Max Zs Values Table (BS 7671 18th Edition)
Every circuit has to disconnect fast enough under an earth fault, and the number that proves it is Zs — here are the full maximum Zs tables for BS EN 60898 MCBs and BS EN 61009 RCBOs to the 18th Edition, plus the formula behind them.
Every circuit you test and certify has to disconnect fast enough under an earth fault, and the number that proves it is Zs — the earth fault loop impedance. This guide gives the full maximum Zs tables for BS EN 60898 MCBs and BS EN 61009 RCBOs to the 18th Edition.
It also covers the formula behind the tables so you can work any value out from scratch, and the 80% rule for comparing a measured reading against the limits.
Key takeaways
- Zs = Ze + (R1 + R2) — the total earth fault loop impedance for a circuit.
- Max Zs formula: Zs ≤ (Cmin × U0) / Ia, with Cmin = 0.95, U0 = 230V, so calculated at 218.5V.
- Ia is 5×In for Type B, 10×In for Type C, 20×In for Type D.
- 18th Edition values are ~5% lower than 17th Edition because of the 0.95 voltage factor.
- Compare a cold measured Zs against 80% of the tabulated limit — or correct the other way, never both.
What Zs is
Zs is the total impedance of the earth fault loop for a circuit: from the transformer winding, along the line conductor to the point of the fault, and back through the circuit protective conductor and the earthing arrangement to the source. It breaks down as Zs = Ze + (R1 + R2), where Ze is the external loop impedance (typically declared at up to 0.35Ω for TN-C-S and 0.8Ω for TN-S) and R1+R2 is the resistance of the circuit's line conductor and CPC.
The lower the Zs, the higher the fault current, and the faster the protective device disconnects. Maximum Zs values are the highest loop impedance at which the device still trips within the required time — 0.4 seconds for most final circuits up to 63A, 5 seconds for distribution circuits.
The 18th Edition formula
For MCBs and RCBOs, the maximum Zs comes straight from the device's instantaneous trip current: Zs ≤ (Cmin × U0) / Ia. Here U0 is 230V nominal line-to-earth voltage; Cmin is 0.95, the minimum voltage factor introduced in the 18th Edition to allow for the supply sitting below nominal; and Ia is the current that guarantees instantaneous operation — 5 × In for Type B, 10 × In for Type C, 20 × In for Type D.
So for a B32: Zs = (0.95 × 230) / (5 × 32) = 218.5 / 160 = 1.37Ω. Pre-18th Edition tables were based on the full 230V; multiplying by Cmin = 0.95 knocks roughly 5% off every value — the old B32 figure of 1.44Ω became 1.37Ω. All the tables below are calculated at 0.95 × 230V = 218.5V, consistent with BS 7671:2018.
Wrong direction to be wrong
If your memorised numbers date from the 17th Edition, they're now slightly generous — the wrong way to be wrong. Use the 0.95 factor values.
Max Zs tables — BS EN 60898 MCBs
Because an MCB relies on its instantaneous magnetic trip to meet earth-fault disconnection times, the same maximum Zs applies whether the required disconnection time is 0.4s or 5s. Each value below gives the design maximum followed by the measured (80%) figure to compare a cold reading against.
Type B (Ia = 5 × In): 6A — 7.28Ω design, 5.83Ω measured; 10A — 4.37Ω, 3.50Ω; 16A — 2.73Ω, 2.19Ω; 20A — 2.19Ω, 1.75Ω; 25A — 1.75Ω, 1.40Ω; 32A — 1.37Ω, 1.09Ω; 40A — 1.09Ω, 0.87Ω; 50A — 0.87Ω, 0.70Ω.
Type C (Ia = 10 × In): 6A — 3.64Ω design, 2.91Ω measured; 10A — 2.19Ω, 1.75Ω; 16A — 1.37Ω, 1.09Ω; 20A — 1.09Ω, 0.87Ω; 25A — 0.87Ω, 0.70Ω; 32A — 0.68Ω, 0.55Ω; 40A — 0.55Ω, 0.44Ω; 50A — 0.44Ω, 0.35Ω.
Type D (Ia = 20 × In): 6A — 1.82Ω design, 1.46Ω measured; 10A — 1.09Ω, 0.87Ω; 16A — 0.68Ω, 0.55Ω; 20A — 0.55Ω, 0.44Ω; 25A — 0.44Ω, 0.35Ω; 32A — 0.34Ω, 0.27Ω; 40A — 0.27Ω, 0.22Ω; 50A — 0.22Ω, 0.17Ω.
These apply to MCBs and RCBOs only — fuses to BS 88, BS 3036 and BS 1361 have entirely different time–current characteristics and their limits must be taken from the BS 7671 tables directly.
RCBOs and RCD-protected circuits
BS EN 61009 RCBOs use the same B/C/D instantaneous trip characteristics for their overcurrent element, so the tables above apply to an RCBO exactly as they do to the equivalent MCB — a B32 RCBO has the same 1.37Ω limit as a B32 MCB.
Where a 30mA RCD provides the fault protection (as on most domestic circuits since the 18th Edition), BS 7671 permits a maximum Zs of 1667Ω for the RCD to meet disconnection times. But that is not a licence for a sloppy loop: good practice is that Zs should still meet the overcurrent device's value where practicable, so the circuit doesn't depend entirely on the RCD functioning. Record the measured Zs either way.
The 80% rule and measuring Zs
The tabulated limits assume conductors at operating temperature — around 70°C for thermoplastic insulation — because that's the worst case during a fault. You test cold, and copper resistance rises with temperature. The common site practice is measured Zs (cold) ≤ 0.8 × tabulated maximum — that's where the 80% column comes from. The alternative is to correct the other way: multiply a cold R1+R2 by 1.2 to approximate operating temperature, then compare against the full limit. Use one correction or the other, never both.
To measure Zs on site there are two accepted routes. A live loop test: with the installation energised, test line-to-earth at the furthest point with a loop impedance tester, using a no-trip mode on RCD-protected circuits. Or by calculation: Zs = Ze + (R1+R2) — measure Ze at the origin, take R1+R2 from dead continuity testing, and add them. The recorded Zs also sets the earth fault current for the prospective fault current and adiabatic checks.
Guidance only
All values here are for guidance. Always verify against the current edition of BS 7671, the IET On-Site Guide and the device manufacturer's data before certifying.
Frequently asked questions
Why are 18th Edition max Zs values lower than older ones?
The 18th Edition applies a minimum voltage factor Cmin of 0.95 to allow for the supply sitting below nominal, so every value is calculated at 218.5V instead of 230V — roughly 5% lower. A B32 fell from 1.44Ω to 1.37Ω.
What's the max Zs for a B32 MCB?
1.37Ω design maximum, calculated as (0.95 × 230) / (5 × 32). Compare a cold measured reading against 80% of that — 1.09Ω — or correct R1+R2 up by 1.2 and compare against the full 1.37Ω.
Do these values apply to RCBOs?
Yes. A BS EN 61009 RCBO uses the same B/C/D overcurrent trip characteristic as the equivalent MCB, so a B32 RCBO has the same 1.37Ω limit. Where a 30mA RCD provides fault protection, BS 7671 permits up to 1667Ω, but good practice still meets the device value.
From guidance to action
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