The distance impedance relays are normally used for phase- to-phase faults as for such faults the loop impedance between the phase is obtained which remains constant, but in case of earth fault the loop impedance consists of impedance of one line and the impedance of the earth fault which is a variable factor. Practically it has been estimated that for earth fault loop impedance is approximately 1.5 times the impedance of earth phase (phase and neutral), however for a multiple earth system this factor may be taken 1.25. Hence for earth faults special connections can be made, so the impedance relay may be used for both phase and earth fault

From this it appears that the system will require two – impedance relays with voltage restraint elements energized, one for phase faults and the other for earth faults. However with the use of earth and phase fault setting resistances, and transfer relay, only one impedance relay set can he used as illustrated in fig 13. Normally the relay is connected for phase-to-phase fault, when an earth fault occurs, the earth fault resistance acts into the circuit through transfer relay.

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