In previous articles Single Earth Electrode and Earthing Resistance Calculation, I have presented methods of calculating the earth resistance using IEEE or IEC methods.
AS 1768 Appendix C recommends that the resultant earthing resistance may be calculated by using the ordinary equation for resistances in parallel if the separation between earthing electrodes is greater than the electrode length. It also added that for practical purposes, the separation between vertical earthing electrodes can be taken as twice the length of the earthing electrode.