The influence of seasonal soil moisture on the behavior of soil resistivity and power distribution grounding systems

• Laboratory tests on several soil samples showing the influence of the soil moisture on the resistivity.
• An equation which can be applied for estimating the resistivity of clayey soils for a range of moisture contents.
• Analysis of the correlation between soil resistivity and monthly rainfall.
• An equation for the estimation of the soil resistivity corresponding to the critical condition of accumulated precipitation based on a measurement performed under a known condition.
• Analysis of the influence of seasonality on the apparent resistivity of different soil types.
• Analysis of the effect of the precipitation conditions on the apparent soil resistivity for grounding systems of a wide range of dimensions.

This paper discusses the influence of the moisture content on the seasonal behavior of both soil resistivity and ground resistance typical of power distribution grounding systems. Although field surveys indicate a wide variation in the moisture content only for the topmost soil layer, laboratory tests show that the resistivities of several types of soil are greatly affected. As a consequence, the ground resistance of systems with small dimensions may vary significantly and compromise the lightning performance of protection systems. The strong correlation, identified from measurements in this work, between soil moisture and, thus, soil resistivity, and monthly rainfall allows extrapolating resistivity measurements for the most critical conditions providing data that can be used to improve the design and performance of grounding systems.