About the Fence Earthing Design

Solar substation fence earthing design ensures that the metallic perimeter fence of a solar plant switchyard is bonded to the earthing grid in a way that limits the touch voltage on the fence during an earth fault — preventing electric shock to persons touching the fence from outside the switchyard boundary.

How to calculate it

1. 1Bond the fence to the earthing grid at every 20–30 m interval using a bare copper conductor (minimum 50 mm²) or a galvanised steel earth rod.
2. 2Install an isolating joint (insulating link) in the fence at the main gate to prevent earth current from flowing along the fence to the gate hardware — otherwise the gate latch becomes a touch-voltage hazard for security personnel.
3. 3Calculate the touch voltage on the fence: V_touch_fence ≈ I_fault × (R_grid + ρ / (2π × D)), where D is the distance of the fence from the grid edge.
4. 4Verify V_touch_fence ≤ E_touch_limit (from IEEE 80 step/touch potential analysis) for the soil resistivity and fault duration at the site.
5. 5If the fence voltage exceeds the limit: extend the earth grid horizontally under and beyond the fence, install a gravel surface layer adjacent to the fence, or use a parallel earth wire buried 0.5 m outside the fence.

Worked example

A 33 kV, 10 MW switchyard: I_fault = 5 kA, R_grid = 0.8 Ω, fence 1 m from the grid perimeter, soil ρ = 100 Ω·m. V_fence ≈ 5000 × 0.8 = 4000 V GPR (ground potential rise). Touch voltage at fence depends on grid mesh design; a typical analysis shows fence touch voltage of 800–1500 V. For fault duration 0.3 s and ρ_s = 3000 Ω·m (gravel), E_touch_limit ≈ 1166 V — fence touch voltage must be below this limit.

What you can calculate

• Fence voltage limit
• IS 3043
• Transferred potential
• Isolation

Standards & references

Frequently asked questions