Part 5 of the earthing series. The joint type sets the maximum temperature, and the maximum temperature sets the conductor size: 142 mm² with bolted joints, 113 mm² with welded joints.
| Fault current | 25 kA, through the earthing conductor |
| Duration | 1 s (back-up protection clearing time) |
| Material | Bare copper, initial temperature 30 °C |
Steel: K = 78, β = 202; the zinc coating limits the temperature.
Use the back-up protection clearing time, not the primary relay time, because the conductor must survive if the main protection fails. A 0.5 s duration would reduce the bolted-joint size to 101 mm².
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Bolted joints (250 °C) | S ≥ 142 mm² | 150 mm² selected | ✓ PASS |
| Welded joints (450 °C) | S ≥ 113 mm² | 120 mm² selected | ✓ PASS |
Key insight: The weakest point is the joint, not the conductor. Sizing with a high k (welded) while installing bolted clamps under-sizes the conductor by about 25 %.
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Open Earthing Grid Design (IEEE 80) calculator →The full fault current that can flow in the conductor. IG is only the part that flows into the soil and is used for GPR and touch voltages.
Yes. BS 7430 gives minimum sizes for buried conductors regardless of fault current, for example 25 mm² copper without corrosion protection.