A new article (RU) by Prof. Eduard Meerovich has been published, based on a previously held webinar.
The article (RU) discusses the problem of measuring grounding resistance in conductive soil, especially under conditions of dense urban development. In an insulating medium, the measurement is simple, but in soil the current spreads through the earth, which leads to mutual influence between the grounding electrode under test and the auxiliary electrodes, as well as the need to define the current and reference (zero-potential) points at finite distances. A key factor is the maximum overall size of the grounding electrode, D: the larger it is, the farther the current spreads and the more space is required for a correct measurement setup, which is difficult in a city. The analogy between electrostatics and the electric-current field makes it possible to reduce the required space by placing the electrodes on the same line.
An empirical rule is discussed: placing the potential electrode at a distance of 0.62 from the current electrode (for a hemispherical grounding electrode), which gives an accurate result only for an idealized geometry. For real grounding electrodes, this method overestimates the resistance, but it reduces the required distance for installing the current electrode to about one D and provides a conservatively positive error. In Russian practice, a scheme is standardized with all electrodes on one straight line and the potential electrode in the middle between the electrode under test and the current electrode (STO FGC UES 569477007-29.130.15.105-2011). Its advantages: reduced space to on the order of D, error up to 10%, positive error at Rcurrent = D, and simple layout. Before recording the result, it is recommended to check the line for the absence of large metallic objects by performing a series of measurements while moving the potential electrode and evaluating the smoothness of the resulting curve.
In addition to what was covered in the webinar, the article separately considers a stakeless method of measuring grounding resistance using clamp meters. Key features: the method is not very accurate and yields higher-than-actual values; the error decreases when there is a well-developed alternative grounding system with significantly lower resistance; it is not applicable to single grounding electrodes without the ability to temporarily connect an alternative network. From a standards perspective, the method is supported by GOST R 50571.16-2019 (IEC 60364-6), Section C3, which allows the use of a pair of clamps or a single clamp and explicitly indicates that the result is overestimated. Practical recommendation: traditional schemes are preferable in clean soil and when there is enough free space; clamps are justified in an urban environment with high metallic contamination, where there are no alternatives, provided the unavoidable error is understood and accepted.