On decrement factor how and why X/R correction is used and abused

Abstract

The dominantly reactive nature of electricity networks, coupled with fast protection times, means the short circuit response of the current waveform is rarely symmetrical. In reality, fault current asymmetry produces an increased rms equivalent current due to decaying DC offset modelled by decrement-factor. For earthing systems this larger rms current creates higher earthing voltages and should be considered properly so that worst case hazards and equipment ratings are assessed. The experience of the authors is that decrement-factor is often incorrectly applied to earthing assessments or not at all. Sound engineering argument supports a case for decrement-factor to NOT be applied in areas of earthing system analysis where mutual coupling is concerned, for instance pipeline induction studies and earth return coupling, such as OHEW or cable-sheath. This first paper of ongoing studies is a re-examination of decrement-factor and how such correction should be made correctly to power system earthing analysis.