#Max Zs Values Table: BS 7671 18th Edition (MCBs and RCBOs)

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, the formula behind them so you can work any value out from scratch, and the 80% rule for comparing measured readings against the limits.

#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 (the part outside the installation — 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, measured during dead testing or predicted with the R1+R2 calculator.

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

• U0 = 230V nominal line-to-earth voltage
• Cmin = 0.95, the minimum voltage factor introduced in the 18th Edition — it allows for the supply sitting below nominal
• Ia = 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.

#Why 18th Edition values are lower than older tables

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. If your memorised numbers date from the 17th Edition, they're now slightly generous, which is the wrong direction to be wrong in. The tables below are all calculated at 0.95 × 230V = 218.5V, consistent with BS 7671:2018.

#Max Zs tables — BS EN 60898 MCBs (0.4s and 5s)

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. The "measured (80%)" column is the figure to compare a cold test reading against — more on that below.

#Type B (Ia = 5 × In)

#Type C (Ia = 10 × In)

#Type D (Ia = 20 × In)

These values are calculated from the standard formula consistent with BS 7671:2018 — always confirm against the current edition and the device manufacturer's data. They 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 — a 30mA device will trip at a tiny fraction of the fault current an MCB needs. But that figure is not a licence for a sloppy loop: good practice (and scheme guidance) 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: measured vs tabulated values

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 with the circuit cold, and copper resistance rises with temperature. The common site practice is:

Measured Zs (cold) ≤ 0.8 × tabulated maximum

That's where the fourth column in each table comes from: if your cold reading is within 80% of the design limit, the hot value should still comply. The alternative approach is to correct the other way — multiply a cold R1+R2 by 1.2 to approximate operating temperature, then compare the resulting Zs against the full limit. Use one correction or the other, never both, and treat any result that only just scrapes past with suspicion rather than a tick.

#How to measure Zs on site

Two accepted routes:

1. Live loop test. With the installation energised, test line-to-earth at the furthest point of the circuit with a loop impedance tester. On RCD-protected circuits use a no-trip test mode, and be aware that parallel earth paths through bonded services can make the reading optimistically low.
2. By calculation: Zs = Ze + (R1+R2). Measure Ze at the origin, take R1+R2 from your dead continuity testing, and add them — the Zs from Ze calculator does the sum. This method is often preferable on RCD-protected circuits and lets you verify a design before anything is energised.

Whichever route you take, the recorded Zs for the circuit is compared against the device's maximum, and it also sets the earth fault current for the prospective fault current and adiabatic checks.

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.

#Do it faster with TradePlanr

Skip the table lookup: the free max Zs calculator gives the limit for any Type B, C or D device from 6A to 63A, with the 80% rule applied at a tap — alongside R1+R2 and PFC tools built on the same BS 7671 values. TradePlanr is a job management app for electricians: quotes, scheduling, invoicing and certificates in one place. The calculators are free, and the full app is a flat £9.99 a month.