Earth Fault Loop Impedance: Step by Step
Measure Ze and Zs, understand the loop, and verify the protective device will disconnect fast enough.
BS 7671:2018+A4:2026 Reg 411 & Table 41.3/41.4IET On-Site Guide
Earth-fault loop impedance is the total resistance of the path a fault current takes from the point of fault, back through the earthing and the supply transformer, and out again to the point of fault. The lower it is, the bigger the fault current and the faster the protective device trips. Zs is what you measure to prove the disconnection times can actually be met.
What's in the loop
Zs = Ze + (R1 + R2)Ze = external loop impedance at the origin; R1 = line conductor resistance; R2 = protective conductor resistance of the circuit.
Ze is everything outside the installation (the supply and the earth return). (R1 + R2) is the go-and-return resistance of the final circuit conductors. Add them and you have Zs: the loop impedance at the furthest point of the circuit, which is where it is highest and disconnection is hardest.
Measuring Ze and Zs
1
Establish Ze evidence
Use declared network data or a competent, planned origin test appropriate to the earthing arrangement. Direct measurement can involve exposed live parts and temporary loss of protective paths, so it requires a specific safe system of work and restoration checks.
2
(R1 + R2) by measurement or calculation
Either measure end-to-end continuity of line + cpc, or calculate from the cable's mΩ/m figures and length. This is a dead test.
3
Confirm Zs by an approved method
Prefer calculation or permitted dead-test evidence where suitable. If a live loop test is justified, use a compatible instrument/method at the safest suitable point, control the live-testing risk, and never bypass protective devices merely to obtain a reading.
4
Compare against the limit
Check the measured Zs against the maximum for the protective device and disconnection time (0.4 s for most final circuits ≤ 63 A on TN, 5 s for distribution).
The disconnection-time check
The maximum Zs is whatever value lets enough current flow to operate the device within the required time. The regulations tabulate this per device; you can also derive it:
Zs(max) = (U0 × Cmin) / IaU0 = 230 V nominal, Cmin = 0.95, Ia = current for disconnection in the required time.
| Device (Type B MCB) | Ia (5 × In) | Max Zs @ 0.4 s (tabulated) |
|---|---|---|
| B6 | 30 A | 7.28 Ω |
| B16 | 80 A | 2.73 Ω |
| B32 | 160 A | 1.37 Ω |
| B40 | 200 A | 1.09 Ω |
Type B trips in the magnetic region at 5 × In. Type C needs 10 × In (half the Zs); Type D needs 20 × In.
Apply the temperature correction
Tabulated maximum Zs values already include a factor for conductors being cold at the moment of test (roughly 0.8). If you calculate your own limit from U0/Ia, apply the same correction, or measure and compare to the tabulated figure, which is simpler and safer.
When Zs is too high
- Increase the cpc size (lower R2): often the cheapest fix on a long run.
- Use a device with a lower trip multiple (Type B instead of C) if the load's inrush allows.
- Redesign the protective measure: an appropriately selected RCD may provide fault protection where overcurrent operation cannot meet the required time, but Zs/RA, IΔn, device operation, continuity, and the governing touch-voltage relationship still require verification.
- Recheck Ze: a poor supply earth or a PME fault at the source can push every circuit's Zs over the limit.
Reference material for competent electrical work. Use the sources and jurisdiction stated in this guide, and always confirm the current rules, equipment instructions, and site conditions before relying on a value or procedure.