Estimation of uncertainty regarding soil breakdown parameters

The behaviour of a grounding system under fault currents differs from its steady-state behaviour. It has been observed that, when a high impulse current is injected into the grounding system, its impulse impedance drops because of the ionisation phenomenon. The critical parameter for the ionisation phenomenon is the soil critical electric field, which corresponds to the electric field threshold above which the soil ionisation occurs. In bibliography various attempts have been made aiming at its estimation. However, a physical quantity is determined not only by its value, but also by an estimation of the uncertainty. The aim of this work is the analysis of the components of uncertainty and the estimation of the uncertainty regarding the determination of the breakdown voltage associated with the soil critical electric field. Therefore a series of measurements has been conducted by subjecting dry and wet soil samples to impulse voltages, while recording the voltage and the current. Based on these measurements the random uncertainty is calculated while an estimation of the systematic uncertainty is given taking into account the measuring equipment.