A definitive assessment of the use of primary clearing times for safety criteria targets

Abstract

Earthing by its nature is particularly open to speculation, opinion and contention. One such point of contention is whether primary or backup clearing times should be used in the preparation of appropriate safety criteria. As humans become increasingly susceptible to ventricular fibrillation the longer the heart current duration, the use of back up clearing times often leads to significantly lower allowable touch voltages (and higher associated costs in achieving compliance). On the other hand using primary protection clearing times for the protection of people, while using back up clearing times for the protection of equipment, seems ill balanced. This paper investigates and presents the case for and against the use of primary clearing times, including a quantified risk based assessment.