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Max Zs lookup. Earth fault loop limits, fast.
Pick the device and its rating to see the maximum Zs, the highest reading to accept on test, and whether your own reading passes.
Max Zs lookup
Check my reading
The reading from your loop tester at the furthest point of the circuit.
Quick reference
Maximum Zs in ohms at 230 V. Max is the tabulated figure. 80% is the highest reading to accept when you test at normal room temperature.
| Rating | Type B | Type C | Type D | |||
|---|---|---|---|---|---|---|
| Max | 80% | Max | 80% | Max | 80% | |
| 3 A | 14.57 | 11.66 | 7.28 | 5.82 | – | – |
| 6 A | 7.28 | 5.82 | 3.64 | 2.91 | 1.82 | 1.46 |
| 10 A | 4.37 | 3.50 | 2.19 | 1.75 | 1.09 | 0.87 |
| 16 A | 2.73 | 2.18 | 1.37 | 1.10 | 0.68 | 0.54 |
| 20 A | 2.19 | 1.75 | 1.09 | 0.87 | 0.55 | 0.44 |
| 25 A | 1.75 | 1.40 | 0.87 | 0.70 | 0.44 | 0.35 |
| 32 A | 1.37 | 1.10 | 0.68 | 0.54 | 0.34 | 0.27 |
| 40 A | 1.09 | 0.87 | 0.55 | 0.44 | 0.27 | 0.22 |
| 45 A | 0.97 | 0.78 | 0.49 | 0.39 | 0.24 | 0.19 |
| 50 A | 0.87 | 0.70 | 0.44 | 0.35 | 0.22 | 0.18 |
| 63 A | 0.69 | 0.55 | 0.35 | 0.28 | 0.17 | 0.14 |
| 80 A | 0.55 | 0.44 | 0.27 | 0.22 | 0.14 | 0.11 |
| 100 A | 0.44 | 0.35 | 0.22 | 0.18 | 0.11 | 0.09 |
| 125 A | 0.35 | 0.28 | 0.17 | 0.14 | 0.09 | 0.07 |
RCDs in TT systems
50 V divided by the rated residual current. Aim for 200 Ω or less in practice.
| Rated residual current | Max Zs (Ω) |
|---|---|
| 10 mA | 5,000 |
| 30 mA | 1,667 |
| 100 mA | 500 |
| 300 mA | 167 |
| 500 mA | 100 |
Common questions
What is Zs?
Zs is the earth fault loop impedance. It is the total impedance of the path fault current takes: out along the line conductor to the fault, back through the protective conductor and the earthing, and round through the supply transformer. The lower it is, the more current flows in a fault and the faster the protective device opens.
How do Ze, R1+R2 and Zs fit together?
Ze is the part outside the installation, from the supply. R1+R2 is the resistance of the circuit's line conductor and protective conductor added together. Zs is the whole loop, so Zs = Ze + (R1+R2). You can measure Zs with a loop tester, or work it out from a measured Ze and R1+R2.
Why accept only 80% of the tabulated value?
The tabulated values assume the conductors are at their maximum operating temperature, usually 70 °C for PVC cable. You test a circuit cold, when its resistance is lower. Conductors get hotter at full load and hotter still while fault current flows, so a cold reading has to leave room for that rise. The 0.8 factor is a rule of thumb for it. Cmin already covers changes in supply voltage.
Which disconnection time applies?
In a TN system, final circuits up to 63 A with one or more socket-outlets, and up to 32 A supplying only fixed equipment, must disconnect within 0.4 s. Distribution circuits and other final circuits are allowed 5 s. The circuit-breaker values here meet both, because they use the current that trips the breaker instantly. TT systems have shorter times, set out in Table 41.1.
What about fuses?
This lookup covers circuit-breakers, RCBOs and RCDs only. Fuses have their own limits, in BS 7671 Table 41.2 for 0.4 s and Table 41.4 for 5 s. For any device, the manufacturer's data may give a higher limit you can use instead.
Sources
- IET: maximum Zs values. The formula Zs = U0 × Cmin ÷ Ia, Cmin of 0.95 and the 1.37 Ω figure for a 32 A Type B.
- IET Wiring Matters: determining the maximum earth fault loop impedance for protective devices (March 2023). Instant-trip current for 0.4 s and the 0.8 temperature factor.
- IET Wiring Matters: why are the values of maximum earth fault loop impedance different? (May 2024). Conductor temperature and the 0.8 factor.
- IET: Amendment 4:2026 to BS 7671:2018, published 15 April 2026.
- ECA: Amendment 4. The previous version was withdrawn on 15 October 2026.
- BEAMA: earth fault loop impedance testing and recording. Table 41.5 gives 1667 Ω for a 30 mA RCD.
- Voltimum: what reading should I be getting? The note to Table 41.5 that an electrode resistance above 200 Ω may not be stable.
Values calculated from the BS 7671 formula. Always check them against your copy of the current edition and the device manufacturer's data. Related: RCD type selector and earthing systems.