Finite difference time domain simulation of earth electrodes soil ionisation under lightning surge condition

The non linear effects of soil ionisation on the behaviour of earth electrodes are investigated by a finite difference time domain numerical scheme. A time variable soil resistivity is used in order to simulate the soil breakdown; step by step the resistivity value is controlled by the local instantaneous value of the electric field compared with the electrical strength, fixed for the soil. No a priori hypothesis about the geometrical shape of the ionised zone around the electrodes has to be enforced. Simulation results related to complex earth electrodes of limited extension are reported. The model has been validated by comparing the computed results with data available in technical literature. A discussion about dangerous earth voltages is reported, by underlining the role of soil ionisation. Electrodes of larger extension can be straightforwardly simulated by means of larger computational resources.