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Soil Resistivity

Soil Resistivity Survey, Analysis and Earth Termination System Design

Soil resistivity surveys provide valuable information in the design of the earth termination system.

The soil resistivity values prevailing on a site are utilised to determine the various layers of soil, corrosiveness and homogeneousness of the soil. The GPR (grid potential rise) and the step and touch voltages are calculated using the soil resistivity values.

The method of determining the soil resistivity distribution of the earth on the site for the purpose of designing an earthing system is the four-electrode sounding method called the ‘Wenner’ method of sounding:

  • Carry out on-site soil resistivity surveys – depending on the size of the site, various surveys are conducted to a sufficient depth to enable calculation of optimum rod lengths. All surveys are to be checked with traverse readings.
  • Analyse resistivity readings and calculate the following:
    • Resistivity in Ohms and Ohm/metres
    • Determine top soil depth – layered soil conditions
    • Appraise corrosiveness based upon soil resistivity & other site factors
  • Design earthing system based upon application, soil conditions (rock etc.), resistivity values and other external factors (corrosiveness etc.).
  • Calculate D.C. resistance of earthing system.
  • Analyse Grid Potential Rise and Step and Touch Potentials to ensure safety factors for the earthing system.

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