A buried ring around a small building, checked against the 10 Ω usually aimed for in lightning protection earthing. A 12 m ring falls short; a 20 m ring passes.
| Soil resistivity | 200 Ω·m (measured, Wenner method) |
| Conductor | Bare copper, 10 mm diameter |
| Burial depth | 0.6 m |
| Target | 10 Ω (low-frequency earth resistance aimed for in BS EN 62305-3) |
| Options | Ring of 12 m diameter (close to the building) or 20 m diameter (around the site) |
BS 7430:2011 does not give a ring formula, so the ring-of-wire formula from IEEE Std 142 (Dwight) is used, with s equal to twice the burial depth.
D is the ring diameter, d the conductor diameter and h the burial depth.
Instead of a larger ring, earth rods can be added to the 12 m ring, driven deep where the soil is wetter. The conductor size has only a small effect: doubling the diameter lowers the 12 m ring by about 0.6 Ω.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| 12 m ring | ≤ 10 Ω | 10.86 Ω | ✗ FAIL |
| 20 m ring | ≤ 10 Ω | 7.03 Ω | ✓ PASS |
Key insight: Ring resistance falls roughly in inverse proportion to the ring diameter. Size matters far more than conductor thickness.
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Open Ground Ring Resistance Calculator calculator →BS 7430:2011 gives formulas for rods, plates, straight strips, meshes and some star and cruciform layouts, but not for a ring. IEEE Std 142's ring-of-wire formula is the usual reference.
BS EN 62305-3 recommends a low earth resistance, if possible below 10 Ω, for a lightning protection earth. Electrical safety requirements for the installation are set separately, for example by disconnection times and touch voltages.