Abstract-- The scope of this paper is the investigation of the variation of the soil resistivity and the grounding system resistance during the year. Measurements of soil resistivity variation as a function of time, at certain characteristic locations, have been carried out. The annual change of multi-layer earth structure parameters has also been calculated by the analysis of data of measurements. The calculation of the earth layer structure parameters is in fact an optimization problem. On this purpose, data treatment has been based on a methodology which uses a genetic algorithm model. The proposed methodology provides evidence of highly accurate results in the calculation of multi-layer soil parameters. These results are used as input data for the simulation of grounding systems behavior for this particular soil type. In this way, an optimized design of the grounding system under consideration becomes possible, eventually resulting in minimized step-voltage and touch- voltage built-up. Index Terms--Ground resistance, Multi-layer soil structure, Soil resistivity measurements, Step voltage, Touch voltage. I. INTRODUCTION The design and construction of a grounding system requires the knowledge of earth structure, since grounding system location is defined by the home site that it protects. Measurements of soil resistivity that have been carried out in various regions throughout the country, showed that in the analysis we should consider at least a two-layer earth structure. Further on, the repetition of measurements Contact Address: Dr. Ioannis F. Gonos High Voltage Laboratory, School of electrical and Computer Engineering, National Technical University of Athens 9 Iroon Politecniou St., Zografou Campus, GR15780, Athens, Greece E-mail: igonos@ieee.org makes obvious that soil resistivity varies upon time and depends on the season of the year, reaching maximum values in the summer months. The final setup of the grounding system is determined by multi-layer earth structure parameters, like soil resistivity and depth of each layer. The calculation of such parameters is necessary in order to achieve a safe grounding system with minimized material and construction cost. The permitted limits for the step and the touch voltage (in V) are given by the following equations [1]: ( ) 1000 6 w step s s s C E C t ρ = + ⋅ ⋅ (1) ( ) 1000 1.5 touch s s s C w E C t ρ = + ⋅ ⋅ (2) where t s is the duration of the shock current (in seconds) and C w is a constant with values 0.116 / 0.157 for a human body weight 50kg / 70kg respectively. C s is a derating factor, which is given by the following empirical equation [1]: 0.09 1 1 2 0.09 s s s C h ρ ρ   ⋅ −     = − ⋅ + (3) where  s and h s the soil resistivity and the thickness of the surface’s material (in Ohms·m and m, respectively). However, a thin layer (8 – 15cm) of a high resistivity material is often spread on the earth’s surface above the grounding system to increase the limit of the corresponding touch voltage. In case of absence of such surface material then C s = 1 and  s = . II. CALCULATION OF THE SOIL STRUCTURE The soil type affects the behavior of a grounding system significantly. The impact of soil type can be Variation of Soil Resistivity and Ground Resistance during the Year I.F. Gonos, National Technical University of Athens, A.X. Moronis, TEI of Athens, and I.A. Stathopulos, National Technical University of Athens -740- V-21 28th International Conference on Lightning Protection