Wire-pilot relaying does not provide back-up protection. Separate overcurrent or distance
relays are used for this purpose. When wire-pilot relaying is applied to an existing line, it is
often the practice to use the existing relaying equipment for back-up protection.
Distance relays may be used for back-up protection even though the line is too short to use
distance relays for primary protection. In such a case, the high-speed zone would be made
inoperative.
When directional-overcurrent relays are used for back-up protection, the requirements on
the voltage source are the least severe, and uncompensated low-tension voltage can be
used. It will be noted that the conventional type of a-c wire-pilot-relaying equipment does
not use any a-c voltage.