Understanding the impulse response of tower grounding systems is a key factor in the determination of the lightning performance of transmission lines and the safety voltages near high voltage towers. This paper reports the results of an experimental study on the impulse characteristics of full-scale transmission line tower footings. The ground potential distribution both at the ground surface and below ground was examined under impulse currents and variable-frequency AC currents. Prospective impulse step and touch voltages were determined by measurement of the ground surface potential distribution near the tower base. The behaviour of the impulse resistance of the tower footings under highmagnitude impulse currents was investigated. The non-linear characteristics of the impulse resistance with peak current are demonstrated for the entire tower base and for single tower footings. The soil ionisation inception is found to be dependent on the rate of rise of applied current. The dynamic impulse resistance was used to describe the dynamics of soil ionisation.
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