Generalised equation of soil critical electric field EC based on impulse tests and measured soil electrical parameters

Abstract

The effective radius of the grounding electrode under high-impulse current becomes higher than the outer radius of the electrode because of soil ionisation. Because of this reason, the ground-impulse impedance is different from the 60-Hz grounding resistance, which is normally measured at low current. The impulse impedance for any given impulse current can be estimated if the critical electric field of the soil is known. It is therefore necessary to have a generalised soil critical electric field equation for grounding systems design as well as lightning performance study. A generalised equation for soil critical electric field Ec is proposed here as a function of soil dielectric constant kg and conductivity σg. Extensive impulse tests and electrical measurements were performed with different soils under different moisture contents. Dielectric constant kg was measured by the wave propagation technique using a soil-filled coaxial transmission line. Effect of waveshape on critical electric field Ec was statistically analysed for different soils.