In short. The resistance of an earth electrode depends on the soil’s resistivity and the electrode’s size and arrangement. Measure the soil first, design the electrode system to meet the target resistance and touch-voltage limits, and test it after installation and periodically.
Soil resistivity
Resistivity varies widely with soil type, moisture, temperature and salt content. Rock and dry sand are high; wet clay is low. It also varies with depth and with the season, so a single reading in the wet season may understate the dry-season resistance.
Measurement
- Wenner four-pin method: four equally spaced probes; apparent resistivity is calculated from the measured resistance and the spacing. Vary spacing to see resistivity with depth.
- Take readings along more than one line.
- Record weather and moisture.
- Interpret with a soil model, not as a single number.
Electrode types
| Type | Notes |
|---|---|
| Driven rods | Simple, effective in layered soils where deeper layers are conductive |
| Plates | Larger contact area, used in some installations |
| Buried horizontal conductor (strip or ring) | Good for surface area and touch-voltage control |
| Foundation (concrete-encased) electrodes | Uses structure steel, widely used in new buildings |
| Chemical or backfill-enhanced electrodes | Used where soil is poor, with maintenance needs |
Design considerations
- Adding rods in parallel reduces resistance, with diminishing returns if they are too close; spacing of at least about the rod length is a common guide.
- Depth reaches moist layers, stable in dry seasons.
- Material and corrosion life, especially with mixed metals.
- Fault current, clearing time and touch-voltage limits, not only the resistance value.
- Interconnection with the building earthing and lightning system. See earthing architecture.
Testing after installation
- Fall-of-potential test with probes placed correctly out of the influence zone.
- Clamp-on tests for loops, where applicable.
- Record the result with weather and date.
- Retest periodically and after significant works.
Illustrative example
A site on rocky ground measures high resistivity at the surface. A design with deep-driven rods fails to reach conductive layers, so the engineer proposes a horizontal ring conductor with several rods and moisture-retaining backfill, then verifies the result with a fall-of-potential test. The record includes the season, so the next test can be compared fairly.
Why soil resistivity is measured first
Electrode resistance depends mainly on the soil around it. Dry rocky ground, common around parts of Hyderabad, has much higher resistivity than moist clay. Resistivity also swings with season, so a pit measured after monsoon can read far better than the same pit in May.
Soil resistivity ranges (indicative)
| Soil | Approx. resistivity (ohm-metre) |
|---|---|
| Wet organic soil, moist clay | 10 to 100 |
| Sandy or gravelly soil | 100 to 1,000 |
| Rock, dry laterite | 1,000 and above |
Single rod estimate
R ≈ (ρ ÷ (2π L)) × (ln(4L ÷ d) − 1)
For ρ = 100 ohm-m, L = 3 m, d = 0.0165 m: R ≈ (100 ÷ 18.85) × (ln(727) − 1) ≈ 5.31 × 5.59 ≈ 30 ohm. Several rods spaced apart, or longer rods, reduce this. Illustrative only.
Ways to reduce resistance
- Parallel rods spaced at least their length apart.
- Deeper rods to reach moist layers.
- Chemical or conductive backfill compounds where permitted.
- Earth mats or grids for large sites.
Measurement
Use the Wenner four-pin method for soil resistivity and fall-of-potential for electrode resistance. Record season and weather. Maintain test pits and inspection chambers. See earthing architecture.
Common mistakes
- Testing probes too close to the electrode.
- Designing from wet-season data only.
- Mixing dissimilar metals that corrode.
What to send us
Send the site, soil data and fault current. Send your application details.
Questions engineers ask
What earth resistance is required?
It depends on the system, regulations and touch-voltage limits. There is no single number for all sites.
Does adding more rods always help?
Yes up to a point, but closely spaced rods interfere with each other.
Why does earth resistance change?
Seasonal moisture, temperature and corrosion.
Are chemical electrodes necessary?
Only where other methods cannot reach the target, and they need inspection.
This guide gives general principles. Design requires site measurements and local rules.