Max Zs calculator for circuit-breakers and RCBOs
Maximum earth fault loop impedance for type B, C and D circuit-breakers and RCBOs in TN systems, with the value to compare test readings against.
Every result has a reason.
Enter your values and calculate to see the result, formula and working here.
Zs ≤ U0 × Cmin ÷ IaTN earthing systems (TN-C-S, TN-S) on a 230 V supply, for circuit-breakers to BS EN 60898 and the overcurrent (B, C or D) characteristic of RCBOs to BS EN 61009, at the ratings Table 41.3 lists. Where an RCBO's residual-current function provides fault protection, a different limit applies (Table 41.5); TT systems are not covered. Table 41.1 sets 0.4 s for final circuits up to 63 A with socket-outlets or 32 A supplying only fixed equipment (Regulation 411.3.2.2); Regulation 411.3.2.3 allows up to 5 s for other circuits in TN systems. Types B and C have one value for both. Fuses are not covered because their Ia comes from each fuse's time-current curve, which this calculator does not hold. A manufacturer's data can allow a different limit for a specific device. A Zs within the limit is one part of fault protection, not a full assessment of the circuit.
What the numbers mean.
Regulation 411.4.4 requires Zs × Ia ≤ U0 × Cmin, where Ia is the current that makes the device disconnect within the required time and Cmin = 0.95 allows for the supply voltage being below nominal. For 0.4 s, Ia is the top of the instantaneous tripping range: 5 × In for type B, 10 × In for type C and 20 × In for type D. For type D at 5 s, Table 41.3 uses 10 × In. Table 41.3 values assume conductors at their operating temperature. Test readings are compared with 0.8 × U0 × Cmin ÷ Ia (Appendix 3). The IET's On-Site Guide and Guidance Note 3 print that figure for a 10 °C ambient, rounded up to two decimal places, and give none for type D at 0.4 s above 32 A. Guidance Note 3 Table A7 corrects it for other temperatures: lower when colder than 10 °C, higher when warmer. This calculator does not apply that correction.
Formula reference: IET Wiring Matters · Why maximum earth fault loop impedance values differ ↗A worked example
Type B, 32 A: Ia = 5 × 32 A = 160 A. 0.95 × 230 V ÷ 160 A = 1.365625 Ω, which Table 41.3 prints as 1.37 Ω. For a test reading, 0.8 × 1.365625 Ω = 1.0925 Ω, which the IET's On-Site Guide and Guidance Note 3 print as 1.10 Ω for a 10 °C ambient.
Validation, precision and limitations