Soil ionisation under lightning impulse voltages

Experiments have been carried out to study soil ionisation and the variation of the critical ionisation gradient against the soil resistivity. Compared with relevant experimental results of other researchers a satisfactory convergence has been proved. An accurate estimation of the critical ionisation gradient has been achieved, of the order of 200 kV/m, following the finding that the critical voltage gradient for wet soil decreases to approximately 35% of the corresponding value for dry soil.