SOIL RESISTIVITY IN EARTHING DESIGN – SAFETY and RELIABILITY
Soil resistivity is a measurement of the resistance or electric current that the soil conducts. The design of a system based on passing current across the surface of the earth is a key component.
Soil resistivity plays a vital role in the design of an earthing system. For substations, the key design acceptance (safety) criteria of touch and step potentials are greatly influenced by soil resistivity. For other grounding systems such as industrial plants, commercial and residential buildings, the acceptance criteria of overall grounding resistance and current density on the electrode surface are also dependent on soil resistivity.
Soil resistivity can be measured in a variety of methods. Electrical profiling is the best approach for assessing soil resistivity for the design of earthing and safety requirements. Wenner's Four-Point Method is the most popular and user-friendly method. In this method, four spikes with equal spacing are connected to the soil resistivity meter. The current is injected through the extreme (current) spikes and the potential drop is measured between the inner (potential) spikes.
The resistivity of the soil is dependent on salt content, moisture, and temperature. The resistivity cannot stay uniform throughout the life of the system as the soil can be eroded due to wind, rain, floods, and manmade actions.
Parameters Impacting Soil Resistivity Measurement
- Presence of loose/backfilled soil
- Presence of buried metals like water, gas, other process pipelines and cables
- Presence of live HT lines above the test setup which may induce current in the cables connecting the spikes
- Presence of water table near the measurement area or any earth grid of a substation
- Distance of any such earth grids and water table from the soil resistivity measurement area should be at least the spacing between the spikes
If the largest variation in a given direction exceeds 30%, the soil is deemed non-uniform and a multilayer soil model shall be used for designing. While using a simple average (arithmetic mean) of measured soil resistivity values for design, the margin of error in safety parameters due to soil resistivity can easily exceed 100%.
The standards have provided formulas to determine approximate values of touch and step potentials for uniform layer and two-layer soils. For multi-layer soil, it is necessary to use software simulation models as prescribed in IEEE 80 Annex H. The CDEGS software by SES has multi-layer modelling capability and provides accurate and optimum design with least possible error.
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