Chapter 8 - Circuits with nonsinusoidal currents

Abstract

Contemporary low-voltage electrical installations are relatively heavily saturated with electronic or power electronics devices. In circuits containing such devices, nonsinusoidal earth fault currents may flow. The earth fault current may be of a unidirectional type of various pulsations or may be in the form of the alternating current composed of harmonics. Real earth fault currents may differ from those normalized by international standards, according to which residual current devices are tested by manufacturers before placing the product on the market. The aim of this chapter is to show the behavior of residual current devices under various nonsinusoidal residual currents, especially waveform shapes different than normalized but occurring in real circuits. The results of the tests presented in the chapter indicate that the tripping current of residual current devices, in presence of strongly distorted residual currents, may significantly differ from the normalized tripping current expected for the 50/60Hz sinusoidal waveform. In some cases, it may lead to ineffective protection against electric shock. The sources of different tripping thresholds of residual current devices, under nonsinusoidal waveforms, are explained as well.