Earth fault loop impedance calculator (Zs = Ze + R1 + R2)
Add Ze and R1 + R2 to get Zs, then compare it with the maximum for a type B, C or D circuit-breaker or RCBO in a TN system.
Every result has a reason.
Enter your values and calculate to see the result, formula and working here.
Zs = Ze + (R1 + R2)Adds the entered Ze and R1 + R2. The comparison uses the maximum measured Zs that the IET's On-Site Guide and Guidance Note 3 print for circuit-breakers and the overcurrent characteristic of RCBOs in TN systems at 230 V, for a 10 °C ambient (0.8 × U0 × Cmin ÷ Ia, rounded up to two decimal places), or the unrounded figure for ratings they do not list. It applies no temperature correction. Warmer readings may be allowed a slightly higher limit (Guidance Note 3 Table A7), so for readings at 10 °C or above the comparison errs on the safe side; readings below 10 °C need a lower limit, so no comparison is shown for them. Choose 5 s only for circuits Table 41.1 does not cover. Adding Ze and R1 + R2 ignores circuit reactance, which the IET treats as negligible for cables under 16 mm²; for larger cables use a full impedance calculation. RCD fault protection, TT systems and fuses are not covered. Parallel earth paths lower measured values. The comparison is one part of fault protection, not a full assessment of the circuit.
What the numbers mean.
Zs is the impedance of the whole fault path: the supply's own loop (Ze) plus the circuit's line and protective conductors (R1 + R2). BS 7671 limits Zs so that a fault draws enough current to disconnect the device in time. Test readings are compared with a lower figure than Table 41.3, 0.8 × U0 × Cmin ÷ Ia, because conductor resistance rises when the circuit is loaded and warm.
Formula reference: IET Wiring Matters · Why maximum earth fault loop impedance values differ ↗A worked example
Ze = 0.35 Ω and R1 + R2 = 0.39 Ω give Zs = 0.74 Ω. For a type B 32 A breaker the maximum measured Zs is 1.10 Ω, so Zs is about 67.3% of it.
Validation, precision and limitations