Physical model of surge-current characteristics of buried vertical rods in the presence of soil ionisation

In this paper a physical model to describe the non-linear surge characteristics of vertical rods is introduced. The model takes into account the creation of ionised discharge channels around the rod. The resistance of these channels as a function of the current flowing through them is developed by taking into account the heat generation in and dissipation out of the channels and the temperature dependent conductivity of air. The total resistance of the rod is evaluated by taking into account the fraction of current flow in soil and along ionised air channels. The model contains three unknown parameters, which were estimated by comparing simulations with experiments. The results show that more or less the same set of model parameters can be used to predict the temporal variation of the surge impedance of ground electrodes under different experimental conditions.