Although the protection for cable feeders (only non-pilot wire type) can be used for transmission lines also, but generally such protections for transmission lines are not used as the transmission lines are longer and work at much higher voltage and there are frequent faults. For protection of transmission lines usually distance protections are used, the principle of operation of these relays as explained earlier depends on the fact that under fault conditions the ratio of voltage to current remains constant and is independent of the fault current and its power factor. As, a matter of fact such relays are designed to measure the impedance of the line (in some cases only reactance) or the ratio of voltage to current up to the fault point and it indicates the distance over which the fault has occurred. These relays under normal condition are used for phase-to-phase fault unless some special connections are made.

There are two types of distance relays:

(i) Definite distance relay.

(ii) Time-distance relay.

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