A brief survey of past and present impedance relays is given, with particular reference to their immunity from interference owing to non-power-frequency components in their input signals. The frequency characteristic of a suitable digital filter is chosen on the basis of a spectral analysis of a 10 ms section of typical fault waveforms. The chosen filter is used in conjunction with a sampling system of measurement to realise a quadrilateral impedance characteristic. Results of an off line simulation of fault conditions are presented, in the form of typical fault waveforms at the relay input, and the relay operating time for various angles of fault incidence.