Free Zs Values Lookup (BS 7671 Max Earth Fault Loop Impedance)
Instantly look up the maximum earth fault loop impedance (Zs) for any protective device under BS 7671. Pick the device type and rating to get the table value and the 80% measured limit.
A free BS 7671:2018+A2:2022 (18th Edition) maximum Zs lookup for UK electricians. Select the protective device (Type B/C/D MCB or RCBO, BS 3036 rewirable or BS 1361 cartridge fuse) and its rating to get the tabulated maximum earth fault loop impedance from Tables 41.2 and 41.3, plus the corrected 80% figure your on-site measured reading is compared against.
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Get a free demoHow to use the Zs Values Lookup (BS 7671 Max Earth Fault Loop Impedance)
- 1Select the protective device type: Type B, C or D MCB/RCBO, or a BS 3036 rewirable or BS 1361 cartridge fuse.
- 2Select the device rating in amps.
- 3Read the tabulated maximum Zs: the BS 7671 limit for that device at the standard disconnection time.
- 4Compare your test instrument reading against the 80% measured figure, not the raw table value.
- 5If your measured Zs is above the 80% limit, the circuit does not meet BS 7671 and needs investigation.
How to work it out yourself
Maximum Zs comes straight from BS 7671:2018+A2:2022 (18th Edition) Tables 41.2 (fuses) and 41.3 (circuit-breakers), not from a formula you work out on site. Each value is the loop impedance at which the device still clears an earth fault inside the required disconnection time.
- A Type B 32A MCB has a tabulated maximum Zs of 1.37 Ω for 0.4s disconnection.
- Apply the 0.8 rule: 1.37 × 0.8 ≈ 1.09 Ω. The figure your measured reading must stay under.
- Measure 0.95 Ω on site → within limit, circuit passes. Measure 1.20 Ω → over the 1.09 Ω limit, investigate.
The 0.8 factor is a widely used on-site rule of thumb. The values here already apply the current Cmin (0.95) correction, so use them as your working limits rather than older uncorrected tables. Always confirm against the edition of BS 7671 current at the time of the work.
Typical UK benchmarks
| Type B 6A max Zs (final circuit, 0.4s) | BS 7671:2018+A2:2022 (18th Edition) Table 41.3, Cmin 0.95 |
| Type B 32A max Zs (final circuit, 0.4s) | BS 7671:2018+A2:2022 (18th Edition) Table 41.3, Cmin 0.95 |
Disclaimer
This tool gives an estimate for general guidance only, based on typical UK figures and the details you enter. It is not a formal quote, survey, or professional advice. Always confirm with a qualified tradesperson before relying on any figure, as regional prices, site conditions, and current standards vary.
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Get a free demoFrequently asked questions
What is Zs in electrical testing?
Zs is the earth fault loop impedance: the total impedance of the fault current path from the point of a fault back through the supply transformer and out again. A low enough Zs guarantees enough fault current flows to operate the protective device within the required disconnection time (0.4s for most final circuits, 5s for distribution circuits).
What is the 0.8 rule of thumb for Zs?
The tabulated maximum Zs values in BS 7671 assume conductors at their normal operating temperature. On site you usually test a cold circuit, so measured impedance is lower than under fault. The '0.8 rule' multiplies the tabulated value by 0.8 to give a practical measured limit your test reading should stay below. The values on this page apply that correction (and the current Cmin 0.95 factor) for you.
How do you measure Zs on site?
Use a loop impedance tester at the point furthest from the origin on the circuit (the point of highest Zs). Most testers offer a low-current 'no-trip' mode so you can measure on RCD-protected circuits without tripping them. Record the reading and compare it against the maximum measured Zs for the circuit's protective device and rating.
Where do the maximum Zs values come from?
They come from BS 7671 Tables 41.2 (BS 3036 and BS 1361 fuses) and 41.3 (Type B/C/D circuit-breakers). Each value is derived from the device's disconnection characteristic (a Type B breaker trips instantly at 5×In, a Type C at 10×In and a Type D at 20×In) and the design voltage with the Cmin factor applied.
Why is the maximum Zs lower for a Type C than a Type B?
A Type C circuit-breaker needs a higher fault current to trip instantly (10×In versus 5×In for a Type B). To push that larger current through the fault loop, the loop impedance has to be lower, so the maximum permissible Zs for a Type C is roughly half that of the same-rated Type B, and a Type D lower still.
What happens if measured Zs is too high?
If measured Zs exceeds the maximum for the device, the protective device may not disconnect within the required time, leaving a dangerous touch voltage present during a fault. On an EICR this is typically coded C2 (potentially dangerous). Causes include long cable runs, undersized CPCs, or a poor supply earth. Each needs investigating rather than simply uprating the device.
