A 30m x 20m grid with a perfectly reasonable-looking resistance and GPR still fails the actual touch and step voltage safety checks by a wide margin.
| Grid size | 30 m x 20 m, 4 x 3 mesh, burial depth 0.6 m |
| Ground rods | 4 rods, 3 m long |
| Soil resistivity | 100 Ω·m (no surface layer) |
| Symmetrical fault current | 10,000 A (current division & decrement factors = 1) |
| Fault clearing time | 0.5 s |
| Body weight category | 70 kg |
These are the maximum voltages a person is allowed to experience — a completely separate quantity from GPR itself, which is why a high GPR alone doesn't automatically mean a failed design.
This is the calculation GPR alone can't substitute for — it depends on mesh geometry (spacing, depth, rod placement), not just total resistance.
| Check | Tolerable limit | Actual | Result |
|---|---|---|---|
| Touch (mesh) voltage | 255.3 V | 6199 V (24x over) | FAIL |
| Step voltage | 355.3 V | 2639 V (7.4x over) | FAIL |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Grid resistance / GPR | n/a (informational — not itself a pass/fail limit) | Rg = 2.285 Ω, GPR = 22,851 V | ✓ PASS |
| Mesh (touch) voltage | ≤ 255.3 V | 6199 V | ✗ FAIL |
| Step voltage | ≤ 355.3 V | 2639 V | ✗ FAIL |
Key insight: Grid resistance and GPR describe the whole grid's behavior relative to remote earth; mesh and step voltage describe what a specific person standing at a specific point actually experiences, which depends heavily on mesh spacing and geometry, not just total resistance. A grid can have an excellent (low) resistance and still be dangerous to stand near if the conductor spacing is too wide — which is exactly the trap a resistance-only check falls into.
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Open Earthing Grid Design calculator →Not on its own — narrowing the mesh from 4x3 to a much finer 16x11 grid (D_avg dropping from 10 m to 2 m) reduces the mesh voltage from 6199 V to about 1656 V, real progress, but still far above the 255.3 V limit. At this fault current and soil resistivity, mesh spacing alone has diminishing returns; other levers have to be combined with it.
Adding a high-resistivity surface layer (e.g. a crushed-rock surface, resistivity ~10,000 Ω·m) raises the tolerable touch-voltage limit dramatically — because Cs and ρs both increase, Etouch rises from 255.3 V to about 2529 V in this case. Combined with a much finer mesh (which still helps reduce Em, just not enough alone), both checks pass: the mesh voltage of about 1656 V is under the new 2529 V touch limit, and the 2843 V step voltage is well under the new step limit of about 9450 V — showing why real designs typically stack several levers together (mesh density, surface treatment, faster protection clearing time, or lower soil resistivity via treatment) rather than relying on any single one.
Both Etouch and Estep are inversely proportional to the square root of clearing time, since IEEE 80's tolerable body-current limit itself assumes a shorter exposure allows a higher survivable current — halving the clearing time (e.g. from 0.5 s to 0.2 s via faster protection) raises both tolerable limits by roughly 58%, which is often cheaper to achieve than physically rebuilding a grid.