Fault loop and tripping calculator

Enter the characteristic and current of the circuit-breaker, the loop impedance of the supply, and the length and cross-section of the cable, and we show whether the protection trips with certainty, how large the fault current is, and how long the cable may be.

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What does the fault loop calculator work out?

If the line conductor touches the protective conductor (the exposed part), the fault current flows in the fault loop: through the supply, the line conductor and the protective conductor. The higher the loop impedance (Zs), the lower the fault current. A circuit-breaker trips with certainty only if the fault current reaches the magnetic (instantaneous) tripping current, so the loop impedance must not exceed a limit.

• Zs = Ze + L · ρ · (1/A + 1/A_PE): the supply (Ze) plus the resistance of the line and protective conductors of the cable; copper, at operating temperature (ρ = 0.0225 Ω·mm²/m).
• Zs,max = 0.95 · 230 / (k · In): the limit for certain tripping. The k is the multiplier of the tripping characteristic according to MSZ EN 60898-1: B 5, C 10, D 20.
• Fault current = 0.95 · 230 / Zs: tripping is certain exactly when Zs is not greater than Zs,max.

The calculator also shows the longest cable run up to which tripping is still certain.

What to watch for

Ze is the least certain figure: measure it at the point of supply or ask the utility. The default (0.35 Ω) is only an estimate. If the protective conductor is thinner than the line conductor, enter it separately, because it increases the loop impedance.

Limits of the calculation

The calculation covers the 230 V line–protective conductor loop, copper conductors, the magnetic (instantaneous) trip and the 0.4 s requirement; for three-phase circuits it also looks at the line–protective conductor loop. It does not take into account the residual-current device, TT and IT systems, or other conductor materials. The result is for guidance only: checking the tripping and carrying out the work are the responsibility of the electrical designer and the installer, with on-site measurement.

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