Part 1 of the earthing series. Five Wenner readings at increasing spacings, converted to apparent resistivity — and what the trend says about the soil layers.
| Test | Wenner four-pin method, equal spacing a, probes driven to a small depth compared with a |
| Readings | a = 1, 2, 4, 8, 16 m; R = 15.0, 6.9, 2.9, 1.2, 0.55 Ω |
Valid when the probe depth is small compared with the spacing.
| Spacing a (m) | R (Ω) | ρa = 2πaR (Ω·m) |
|---|---|---|
| 1 | 15.0 | 94.2 |
| 2 | 6.9 | 86.7 |
| 4 | 2.9 | 72.9 |
| 8 | 1.2 | 60.3 |
| 16 | 0.55 | 55.3 |
Resistivity falls as spacing rises, and the spacing roughly indicates the depth being sampled. The soil is therefore more conductive at depth: a drier top layer of about 95 Ω·m over a wetter lower layer of about 55 Ω·m.
For a grid about 40 m across, the spacings comparable to the grid size (8–16 m) matter most for grid resistance, while the top layer governs touch voltage near the surface. A single uniform value of about 95 Ω·m (the top layer) is conservative for touch and step voltages; a two-layer model gives a more economical design.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Apparent resistivity at 1 m | — | 94.2 Ω·m | ✓ PASS |
| Apparent resistivity at 16 m | — | 55.3 Ω·m | ✓ PASS |
Key insight: Never design from a single reading. Measure along at least two directions and several spacings; the trend with spacing is as important as the values.
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Open Earthing Grid Design (IEEE 80) calculator →Ideally in dry conditions, which give the highest (worst-case) resistivity, and away from buried metal pipes or cables that distort the readings.
Then deep rods help less and a larger horizontal grid is usually more effective.