A sampling technique has been developed which allows a comparison of instantaneous values derived at different instants of time, thereby dispensing with the need to phase-shift or mix signals derived from the primary line quantities. Both amplitude and phase comparisons are used to derive a variety of impedance characteristics. The relays combine high sensitivity with a high resistance to interference. Detailed results are presented for a current-polarised relay with a characteristic surrounding the probable fault area.
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P.G. McLaren (1968) studied this question.