Part 3 of the earthing series. A 50 m × 40 m grid with 10 m meshes gives a touch voltage of 1,365 V against a 932 V limit. Halving the mesh spacing brings it to 822 V.
| Soil | Uniform, ρ = 200 Ω·m |
| Grid | 50 m × 40 m, buried 0.6 m, conductor diameter 12.6 mm, 20 rods of 3 m |
| Fault | Earth fault 5 kA, split factor Sf = 0.6, Df = 1 → grid current IG = 3 kA |
| Clearing time | 0.5 s, 70 kg body weight |
| Surface | 100 mm crushed rock, 3,000 Ω·m (wet) |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Touch, 10 m mesh | Em ≤ 932 V | 1,365 V | ✗ FAIL |
| Touch, 5 m mesh | Em ≤ 932 V | 822 V | ✓ PASS |
| Step, 5 m mesh | Es ≤ 3,061 V | 680 V | ✓ PASS |
Key insight: Touch voltage is controlled by conductor spacing, not by grid resistance. A grid can have a low resistance and still be unsafe.
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Open Earthing Grid Design (IEEE 80) calculator →In the corner meshes of the grid. That is why IEEE 80's Em is the mesh voltage at the centre of a corner mesh, and why extra rods and closer spacing at the perimeter help.
Then no person can be exposed to more than the GPR and the design is safe for touch and step without further checks (transferred voltages still need attention).