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÷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.
This paper considers electric shock hazard due to induced sheath voltages in 110 kV power cables. The purpose of this paper is to find an optimal configuration of the power cable system, taking into account electric shock hazard and ability of the system to transfer maximal power. A computer simulations on a computer model of the local power system, comprising high voltage power cables, were carried out. This model enables to analyse various configurations of the metallic cable sheaths bonding and earthing (singlepoint bonding, both-ends bonding, cross-bonding) and their impact on induced voltages in the cable sheaths. The analysis presented in the paper shows, that it is possible to find an optimal configuration of the complicated power cable system, in terms of electric shock hazard, maximal power transfer as well as economic aspects.