Zs, the earth fault loop impedance, is the total impedance of the path a line-to-earth fault current takes through the cpc, earthing conductor, supply earth and transformer winding. BS 7671:2018+A4:2026 sets a maximum Zs for every protective device in Tables 41.2 (fuses, 0.4 s), 41.3 (MCBs and RCBOs to BS EN 60898 and BS EN 61009, 0.4 s and 5 s) and 41.4 (fuses, 5 s), so that a fault produces enough current to disconnect within the time in Table 41.1: 0.4 s for most final circuits on a TN system. The most-quoted figure is 1.37 Ω for a 32 A Type B MCB, or 1.10 Ω once the 80 per cent temperature correction is applied to a site measurement.
- The tabulated maximum Zs is calculated as (230 V × 0.95) ÷ the current that trips the device in time. For MCBs that is 5, 10 or 20 times the rating for Types B, C and D.
- Tables assume conductors at 70 °C. Site measurements are made cold, so compare them against 0.8 × the table value (IET Guidance Note 3 method).
- Ze, the external loop impedance, is typically no more than 0.8 Ω on TN-S and 0.35 Ω on TN-C-S; on TT it is set by the electrode and the RCD, with 21 Ω the commonly cited practical figure.
- Zs can be verified by calculation, Ze + (R1+R2), where a live test is not practicable.
- Always check the figure against the table in the edition of BS 7671 you are certifying to; the method below lets you do that.
What is Zs and why does it have a maximum?
Under BS 7671 Chapter 41, protection against electric shock on most circuits relies on automatic disconnection of supply (ADS): a line-to-earth fault must produce a current large enough to operate the protective device quickly. Fault current is supply voltage divided by loop impedance, so the larger Zs is, the smaller the fault current and the slower the device operates. Each device has a current at which it is guaranteed to disconnect within 0.4 s (or 5 s for distribution circuits), and the maximum Zs is the highest impedance that still lets that current flow.
Zs = Ze + (R1+R2): Ze is the impedance external to the installation (transformer, service cable, DNO earth) and R1+R2 is the resistance of the circuit's line conductor and cpc. Both are measured during initial verification (Chapter 64) and periodic inspection (Chapter 65).
How are the maximum Zs values in BS 7671 calculated?
Since Amendment 3 of the 17th Edition, the tables have applied a factor Cmin of 0.95 to the nominal voltage to allow for the supply being at the bottom of its tolerance. The formula is:
Zs(max) = (U0 × Cmin) ÷ Ia = (230 × 0.95) ÷ Ia = 218.5 ÷ Ia
where Ia is the current that causes the device to operate within the required time. For MCBs and RCBOs to BS EN 60898 and BS EN 61009, Ia is the upper limit of the instantaneous trip band, which is why the 0.4 s and 5 s values are identical:
| MCB type | Instantaneous trip range | Ia used in the table |
|---|---|---|
| B | 3 to 5 × In | 5 × In |
| C | 5 to 10 × In | 10 × In |
| D | 10 to 20 × In | 20 × In |
Worked example: 32 A Type B. Ia = 5 × 32 = 160 A. Zs(max) = 218.5 ÷ 160 = 1.366 Ω, printed as 1.37 Ω. For a 32 A Type C, Ia = 320 A and Zs(max) = 0.68 Ω. For a 32 A Type D, Ia = 640 A and Zs(max) = 0.34 Ω. Because this is a straight calculation you can check any figure in the table with a calculator, or produce one for a rating the table does not list.
What are the maximum Zs values for Type B, C and D MCBs?
The table gives the calculated Table 41.3 figures (230 V, 0.4 s and 5 s) and the 80 per cent comparison figures for site use. Check them against the printed table in your copy of BS 7671; rounding can differ in the last digit.
| Rating (A) | Type B tabulated | Type B × 0.8 | Type C tabulated | Type C × 0.8 | Type D tabulated | Type D × 0.8 |
|---|---|---|---|---|---|---|
| 6 | 7.28 | 5.82 | 3.64 | 2.91 | 1.82 | 1.46 |
| 10 | 4.37 | 3.50 | 2.19 | 1.75 | 1.09 | 0.87 |
| 16 | 2.73 | 2.18 | 1.37 | 1.10 | 0.68 | 0.55 |
| 20 | 2.19 | 1.75 | 1.09 | 0.87 | 0.55 | 0.44 |
| 25 | 1.75 | 1.40 | 0.87 | 0.70 | 0.44 | 0.35 |
| 32 | 1.37 | 1.10 | 0.68 | 0.55 | 0.34 | 0.27 |
| 40 | 1.09 | 0.87 | 0.55 | 0.44 | 0.27 | 0.22 |
| 45 | 0.97 | 0.78 | 0.49 | 0.39 | 0.24 | 0.19 |
| 50 | 0.87 | 0.70 | 0.44 | 0.35 | 0.22 | 0.17 |
| 63 | 0.69 | 0.55 | 0.35 | 0.28 | 0.17 | 0.14 |
All values in ohms. The table applies equally to RCBOs to BS EN 61009 with the same B, C or D characteristic, because the overcurrent element behaves in the same way.
Why is measured Zs compared against 80 per cent of the table value?
The values in Tables 41.2 to 41.4 are for conductors at their maximum normal operating temperature, 70 °C for thermoplastic cables, the worst case for a fault on a loaded circuit. Copper resistance rises by roughly 0.4 per cent per degree, so a conductor at 70 °C has about 20 per cent more resistance than one at 20 °C.
You test with the installation cold. To make a like-for-like comparison, IET Guidance Note 3 gives two options:
- The rule of thumb: multiply the tabulated maximum by 0.8 and compare the cold measurement against that. 1.37 Ω × 0.8 = 1.10 Ω for the 32 A Type B.
- Correct the measurement: Zs(corrected) = Ze + (R1+R2) × 1.2 (for 70 °C thermoplastic) and compare against the full table value. This is more precise where Ze is a large share of Zs, because Ze is not affected by the circuit's conductor temperature.
Either is acceptable. The rule of thumb is conservative and is what most inspectors use, which is why the "maximum permitted Zs" column on most certificate forms shows the 80 per cent figure. See Schedule of Test Results: How to Fill In Every Column.
What about fuses to BS 3036, BS 88-3 and BS 1361?
Fuses have no flat instantaneous trip band, so their maximum Zs values come from time-current curves and differ between 0.4 s (Table 41.2) and 5 s (Table 41.4). The 0.4 s disconnection current is far above the rating: a 30 A BS 3036 rewirable fuse needs in the order of 200 A to blow within 0.4 s, which is why its 0.4 s maximum Zs is only about 1.04 Ω. BS 1361 cartridge fuses have been superseded by BS 88-3 and share the same values. Because these figures come from curves rather than a formula, read them from Table 41.2 or 41.4 in your edition, and apply the same 0.8 factor to the site measurement.
What is a typical maximum Ze for TN-S, TN-C-S and TT supplies?
Ze is the external earth fault loop impedance measured at the origin with the installation's main earthing conductor disconnected (see Earth Fault Loop Impedance Testing: Ze, Zs and Ipf Explained). BS 7671 does not set a maximum Ze; it is the whole loop, Zs, that matters. The commonly quoted maxima come from Energy Networks Association guidance on what a DNO will normally declare:
| Earthing system | Typical declared maximum Ze | Notes |
|---|---|---|
| TN-S | 0.8 Ω | Separate earth via cable sheath |
| TN-C-S (PME) | 0.35 Ω | Neutral and earth combined in the supply |
| TT | 21 Ω (with RCD) | Electrode resistance dominates; 200 Ω is the IET stability threshold |
A measured Ze well above the declared figure does not fail against BS 7671 on its own, but it eats into the Zs budget for every circuit and usually indicates a supply earth problem that belongs with the DNO. On TT systems the check is RA × IΔn ≤ 50 V, where RA is the electrode resistance and IΔn the rated residual current of the RCD providing fault protection; with a 30 mA device this gives a theoretical 1667 Ω, but electrode readings above 200 Ω are regarded as unlikely to remain stable, and the 21 Ω figure is simply the maximum Ze a DNO will typically declare for a TT supply, not a BS 7671 limit. Record the actual electrode resistance on the certificate.
Can Zs be verified by calculation instead of measuring?
Yes. Ze is measured once at the origin; R1+R2 is measured dead at the furthest point of each circuit during continuity testing. Adding them gives a calculated Zs for any circuit where a live loop test is not practicable, for example a circuit that cannot be energised or where a no-trip tester still trips a sensitive RCD.
Worked example: TN-C-S supply, Ze 0.28 Ω. Ring final in 2.5/1.5 mm² with R1+R2 at the furthest socket 0.42 Ω. Calculated Zs = 0.28 + 0.42 = 0.70 Ω against 1.10 Ω (80 per cent method) for the 32 A Type B; or, by the correction method, 0.28 + (0.42 × 1.2) = 0.78 Ω against 1.37 Ω. Both pass. A measured Zs at the same socket should land close to 0.70 Ω.
The calculation also sanity-checks a live reading that looks wrong. A measured Zs lower than Ze is a parallel earth path (bonded pipework, SWA armour), not a better circuit. A measured Zs well above Ze + (R1+R2) is a high-resistance joint that the continuity test passed at low current.
How Certio helps
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