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Home » Ask the Experts Electrical Earthing Blog » Understanding Soil Resistivity & Soil Resistivity Testing Methods

05/08/2016 By Ian Leave a Comment

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.


Understanding Soil Resistivity

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

Soil Resistivity Testing Methods

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

  • Wenner method
  • Schlumberger method
  • Driven Rod

Webinar series – Soil Resistivity

Did you know you can also register for free to watch webinar replays from the soil resistivity series

  • Part 1 – Testing, avoiding the common mistakes
  • Part 2 – How to process the data
  • Part 3 – How the seasonal changes affect the electrode performance/behaviour

Filed Under: Soil Resistivity Tagged With: BS7430, Soil Resistivity Testing, Soil Resistivity Testing Methods

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Do you want to understand more about Electrical Earthing System Design – Greymatters Academy is our Earthing Training site take a look or access your free trial here.

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Soil Resistivity Testing Methods Wenner 4 Probe Test

Soil Resistivity Testing Methods a popular post. Originally published in 2013 and now updated. A Wenner probe test is a geotechnical investigation method used to determine the electrical resistivity of the soil. Soil resistivity testing can be carried out using different methods. The Wenner 4 Probe test is one of the most common soil resistivity […]

Schlumberger Array

The Schlumberger array is a geotechnical investigation method which determines the electrical resistivity of the soil. This is similar to the Wenner probe test, but it uses multiple current electrodes rather than just two. This allows for a more detailed and accurate measurement of soil resistivity. Soil resistivity testing carried out using different methods, this […]

Driven Rod Method

The driven rod test is a geotechnical investigation method which determines the resistance of soil to the penetration of a rod or other penetrometer.  The Soil resistivity is determined using different Soil Resistivity Testing methods. However the Driven Rod, described below is the last of the three most popular Soil Resistivity Testing methods we use […]

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About Ian

This post is written by Ian Griffiths, Principal Engineer at GreyMatters, an Earthing & Lightning Consultant of 28 years, one of the top 1% accredited CDEGS and XGSLab consultants and professional advisor to international utility companies, data centre and infrastructure developers.

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