Method of earth fault loop impedance measurement without nuisance tripping of RCDs in 3-phase low-voltage circuits

Abstract

Verification of electrical safety in low-voltage power systems includes the measurement of earth fault loop impedance. This measurement is performed to verify the effectiveness of protection against indirect contact. The widespread classic methods and meters use a relatively high value of the measuring current ( 5 (cid:4) 20 ) A, so that they are a source of nuisance tripping of residual current devices (RCDs). The meters dedicated to circuits with RCDs usually use an extremely low value of current (lower than 15 mA), which in many cases it is not acceptable in terms of the measurement accuracy. This paper presents a method of earth fault loop impedance measurement in 3-phase circuits, without nuisance tripping of RCDs – the concept of measurement, the meter structure and the experimental validation. The nuisance tripping is avoided in spite of the use of measuring current value many times higher than that of the rated residual current of RCDs. The main advantage of the proposed method is the possibility of creating values of measuring current in a very wide range, what is very important with regard to accuracy of the measurement.