The main purpose of the work is to identify the optimal domains for the earth grounding with horizontal, pipe-type electrodes. This continues a previous work, which summarizes the problem of horizontal earth grounding, brings together the calculation relationships for their design sizes and presents the optimal applicable criteria. The contributions of the present research begin with the framing of the earth grounding in the more general case of the penetration of the electromagnetic field in massive conductors, which is why the depth of penetration of the electromagnetic field in the main types of soils is determined. The relatively small dimensions of the earth groundings in relation to the penetration depth are noted, but they also warn against the existence of possible obstructions. The optimal criteria, i.e. minimum in this case, are the area of the footprint on the ground, the total mass of the metallic materials and the investment costs with the earth grounding. Their application on the physical model of the horizontal earth groundings, led to the identification of optimal domains, bipolarly grouped at the smaller dimensions of the electrode lengths, but at the lower and upper limits of the distances between the electrodes respectively.