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+A2:2022 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
Ze at the origin
Safe-isolate, disconnect the main earthing conductor from the MET (so parallel paths don't lower the reading), and measure between the incoming line and earth. Reconnect immediately after.
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
Zs live measurement
With the circuit energised, use a loop tester at the furthest accessory. On RCD-protected circuits use the no-trip (low-current) function, or link out the RCD, so the test doesn't trip it.
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.
- Fit an RCD: on TT systems, and where loop impedance can't meet the overcurrent limit, a 30 mA RCD provides the required disconnection regardless of Zs.
- 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 qualified electrical work. Figures follow BS 7671 and the IET On-Site Guide unless stated; always confirm against the current edition and the standard that applies to your installation.