Zs Earth Fault Loop Impedance Calculator

Zs is the total earth fault loop impedance — the impedance of the complete fault current path from the supply transformer, through the line conductor, through the fault, and back via the earth path. BS 7671 requires Zs to be low enough that the protective device (MCB/fuse) disconnects within the required time (0.4s for socket circuits, 5s for distribution circuits). If Zs is too high, the fault current will be too low to trip the device quickly, creating a shock hazard. For a more detailed analysis, see our Earth Fault Loop Impedance Calculator.

Ze (external earth fault loop impedance) is measured at the origin of the installation with the main earthing conductor disconnected — this gives the impedance of the supply network. R1+R2 is measured using an insulation/continuity tester on the circuit with the supply isolated: R1 is the line conductor resistance and R2 is the CPC (circuit protective conductor) resistance. You can also measure Zs directly at the furthest point of the circuit, but the Ze + R1+R2 method allows verification of each component.

When you measure R1+R2 during testing the conductors are cold — at the ambient test temperature, which BS 7671 Appendix 4 references to 20°C. In service the conductors run hotter: thermoplastic (PVC) is rated to 70°C, thermosetting (XLPE) to 90°C. Copper resistance rises by about 0.4% per °C (a 0.004/°C coefficient referenced to 20°C), so correcting a cold measurement up to 70°C multiplies R1+R2 by roughly 1.20. This calculator's Ct = [1 + 0.004 × (operating − 20)] ÷ [1 + 0.004 × (ambient − 20)], which gives exactly ×1.20 for a 20°C test and a 70°C PVC conductor. Correcting to operating temperature gives the worst-case impedance that governs whether the protective device disconnects in time.

For Type B MCBs per BS 7671:2018+A4:2026 Table 41.3 (0.4s disconnection, with Cmin=0.95 — values unchanged from A2:2022): 6A = 7.28Ω, 10A = 4.37Ω, 16A = 2.73Ω, 20A = 2.19Ω, 25A = 1.75Ω, 32A = 1.37Ω, 40A = 1.09Ω, 50A = 0.87Ω, 63A = 0.69Ω. Type C values are half of Type B, and Type D values are a quarter of Type B, because they require progressively higher fault currents (5×In, 10×In and 20×In respectively) to trip magnetically.

No — that double-counts. The Table 41.3 maximum Zs values already assume the conductor is at its operating temperature (e.g. 70°C for PVC). There are two equivalent GN3 methods for bridging the gap between your cold test measurement and that hot limit, and you use ONE of them. Method 1 (the 0.8 'rule of thumb'): leave your measured Zs as-is and compare it against 0.8 × the tabulated maximum. Method 2 (temperature correction): correct your measured Zs up to operating temperature (≈ ×1.20 for PVC) and compare it against 100% of the tabulated maximum. Both account for the same rise in conductor resistance. Applying the 0.8 factor and the temperature multiplier at the same time is over-conservative and can flag a compliant circuit as a fail. This calculator lets you pick one method so it never double-counts.