Maximum Zs values — lookup table
Maximum permitted earth-fault loop impedance (Zs) for final circuits at 230 V with the BS 7671:2018+A2:2022 voltage factor Cmin = 0.95 — the U0 × Cmin ÷ Ia method for B, C and D type MCBs.
How the table is built
For a circuit-breaker, the maximum Zs = U0 × Cmin ÷ Ia, where Ia is the current causing the protective device to operate within the required disconnection time (0.4 s for final circuits ≤ 63 A in TN systems). For MCBs to BS EN 60898: type B trips at 5×In, type C at 10×In, type D at 20×In. With U0 = 230 V and Cmin = 0.95, the numerator is 218.5 V.
| Rating | Type B (max Zs) | Type C (max Zs) | Type D (max Zs) |
|---|---|---|---|
| 6 A | 7.28 Ω | 3.64 Ω | 1.82 Ω |
| 10 A | 4.37 Ω | 2.19 Ω | 1.09 Ω |
| 16 A | 2.73 Ω | 1.37 Ω | 0.68 Ω |
| 20 A | 2.19 Ω | 1.09 Ω | 0.55 Ω |
| 25 A | 1.75 Ω | 0.87 Ω | 0.44 Ω |
| 32 A | 1.37 Ω | 0.68 Ω | 0.34 Ω |
| 40 A | 1.09 Ω | 0.55 Ω | 0.27 Ω |
| 50 A | 0.87 Ω | 0.44 Ω | 0.22 Ω |
Reading the results on a schedule
- 80% rule: tabulated maxima assume conductors at operating temperature. Compare your measured Zs against 0.8 × the table value — a measured 1.25 Ω on a B32 passes, a measured 1.40 Ω fails.
- Zs = Ze + (R1+R2): measuring Ze at the origin and adding the circuit's R1+R2 from the ring final / radial test is an accepted alternative to a direct Zs measurement — record which method was used.
- Older tables: pre-2018 printings used Cmin = 1.0 (e.g. B32 = 1.44 Ω). Cite the current standard's figure on new reports.
- Fuses and RCBOs: BS 1361 / BS 3036 fuse values and RCBO trip characteristics come from the relevant appendix or manufacturer data — don't reuse MCB figures for them.
RCD-only fault protection (typical TT)
Where an RCD is the fault-protection device rather than the MCB, the maximum Zs is 50 V ÷ IΔn: about 1,667 Ω for a 30 mA RCD. That is why TT installations with high electrode resistance can still comply — but the report must also carry the RCD's operating time at IΔn, and disconnection-time compliance is checked against the RCD, not the breaker.
FAQ
What is the maximum permitted Zs for a 32 A type B MCB?
Why do measured Zs values only need to be 80% of the maximum?
What is the relationship between Zs, Ze and R1+R2?
Does a 30 mA RCD remove the Zs limit?
Basis & sources
Primary sources only — the statute, form or paper this page is built on.
- IET Model Forms (BS 7671:2018+A4:2026) — current standard and model forms
- Approved Document P (2013 edition, effective 6 April 2013) — Part P approved document