Understanding Soil Resistivity & Soil Resistivity Testing Methods

Understanding Soil Resistivity is critical to Electrical Earthing System Design.

Soil Resistivity

Soil resistivity is a measure of how much the soil resists the flow of electricity. So an understanding of the soil resistivity and how it varies with depth in the soil is necessary to design the grounding – earthing system in an electrical installation.

Why is Understanding Soil Resistivity Critical to Electrical Earthing System Design?

Soil Resistivity is most critical when determining the design of the grounding – earthing system for new installations (green field applications). To meet your ground – earth resistance requirements, ideally, you locate a location with the lowest possible resistance.

Overcoming poor soil conditions with creative earthing system design was discussed in an earlier post. Because soil composition, moisture content, and temperature all impact the soil resistivity. Soil is rarely homogeneous and the resistivity of the soil will vary geographically and at different soil depths.

Seasonal Variation to Soil Resistivity

Moisture content and hydrology can change seasonally, and will vary according to the nature of the sub layers of earth, and the depth of the permanent water table. Soil and water are generally more stable at deeper strata, it can be useful that the ground – earth rods be placed at a depth to penetrate into the lower resistivity layers.  Occasionally, at the water table if practical. Also, ground earth rods should be installed where there is a stable temperature, i.e. below the frost line.

For a grounding – earthing system to be effective, it should be designed to withstand the worst possible conditions. Thus measuring and understanding soil resistivity is critical.

Measuring Soil Resistivity

Because soil quality may vary greatly with depth and over a wide lateral area, estimation of soil resistivity based on soil classification provide only a rough approximation. Actual soil resistivity measurements are required to fully qualify the resistivity and its effects on the overall electrical system.

Soil resistivity testing methods are covered in the article link below.

Wenner method

Schlumberger method