Deep earth electrodes in highly resistive ground: frequency behaviour

To reach a low ground resistance value in very resistive soil, long vertical rods are often used. But due to the inductive behaviour of long electrodes, high frequency ground impedance could be much greater than its resistance at low frequency. Based on field measurements, this paper shows the existence of a critical length for ground rods or wires. An earth electrode shorter than the critical length has HF impedance lower than its resistance (capacitive behaviour). On the other hand, for greater lengths, the HF impedance increases as a function of the frequency (inductive behaviour). A simple modelling confirms these results.