About the Earth Fault Current
Earth fault current calculation determines the magnitude of a single-line-to-ground fault current in a solar plant's HT system — the most common fault type in overhead and underground HV cables — which governs the earth fault relay setting, the earthing system design, and the step/touch potential hazard.
Formula
I_ef = V_line / (√3 × (Z1 + Z2 + Z0) / 3)
- V_line
- Line-to-line voltage (V)
- Z1
- Positive-sequence impedance of the system (Ω)
- Z2
- Negative-sequence impedance (≈ Z1 for most networks)
- Z0
- Zero-sequence impedance (determined by transformer earthing)
How to calculate it
- 1Identify the transformer earthing: solidly earthed (Z0 small → high earth fault current), resistance earthed (Z0 high → limited earth fault current), or unearthed (Z0 → ∞ → negligible earth fault current).
- 2Collect sequence impedances from the utility's short-circuit data or calculate from transformer parameters and line impedances.
- 3Apply the formula above to get maximum earth fault current (bolted fault at the busbar).
- 4Calculate minimum earth fault current (fault at the far end of the feeder, maximum impedance path) for relay sensitivity checks.
- 5Size the earth fault relay pick-up at 10–20% of the maximum earth fault current to ensure sensitivity.
Worked example
A 11 kV solidly-earthed system with Z1 = Z2 = 0.5 Ω and Z0 = 1.0 Ω: I_ef = 6350 / ((0.5 + 0.5 + 1.0)/3) = 6350 / 0.667 = 9,520 A. Earth fault relay set at 10% = 952 A, TMS 0.1 — clears in 0.14 s.
What you can calculate
- Neutral earthing
- SLG current
- GPR estimate
- CEA guidelines
Standards & references
Frequently asked questions
Why is the zero-sequence impedance so important for earth fault calculations?
Zero-sequence current only flows through paths that include the earth — transformer neutrals, earth conductors, and the ground itself. The zero-sequence impedance Z0 of the system is determined almost entirely by the transformer neutral earthing arrangement. A solidly-earthed neutral has a low Z0 (high earth fault current); a resistance-earthed neutral inserts a large Z0 (limits earth fault current to a few amps or tens of amps). The fault level governs both the relay sensitivity and the earthing grid design.
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