping is obtained without infeed being required at the remote end excpet where guard relays are included in the scheme. The pilot wire information link is vital for the stability of pilot wire feeder protections. Supervision is by the injection of a d.c. supply at one (send) end, with a continuously energised, polarised, relay monitoring this supply at the other (receive) end. Thus an open or short circuit, or crossing of the pilot wires will cause the receive relay to be de-energised.