Zs is the earth fault loop impedance of a circuit: the total impedance, in ohms, of the path a line-to-earth fault current takes from the fault, back through the circuit protective conductor and the earthing, through the supply and the transformer winding, and out along the line conductor to the fault again. It decides whether the fuse or circuit-breaker trips fast enough. BS 7671 Table 41.3 sets the maximum for a 32 A Type B breaker at 1.37 Ω; tested cold, the reading should be no more than about 1.10 Ω.
- Zs = Ze + (R1 + R2): the supply's external impedance plus the circuit's line and cpc resistances.
- The maximum Zs comes from BS 7671 Tables 41.2 to 41.4 for the device protecting the circuit, worked out as Cmin × U0 ÷ Ia, with Cmin = 0.95.
- The tables assume hot conductors. Compare a cold reading with 80% of the tabulated figure, or with the corrected tables in the On-Site Guide and IET Guidance Note 3.
- Measure Zs live at the furthest point of every circuit. A calculated Zs is a cross-check, not a substitute.
- On an EICR, a Zs too high for the device to disconnect in time is normally coded C2, unless an RCD in the circuit provides disconnection.
- On a TT system the overcurrent device rarely achieves disconnection, so an RCD does the job; Table 41.5 gives 1,667 Ω as the maximum for a 30 mA RCD.
What does Zs actually measure?
When a line conductor touches earthed metalwork, current flows round a loop. Ohm's law does the rest: the fault current is roughly the supply voltage divided by the loop impedance. A low Zs means a big fault current, and a big fault current means the breaker trips on its magnetic element, almost instantly. A high Zs means a smaller fault current, which may only bring the device onto its slower thermal curve, and the metalwork sits at a dangerous touch voltage for longer.
The loop has three parts:
- Ze, the external part: the transformer winding, the supply cable and the DNO's earth return (the sheath on TN-S, the combined PEN conductor on TN-C-S, the ground and the electrode on TT). See the Ze guide.
- R1, the resistance of the circuit's line conductor from the board to the furthest point.
- R2, the resistance of the circuit protective conductor over the same run.
R1 + R2 is the figure you measure dead during continuity testing, which is why Zs = Ze + (R1 + R2) is such a useful check. If the live Zs and the sum disagree by more than a few hundredths of an ohm, something is wrong: a loose termination, a parallel path, or a mistake in one of the readings.
How is the maximum Zs worked out?
BS 7671 Regulation 411.4.4 requires Zs × Ia ≤ U0 × Cmin, where Ia is the current that operates the device within the required disconnection time, U0 is 230 V and Cmin is 0.95, a factor that allows for the supply voltage running low. As the IET explained in Wiring Matters (Issue 94, March 2023), the values in Table 41.3 come straight out of that formula.
For a circuit-breaker to BS EN 60898 or an RCBO to BS EN 61009-1, Ia for 0.4 s is the top of the instantaneous band: 5 × In for Type B, 10 × In for Type C and 20 × In for Type D. So a 32 A Type B gives 0.95 × 230 ÷ 160 = 1.37 Ω, and a 32 A Type D gives 0.95 × 230 ÷ 640 = 0.34 Ω.
| Device (0.4 s, U0 230 V) | Ia | Max Zs, Table 41.3 | 80% limit for a cold reading |
|---|---|---|---|
| 6 A Type B | 30 A | 7.28 Ω | 5.82 Ω |
| 16 A Type B | 80 A | 2.73 Ω | 2.18 Ω |
| 20 A Type B | 100 A | 2.19 Ω | 1.75 Ω |
| 32 A Type B | 160 A | 1.37 Ω | 1.10 Ω |
| 40 A Type B | 200 A | 1.09 Ω | 0.87 Ω |
| 16 A Type C | 160 A | 1.37 Ω | 1.10 Ω |
| 32 A Type C | 320 A | 0.68 Ω | 0.55 Ω |
| 32 A Type D | 640 A | 0.34 Ω | 0.27 Ω |
Fuses are in Table 41.2 (0.4 s) and Table 41.4 (5 s). For MCCBs to BS EN 60947-2, which BS 7671 doesn't tabulate, work it out from the manufacturer's settings and curves, as the same Wiring Matters article sets out. The full set of tables, including 5 s values for distribution circuits, is in our maximum Zs values guide.
Disconnection times come from Table 41.1 and Regulation 411.3.2: on a 230 V TN system, 0.4 s for final circuits up to 63 A with sockets and up to 32 A supplying fixed equipment, and 5 s for distribution circuits and larger final circuits.
Why is a cold reading compared with 80%?
The Chapter 41 maximums assume the conductors are at their maximum operating temperature, 70 °C for thermoplastic (PVC) cable. Copper resistance rises by about 0.4% per degree, so a circuit that reads 1.30 Ω at 15 °C will be well over that once it has been carrying load. Regulation 643.7.3 says the measured value has to account for this.
The practical answer is the rule of thumb in the IET On-Site Guide and Guidance Note 3: multiply the tabulated maximum by 0.8 and compare your cold reading with that. The OSG tables already give corrected figures at 10 °C, for example 1.75 Ω for a 20 A Type B, which matches the 80% column above. Some manufacturers publish their own maximums, often higher than BS 7671's because they reflect the real device, and Appendix 3 of BS 7671 allows you to use manufacturer data where you have it. Note which source you used.
How do you measure Zs properly?
Zs is measured live, line to earth, at the furthest point of the circuit: the last socket on a radial, the furthest point on a ring, the last light on a lighting circuit. For a ring, the furthest point is where the dead test gave the highest (R1 + R2), not simply the socket furthest from the board.
| Situation | Test to use | Notes |
|---|---|---|
| Circuit on an MCB or fuse, no RCD | High-current loop test, two-wire | Most accurate and repeatable |
| Circuit on an RCD or RCBO | No-trip loop test | Low current; take two readings, cross-check if near the limit |
| Can't reach the end of the circuit live | Calculate Ze + (R1 + R2) | Record that it was calculated and why |
| Distribution circuit | Zs at the sub-board | Record as Zdb in the board block on the A4 schedule |
Things that spoil the reading:
- Parallel paths. Metal conduit, trunking, bonded pipework and steel back boxes give the fault current other ways home, so a measured Zs can be lower than the sum. That's fine for disconnection, but the dead (R1 + R2) is what proves the cpc itself is continuous.
- Probe and lead resistance. Use the instrument's lead null and a fused plug-in lead or wander lead with a sound connection.
- Supply variation. Loop readings bounce on a busy network. Two readings within a few hundredths of each other are worth more than one.
- The wrong earthing. Never measure Zs with the main earthing conductor disconnected; that is the setup for Ze only, and the installation must be isolated for it.
For a reading that is high or keeps changing, work through the high Zs troubleshooting guide.
Worked examples: does it pass?
These use typical (r1 + r2) figures from the OSG resistance tables (2.5 mm² line with 1.5 mm² cpc is 19.51 mΩ/m at 20 °C; 1.0/1.0 mm² is 36.20 mΩ/m).
Ring final, 32 A Type B RCBO, 2.5/1.5 mm² twin and earth, 60 m of cable in the ring. End-to-end r1 is about 0.45 Ω and r2 about 0.74 Ω, so (R1 + R2) at the furthest point is (0.45 + 0.74) ÷ 4 = 0.30 Ω.
| Earthing | Ze | Zs = Ze + (R1 + R2) | Cold limit (80% of 1.37) | Result |
|---|---|---|---|---|
| TN-C-S (PME) | 0.35 Ω | 0.65 Ω | 1.10 Ω | Pass |
| TN-S | 0.80 Ω | 1.10 Ω | 1.10 Ω | On the limit: check, consider correction |
| TT | 60 Ω | 60.3 Ω | 1.10 Ω (MCB) / 1,667 Ω (30 mA RCD) | Disconnection by the RCD |
Lighting radial, 6 A Type B, 1.0/1.0 mm², 40 m to the last point. (R1 + R2) = 40 × 0.0362 = 1.45 Ω. On PME with Ze 0.35 Ω, Zs = 1.80 Ω against a cold limit of 5.82 Ω: plenty in hand.
The TN-S ring is the one that catches people. 0.80 Ω is the typical maximum a DNO quotes for TN-S, so on an older supply with a long ring, the reading sits right on the 80% line. Shortening the ring isn't an option on an EICR, so record what you find, check the RCBO meets Table 41.5, and note the margin.
What about TT systems and RCDs?
On TT, Ze includes the consumer's earth electrode, so Zs is typically tens or hundreds of ohms. No MCB is going to trip magnetically on that, so fault protection comes from an RCD. Regulation 411.5.3 requires RA × IΔn ≤ 50 V, and Table 41.5 turns that into maximum Zs values: 1,667 Ω for a 30 mA RCD and 500 Ω for a 100 mA time-delayed (S-type) device. IET Guidance Note 3 notes that an electrode reading above 200 Ω may not be stable, so treat anything above that as worth investigating even though it is within Table 41.5. See earth electrode testing on TT systems.
BS 7671 also allows an RCD to provide automatic disconnection on a TN system where Zs is too high for the overcurrent device. That makes a high Zs on an RCBO circuit a different conversation from a high Zs on a plain MCB, though the overcurrent device still has to cope with a line-to-neutral fault, which is a separate check (see prospective fault current).
How is a high Zs coded on an EICR?
Where Zs exceeds the maximum for the device and nothing else provides automatic disconnection, the circuit can't be relied on to clear an earth fault in the required time. Electrical Safety First's Best Practice Guide 4 (Issue 7.4, August 2026) treats inadequate fault protection as potentially dangerous, so that is normally a C2. A missing or disconnected cpc on a circuit with exposed metalwork can justify a C1.
| Finding | Typical code | Comment |
|---|---|---|
| Zs above Table 41.3 max, MCB only, no RCD | C2 | Disconnection time not achieved |
| Zs above MCB max but RCD/RCBO meets Table 41.5 | No code for Zs, or C3 with comment | ADS provided by the RCD; record the reasoning |
| Zs within limits but much higher than the last report | FI | Deterioration: find out why |
| Zs unreadable, cpc open circuit, exposed metal | C1 or C2 | Depends on what is energised and touchable |
| Couldn't reach the furthest point | Limitation | Record it; don't write a guess |
The final call is always the inspector's. The observation codes guide covers the wider coding logic, and the ten most common EICR fails shows where Zs sits among them.
Where does Zs go on the certificate?
On an EIC, an EICR and the schedule of test results, Zs is recorded for every circuit in the test results columns, and the maximum permitted Zs for the device goes in the circuit details, so the reader can see the margin at a glance. On a Minor Works certificate, Zs goes in the test results for the circuit you altered or extended. Ze belongs in the supply characteristics, and in the BS 7671:2018+A4:2026 model forms each distribution board has its own Zdb and Ipf in the board block. The schedule of test results guide walks through each column.
Common mistakes with Zs
- Testing at the first socket instead of the furthest point.
- Comparing a cold reading with the full table value.
- Writing a calculated figure without saying it's calculated.
- Copying Zs from the last certificate. Connections deteriorate; that's the point of the test.
- Putting a sub-board's Zdb in the Ze box.
- Forgetting the RCD. A high Zs on an RCBO circuit is not automatically a C2.
How Certio helps
Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is £4.99 a month, charged when you sign up.