This paper deals with the behaviour of a grounding system, under impulse lightning current. The authors have made a large number of experiments in the High Voltage Laboratory of N.T.U.A. and the relevant experimental results were found to agree favorably with those obtained theoretically by using the Electromagnetic Transient Analysis Program (EMTP). Experimental results have been obtained by using variable peak impulse lightning current. The typical waveforms of the injected electric current and the variation of the potential and the impedance upon the time are presented using diagrams which show the compliance between theoretical and experimental data.