Free Zs Values Lookup (BS 7671 Earth Fault Loop)
Look up maximum earth fault loop impedance (Zs) from BS 7671. Choose the device and rating for the table value and 80% measured limit.
Zs Values Lookup (BS 7671 Earth Fault Loop) results
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A free maximum Zs lookup for UK electricians. The current edition is BS 7671:2018+A4:2026. The tabulated values on this page are from BS 7671:2018+A2:2022 Tables 41.2 and 41.3. 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, plus the corrected 80% figure your on-site measured reading is compared against.
How to use the Zs Values Lookup (BS 7671 Earth Fault Loop)
- Select the protective device type: Type B, C or D MCB/RCBO, or a BS 3036 rewirable or BS 1361 cartridge fuse.
- Select the device rating in amps.
- Read the tabulated maximum Zs: the BS 7671 limit for that device at the standard disconnection time.
- Compare your test instrument reading against the 80% measured figure, not the raw table value.
- If your measured Zs is above the 80% limit, the circuit does not meet BS 7671 and needs investigation.
How the Zs Values Lookup (BS 7671 Earth Fault Loop) works
Maximum Zs on this page comes from BS 7671:2018+A2:2022 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. The current edition is BS 7671:2018+A4:2026.
- 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 Cmin (0.95) correction from those BS 7671:2018+A2:2022 tables, so use them as your working limits rather than older uncorrected tables. The current edition is BS 7671:2018+A4:2026.
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
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Frequently 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.
