The touch voltages obtained in the safety calculation report and touch voltages plots indicate that our initial design is quite far from providing a safe ground grid design: touch voltages exceed the safe limit at most locations throughout the substation. The highest values occur in the corner meshes of the grounding grid, which suggests that there is a need to have more conductors towards the edge of the grounding system than towards the central portion. This observation is consistent with analytical and experimental results. However, the optimum or most efficient conductor compaction at the periphery of a grounding system depends on many factors, particularly on earth structure characteristics. Moreover, practical considerations often introduce additional constraints, which must be accounted for. In general, however, the following crude rules of thumb can be used as a preliminary set of guidelines:
When the surface (shallow depth) soil resistivity is small compared to that of the deeper layers (those which are not in contact with the grounding system), use grids with more conductors at the edge than in the central area (exponentially-spaced conductors). The degree of conductor clustering (compactness) at the periphery of the grid should increase with an increase in the contrast between the surface and deep layer resistivities.
When the surface soil resistivity becomes larger than that of the deeper soil layers, the clustering (compactness) ratio should decrease towards a uniform distribution of conductors in the case where the contrast ratio is significant (5 or more) and the thickness of surface layers is small compared to the size of the grounding system (1/5 or less).
Finally, when the surface soil resistivity is quite large compared to that of the deeper layers and its thickness is small enough so that use of ground rods penetrating into the deeper layer is efficient, a number of ground rods should be installed wherever possible to reduce the GPR, touch and step voltages instead of using unequally spaced conductors.
Based on the soil model and the initial design, we will illustrate the first methods in this study. We will also increase the total number of conductors in the grid. The improvements will be carried out by modifying the grid itself to use a denser exponential design.