In AC circuits one must consider essentially the impedance of an earthing ZE, which is the impedance
between the earthing system and the reference earth at a given operating frequency. The reactance of the
earthing system is the reactance of the earthing conductor and of metal parts of the earth electrode. At low
frequencies - the supply frequency and associated harmonics - reactance is usually negligible in
comparison to earthing resistance, but must be taken into account for high frequencies such as lightning
transients. Thus, for low frequencies, it is assumed that the earthing impedance ZE is equal the dissipation
resistance RD, which is in turn assumed to be approximately equal to the earthing resistance, R: