Earth Resistivity and Grounding Design – A brief history and the missing link

In 1915 Wenner (Frank Wenner 1873-1954) of the National Bureau of Standards’ electrical resistance measurement section published a landmark bulletin describing a method of measuring earth resistivity. That method we now obviously call the Wenner Method. Wenner devised his method as a way of injecting a current into the ground via two electrodes insulated to a known depth and then measuring the voltage response at two other similar electrodes either within the first two, or outside, aligned and equidistant. The purpose for conducting such a test was for Electrical Prospecting, ideally used to identify oil and ore deposits potentially worth extracting. Another well-known and alternative earth resistivity measurement method is the Schlumberger method (Conrad Schlumberger 1878-1939) and was also developed in the early 1900’s as a means of improving the outcomes of geological surveying and prospecting, particularly for petroleum exploration, with outstanding success!

It is quite likely that neither gentleman ever thought that their testing methods would be used to measure earth resistivity for earthing/grounding design. The purpose of which is to develop one of the cornerstone building blocks of a safe electrical system, that is the electrical resistivity of the earth in which we design and build substation grids. When did these geo-physicist’s names enter the common vernacular of electrical power system earthing and grounding?

This paper presents a practitioner’s re-proof of the basic electrical theory that led to the development of these test methods. It also discusses the correct application of soil electrical resistivity models for earthing/grounding design and helps to refine the method by which surface resistivity is assessed, which is an important and often ill-considered factor in the development of safety criteria for electrical safety.

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