Part 4 of the earthing series. The same grid fails without crushed rock (limit 289 V) and passes with 100 mm of it (limit 932 V). How the surface layer factor Cs works.
| 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 |
| Layout | 5 m mesh (11 × 9 conductors) from Part 3 — mesh voltage Em = 822 V, step Es = 680 V |
The person stands on native soil, ρs = ρ = 200 Ω·m, so Cs = 1.
| Case | Em (V) | Touch limit (V) | Es (V) | Step limit (V) |
|---|---|---|---|---|
| No surface layer | 822 | 289 | 680 | 489 |
| 100 mm crushed rock | 822 | 932 | 680 | 3,061 |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Touch, no surface layer | ≤ 289 V | 822 V | ✗ FAIL |
| Step, no surface layer | ≤ 489 V | 680 V | ✗ FAIL |
| Touch, crushed rock | ≤ 932 V | 822 V | ✓ PASS |
| Step, crushed rock | ≤ 3,061 V | 680 V | ✓ PASS |
Key insight: Crushed rock does not change the voltages the grid produces — it adds resistance under the person's feet, so they can tolerate much more. It must be kept clean; soil or sand mixed in lowers its resistivity.
Every input in this example is editable in the live calculator — no signup needed.
Open Earthing Grid Design (IEEE 80) calculator →Use the wet value. IEEE 80 lists typical values from about 1,000 to over 10,000 Ω·m depending on the stone; 3,000 Ω·m is a common design value.
Yes. Asphalt has a very high resistivity and is often used for roads inside substations.