Maximum Zs calculator (18th Edition)
Pick your device, get the tabulated maximum Zs and the 80% figure for on-site testing. Type B, C and D MCBs and RCBOs, straight from the 18th Edition formula. Full tables below.
- Device–
- Trip current Ia–
- 80% test value (cold circuit)–
Guidance only. Values follow the 18th Edition formula (U0 = 230 V, Cmin = 0.95) for MCBs/RCBOs to BS EN 60898 and 61009, matching Table 41.3 of BS 7671:2018+A2:2022. Always confirm against the current standard and the device manufacturer's data.
18th Edition maximum Zs tables (MCBs & RCBOs)
Tabulated values in ohms, from Zs = 0.95 × 230 ÷ Ia. For on-site testing of a cold circuit, compare against 80% of these.
| Rating In | Type B (5 × In) | Type C (10 × In) | Type D (20 × In) |
|---|---|---|---|
| 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 |
| 45 A | 0.97 Ω | 0.49 Ω | 0.24 Ω |
| 50 A | 0.87 Ω | 0.44 Ω | 0.22 Ω |
| 63 A | 0.69 Ω | 0.35 Ω | 0.17 Ω |
| 80 A | 0.55 Ω | 0.27 Ω | 0.14 Ω |
| 100 A | 0.44 Ω | 0.22 Ω | 0.11 Ω |
| 125 A | 0.35 Ω | 0.17 Ω | 0.09 Ω |
Where the numbers come from
For automatic disconnection to work, the earth fault loop impedance has to be low enough that the fault current trips the device fast. The 18th Edition formula is Zs ≤ Cmin × U0 ÷ Ia, where Cmin is 0.95 (allowing for supply voltage dipping to 95%), U0 is 230 V, and Ia is the current that fires the instantaneous trip: 5 × In for Type B, 10 × In for Type C, 20 × In for Type D. Once you know the fault current, the adiabatic equation calculator checks the CPC can handle it.
So for the classic B32: 0.95 × 230 ÷ 160 = 1.37 Ω. Cold-circuit test limit at 80%: 1.10 Ω.
Why the 80% rule exists
The tabulated values assume conductors at operating temperature, warm copper has more resistance. When you're testing, the circuit is usually cold, so a measured reading that only just squeaks under the tabulated value could drift over it in service. Knocking the limit down to 80% for your measured comparison builds that margin in. (The alternative is the full temperature-correction calculation, but on site, 80% is the working rule.)
Two ways to get Zs: measure it or calculate it
On a live circuit, you measure it: earth fault loop impedance tester at the furthest point of the circuit, connections proved, GS38 leads. That reading goes on the certificate and gets compared against the 80% figure.
From dead tests, you calculate it: Zs = Ze + (R1 + R2), external loop impedance plus the line and CPC resistance of the circuit. Worked example: a TN-C-S supply with Ze at 0.35 Ω and a measured R1+R2 of 0.72 Ω gives Zs = 1.07 Ω. Against a B32's 80% limit of 1.10 Ω, that's a pass, barely, and exactly the kind of margin worth knowing about before you plasterboard over everything. Typical maximum external values: 0.35 Ω for TN-C-S, 0.8 Ω for TN-S, but always use your measured Ze.
RCDs and TT systems (Table 41.5)
Where an RCD provides the fault protection, which is the normal arrangement on a TT supply, the limit stops being about tripping an MCB magnetically and becomes RA × IΔn ≤ 50 V. That gives dramatically higher permitted values:
| RCD rating IΔn | Maximum Zs (Table 41.5) |
|---|---|
| 30 mA | 1667 Ω |
| 100 mA | 500 Ω |
| 300 mA | 167 Ω |
| 500 mA | 100 Ω |
Don't treat 1667 Ω as a design target though. An earth electrode that high is unstable, dries out in summer, corrodes, drifts. The long-standing rule of thumb is to keep a TT electrode under 200 Ω, and the lower the better.
What about fuses?
BS 88, BS 1361 and rewirable BS 3036 fuses each have their own curves, so there's no single formula like the MCB one, and their 0.4 s and 5 s values genuinely differ. Rather than risk a wrong number here, take fuse values straight from Tables 41.2 and 41.4 of BS 7671 or the On-Site Guide. If you're working on a modern domestic board it's academic anyway, it'll be MCBs and RCBOs, which is exactly what this calculator covers.
Max Zs FAQs
What's the max Zs for a B32 MCB?
1.37 Ω tabulated under the 18th Edition, or 1.10 Ω as your 80% comparison value when testing a cold circuit.
Is max Zs different for 0.4 s and 5 s disconnection?
Not for MCBs and RCBOs. The instantaneous magnetic trip satisfies both disconnection times, so one value covers both. Fuses do differ between 0.4 s and 5 s, and they're not covered by this tool.
Why did the values change between the 17th and 18th Edition?
The 18th Edition bakes the Cmin factor of 0.95 into the formula, which nudged every tabulated value down slightly. A B32 went from 1.44 Ω to 1.37 Ω, for example. If your head still carries the 17th Edition numbers, that's the difference.
My measured Zs is too high, what now?
Rule out the boring stuff first: test lead null, loose connections, parallel paths. Then the real causes: a long run, an undersized CPC, or a high Ze from the supply. Fixes are a bigger CPC or cable, a different device, or, where the design allows, fault protection by a 30 mA RCD, which is how TT systems live with sky-high Ze values.
Does an RCBO have the same max Zs as an MCB?
For fault protection via the overcurrent element, yes, use its curve and rating exactly like an MCB. The RCD element is a separate protective function with its own rules.
How do you measure Zs on site?
Live test with an earth fault loop impedance tester at the furthest point of the circuit, and compare the reading against 80% of the tabulated maximum. Or calculate it from dead tests: Zs = Ze + (R1 + R2). Both routes are accepted, and doing both is a decent sanity check on your own results.
What is the maximum Zs for an RCD-protected circuit?
Where the RCD provides fault protection (TT systems, typically), Table 41.5 applies: 50 ÷ IΔn. That's 1667 Ω for a 30 mA device, 500 Ω for 100 mA, 167 Ω for 300 mA. In practice keep a TT electrode under 200 Ω for stability, the tabulated maximum is a compliance ceiling, not a design target.
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