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The key question addressed in this paper is: for comparative purposes, what is a meaningful measure of the relative performance of an earth electrode under lightning transient conditions? Several methods of quantifying the transient performance are introduced and evaluated using time-domain transient analysis of equivalent circuits. The nonlinear effect of soil ionisation is included and the effect of changing soil resistivity and lightning current waveform is considered. The concept of normalised impulse impedance is introduced to facilitate meaningful comparison between different electrodes and conditions. It is concluded that the equivalent resistance of an electrode gives an acceptable measure of performance as well as being relatively easy to both calculate and measure.