Soil resistivity and reducing earth resistance

Soil resistivity (ρ, in Ω·m) is the resistance of a 1 m cube of soil between opposite faces. It is the main quantity deciding the resistance of an earth electrode, since for a rod electrode R ≈ (ρ / 2πl) · ln(4l/d), where l is the rod length and d its diameter.

Factors affecting soil resistivity

1. Type of soil: clay and marshy soil 10–100 Ω·m; sand and gravel 500–1000 Ω·m or more; rock above 1000 Ω·m.
2. Moisture content: water dissolves salts and carries current; resistivity falls very quickly up to about 15–20% moisture.
3. Dissolved salts (chemical content): more salt means more ions and lower resistivity.
4. Temperature: lower temperature increases resistivity; frozen soil has very high resistivity.
5. Grain size and compactness: fine, closely packed grains give lower resistivity than loose coarse soil.
6. Seasonal variation: resistivity is highest in the dry season, so earth resistance must be designed for that time.
7. Depth (layering): different layers have different values; deeper layers are usually moister and more stable.

Methods to reduce earth resistance

1. Increase the length (depth) of the electrode: reaches moist layers; resistance falls roughly inversely with length.
2. Use multiple electrodes in parallel, spaced at least equal to (preferably twice) their length, or use an earth mat/grid in substations.
3. Increase electrode diameter or use plate/strip electrodes, giving larger contact area (gain is smaller than increasing length).
4. Chemical treatment: surround the electrode with salt and charcoal in layers, or with bentonite or other earth-enhancing compounds.
5. Keep the soil moist: provide a watering pipe in the pit, especially in the dry season.
6. Replace high-resistivity soil near the electrode with low-resistivity soil (clay, loam).
7. Use counterpoise wires (buried horizontal conductors) for tower footings in rocky areas.
8. Ensure good contact and corrosion-free joints: compact backfill and use copper or copper-bonded electrodes.

The aim is usually below 1 Ω for large substations, below 5 Ω for distribution substations and below 10 Ω for tower footings (typical values).