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.
About the Author
A Filipino Engineer, Registered Professional Engineer of Queensland (RPEQ) - Australia and Professional Electrical Engineer (PEE) - Philippines with extensive experience in concept select, front-end engineering, HV & LV detail design, construction and commissioning of Hazardous and Non-Hazardous Area electrical installations in water and waste water pipeline and pumping facilities, offshore platforms, hydrocarbon process plants and pipelines including related facilities. Hazardous area classification and design certification (UEENEEM015B, UEENEEM016B, UEENEEM017B).