## 1 Introduction  
Grounding resistance is one of the most important parameters that determine the suitability of a protection installation. Its measurement is an important issue and many studies have focused on it. Although it is possible to theoretically calculate its value provided the physical configuration and soil parameters are known [1, 2, 3, 4], field measurement is a mandatory task. Among the various methods of measuring the earthing resistance of an operating electrode [5], the well-known fall-of-potential method (FOPM) is one of the simplest and most effective. This method involves externally connecting the electrode to be measured to a current source and another auxiliary electrode located far away at a distance D, forming a circuit that closes as the current passes through the ground from the electrode being measured to the distant auxiliary electrode, that is, a three-pin arrangement. The procedure is completed by measuring the potential difference between the electrode and an auxiliary probe driven into the ground at an appropriate distance \(x\) from it. For a very specific value of this distance, around 0.618D, the ratio between the measured potential difference and the injected current provides the value of the grounding resistance of the electrode. This is known as the 62% rule [6]. This is always satisfied provided that the electrode to be measured is small, \(D\) is very large, and \(x\) is large enough so that from distance \(x\), the electrode behaves like a point source of current at ground level [7]. To measure extensive electrodes associated with large installations, the size of the electrodes presents a problem since it would be necessary to place the auxiliary potential probe at a considerable distance to meet the requirements of the 62% rule, not to mention the auxiliary current electrode, which must be placed even farther away from the electrode, severely compromising the proper functioning of the circuit that closes through the ground loop. This would require a more intense current injection for the measurement method to work properly. Several attempts have been made to avoid the limitations of FOPM when it is not possible to move the auxiliary current electrode far enough away [8; 9; 10; 11]. Some modifications of the FOPM have been proposed [12; 13] and the effect of surrounding electrodes and other conductors on FOPM measurements have been studied [14]. Currently, in the European context, the measurement of grounding resistance for large electrodes in electrical installations with alternating current voltages above 1 kV connected to transmission lines is carried out in accordance with the EN 50522 standard [15]. According to this standard, it is necessary to inject current through the grounding system of a transmission line tower at the substation. The tower must be sufficiently distant so that both electrodes can be considered isolated. This will create a measurable rise in the ground potential at the electrode being tested, which can be used to evaluate the grounding resistance. Despite the apparent simplicity described, the procedure is complex and costly to execute, as it requires handling the transmission line used. In this study, a new modification of the classic FOPM is proposed, which is applicable to situations like those described above without requiring the potential probe to be placed far from the edge of an extensive grounding grid. The only requirement to maintain is that the auxiliary current electrode remains as far away from the mesh as possible. Moreover, the farther this electrode is from the grid, the more precise the measurements obtained. This is achieved by defining a correction factor that must be multiplied by the field measurement result of the resistance to obtain the correct grounding resistance value of the mesh. The correction factor depends on the nature of the grounding electrode and the theoretical potentials it generates around it through current injection.