What is Soil Resistivity and Why Does it Matter?

Soil resistivity is an important parameter in the design and analysis of grounding systems, and it is essential to measure and understand soil resistivity to ensure the safety and reliability of electrical systems.

Soil resistivity is expressed in ohm-meters (Ωm) and is a measure of the resistance of the soil to the flow of electric current. The resistivity of soil is influenced by various factors including:

  • soil type
  • moisture content
  • temperature
  • ionic content
  • compaction

Low Soil Resistivity

A lower soil resistivity means that the soil is more conductive and is better able to conduct electrical current. This is desirable in grounding systems where a low resistivity is required to ensure that electrical charges are safely dissipated into the earth.

High Soil Resistivity

Conversely, a higher soil resistivity means that the soil is less conductive and is less effective at conducting electrical charges. This is not desirable in grounding systems as it can result in higher resistance, leading to increased risk of electrical shock and other safety hazards.

How to Test Soil Resistivity

Soil resistivity can be measured using various techniques such as the Wenner four-point method, Schlumberger method, and the fall-of-potential method.

Measurements are typically performed by placing electrodes in the soil and measuring the potential difference between them. This potential difference is then used to calculate the soil resistivity.

Typical Soil Types and Resistivity Range

Soil Resistivity Ohm-cm (Range)
Surface soils, loam, etc. 100 - 5,000
Clay 200 - 10,000
Sand and gravel 5,000 - 100,000
Surface limestone 10,000 - 1,000,000
Shales 500 - 10,000
Sandstone 2,000 - 200,000
Granites, basalts, etc. 100,000
Decomposed gneisses 5,000 - 50,000
Slates, etc. 1,000 - 10,000