Abstract: A loop impedance meter is provided for testing an AC electrical main supply incorporating a residual current device. The loop impedance meter includes an electrical control circuit for connecting a load resistance intermittently between the AC main supply terminal and the earth terminal to insure a potential difference between those terminals and to provide an indication of the loop impedance of the AC main supply from that potential difference. The control circuit includes an electronic switch in series with the load resistance, and arranged to open and close the electronic switch by applying a plurality of voltage pulses. The width of the voltage pulses is less than a millisecond, causing a series of measurement pulses of different widths to flow through the load resistance. The control circuit is arranged to process the results of measuring the loop impedance by extrapolating to the effective impedance at the AC mains supply frequency.

Abstract: A loop impedance meter for testing an A.C. electrical mains supply, including an electronic control circuit for connecting a load resistance intermittently between the A.C. mains supply terminal and the earth terminal to measure the potential difference between those terminals and to provide an indication of the loop impedance of the A.C. mains supply from that potential difference, wherein the control circuit is arranged to allow a train of short pulses of current to flow through the load resistance and the loop, the pulse train beginning its sequence with a first train of pulses for preconditioning any residual circuit device (RCD) present in the loop to temporarily desensitize it, wherein the first train of pulses is followed by one or more measurement pulses, the pulses of the first train being of generally increasing width.