Ground Testing FAQs
Category: Theory Vol. 2

Why do I need to perform a test in accordance with a method?
Think about it! Most electrical measurements are made on equipment, which by design is tightly controlled and well understood. By consulting a schematic, or even by visual inspection, it isn't too challenging to figure out where to make connections and what might be expected. But when grounded, the planet, in effect, becomes part of the electrical system. This demands some special techniques. How do you know where, or how, to hook up the test leads? What precisely is the instrument measuring? What might be the effect of this or that contingency? What are we actually seeing? These are reasonable concerns.

We cannot control and manipulate the earth as readily as a piece of equipment on a work bench. Instead of trying to adapt the conditions to us, as might be done in a laboratory, we must adapt to the conditions. This is the function that standardized and recognized procedures fill. They are emphatically not devised by the manufacturers of test instruments to accommodate the capabilities of their products or to entrap the users with an esoteric test routine. The opposite is the case: instrumentation is designed according to the requirements of the established procedures. These standards have been developed by field engineers, through agencies like the U.S. Geological Survey, as the most effective means of overcoming the challenges presented by the immensity and infinite variability of the earth as an electrical element. Adherence to an accepted procedure means that the ground electrode has been tested by the most effective means, and the measurement is reliable.

But do you have to stick to a recognized procedure? No, not if you have a basic familiarity with ground testing. The experienced operator will sometimes be satisfied that the test result is reliable and that adequate probe spacing has been accomplished by merely moving the potential probe to make a few judicious repeat measurements. If these are in agreement to the operator's satisfaction (readings can be expected to vary somewhat with change of position but should hold fairly consistently to a central value if they are taken outside of the electrical influence of the test ground), then the average can be accepted as a reasonably close approximation to the true ground resistance. One of the valuable functions of standard procedures is that they objectify the evaluation of test results. The operator's judgment may be reliable but how is that verified if any external agent, such as a client or an independent authority, is involved? Standard procedures make use of mathematics, derived from calculus, to objectively throw out groups of readings that lack sufficient agreement. A series of readings taken with insufficient probe spacing, so that they are on the changing part of the resistance curve, will lack internal cohesion. Those that differ only by the slight variation that is normal after the curve levels off at maximum resistance will agree within an acceptable accuracy specification. The experienced operator may well be able to recognize the difference, but how to convince an independent agent? Standard procedures eliminate this duplicity by providing an objective method that can be confidently referenced in test reports.