Maximum Zs Values BS 7671: Tables 41.2 to 41.5 and the 80% Rule

Zs, the earth fault loop impedance, is the total impedance of the path a line-to-earth fault current takes: through the line conductor, the circuit protective conductor, the main earthing conductor, the supply earth and back through the 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 (circuit-breakers and RCBOs, 0.4 s and 5 s), 41.4 (fuses, 5 s) and 41.5 (RCDs), so that a fault produces enough current to disconnect within the time in Table 41.1. The most-quoted figure is 1.37 Ω for a 32 A Type B circuit-breaker, or 1.10 Ω once the 80 per cent temperature allowance is applied to a cold site measurement. Every figure in the breaker table comes from one formula, which this guide works through so you can check any device in any edition.

What is Zs and why does it have a maximum?

Under Chapter 41 of BS 7671, protection against electric shock on almost every circuit relies on automatic disconnection of supply (ADS). A fault between line and an exposed-conductive-part must drive enough current round the loop to operate the protective device before the touch voltage has time to do harm. Fault current is simply U0 divided by Zs, so the larger the loop impedance, the smaller the fault current and the slower the device. Each device has a current, Ia, at which it is guaranteed to operate within the required time, and the maximum Zs is the highest impedance that still lets Ia flow.

Regulation 411.3.2 sets the times. For final circuits not exceeding 63 A with socket-outlets, and final circuits not exceeding 32 A supplying fixed equipment only, Table 41.1 requires 0.4 s on a TN system and 0.2 s on a TT system at 230 V (Regulation 411.3.2.2). Distribution circuits, and final circuits outside those two categories, get 5 s on TN (411.3.2.3) and 1 s on TT (411.3.2.4). That is why a 40 A shower circuit with no socket is a 5 s circuit, while a 32 A ring final is a 0.4 s circuit.

Zs = Ze + (R1+R2). Ze is the impedance external to the installation (the transformer, the service cable and the DNO's 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). Ze on its own is covered in What is Ze? and the test method in Earth Fault Loop Impedance Testing.

How are the maximum Zs values in BS 7671 calculated?

Regulation 411.4.4 states the requirement as Zs × Ia ≤ U0 × Cmin. Transposed, the maximum is:

Zs(max) = (U0 × Cmin) ÷ Ia = (230 × 0.95) ÷ Ia = 218.5 ÷ Ia

Cmin is the minimum voltage factor. It was introduced by Amendment 3 to the 17th Edition (BS 7671:2008+A3:2015, effective 1 July 2015) to allow for the supply sitting at the bottom of its statutory tolerance, and the IET's note on the change records that 0.95 was chosen because a proposed 0.94 was judged unrealistically low. Every value in Tables 41.2, 41.3, 41.4 and 41.6 dropped to 0.95 times its previous figure; the 32 A Type B went from 1.44 Ω to 1.37 Ω. If a manufacturer's data sheet or an old training handout still shows 1.44 Ω, it predates A3 and should not be used.

For circuit-breakers to BS EN 60898 and RCBOs to BS EN 61009, Ia is the top of the instantaneous trip band, which is why the 0.4 s and 5 s columns are identical:

Worked example, 32 A Type B: Ia = 5 × 32 = 160 A, Zs(max) = 218.5 ÷ 160 = 1.366 Ω, printed as 1.37 Ω. A 32 A Type C has Ia = 320 A and Zs(max) = 0.68 Ω; a 32 A Type D has Ia = 640 A and Zs(max) = 0.34 Ω. So a Type C always allows half the Zs of a Type B, and a Type D a quarter, which is why swapping a C-curve for a B-curve on a long lighting circuit doubles the headroom overnight, provided the load's inrush allows it.

What are the maximum Zs values for Type B, C and D circuit-breakers?

The table below gives the Table 41.3 figures for 230 V (0.4 s and 5 s) and the 80 per cent site comparison figure next to each. The 80 per cent column is 0.8 times the printed table value, rounded to two places, which is how most instruments and certificate forms present it. Check against the printed table in your copy of BS 7671; rounding can differ in the last digit.

All values in ohms. RCBOs to BS EN 61009 with a B, C or D characteristic take the same figures, because the overcurrent element behaves exactly like the equivalent circuit-breaker. The residual element is a separate matter, covered below under Table 41.5.

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 insulation, because that is the worst case for a fault on a fully 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 the same conductor at 20 °C. You test on a cold, lightly loaded installation, so a raw comparison flatters the circuit.

BS 7671:2018 deals with this in Appendix 3, which took over the material from Appendix 14 of the 17th Edition. The formula for a measured value is Zs(m) ≤ 0.8 × U0 × Cmin ÷ Ia, where Ia is the current that operates the device within the Table 41.1 time or within 5 s under Regulation 411.3.2.3. IET Guidance Note 3 (10th Edition, aligned to A4:2026) applies it as the familiar rule of thumb: the measured Zs should not exceed 0.80 of the tabulated maximum. For the 32 A Type B that is 1.37 × 0.8 = 1.10 Ω.

There are two acceptable ways to use it:

• The rule of thumb: multiply the tabulated maximum by 0.8 and compare the cold reading against that. Conservative, quick, and what most inspectors and most instruments do by default.
• Correct the measurement instead: Zs(corrected) = Ze + (R1+R2) × 1.2 for 70 °C thermoplastic cable, then compare against the full table value. This is more accurate where Ze makes up a large share of Zs, because the supply impedance does not warm up with the circuit's load current. This is the route to take when a reading lands between 0.8 and 1.0 times the table value.

Worked example: TN-C-S, Ze 0.30 Ω, 32 A Type B radial, measured Zs 1.18 Ω. Rule of thumb: 1.18 > 1.10, fail. Correction method: R1+R2 is 1.18 − 0.30 = 0.88 Ω, corrected Zs = 0.30 + (0.88 × 1.2) = 1.36 Ω, which is under 1.37 Ω by a whisker. Both methods are legitimate, but a result that close deserves a look at why the circuit is so long or so thin before you sign it off, and High Zs Readings: Troubleshooting walks through the usual causes.

The "maximum permitted Zs" column on the schedule of test results is where the figure goes. The A4:2026 model schedule still asks for it per circuit, and most inspectors enter the 80 per cent value so the comparison with the measured column is like-for-like; see How to Fill In a Schedule of Test Results.

What about fuses to BS 3036, BS 88-2 and BS 88-3?

A fuse has no flat instantaneous band. Its operating current comes from a time-current curve, so the current that clears it in 5 s is a good deal lower than the current that clears it in 0.4 s, and the two tables give different values: Table 41.2 for 0.4 s and Table 41.4 for 5 s. The 0.4 s disconnection current is far above the rating. A 30 A BS 3036 semi-enclosed (rewirable) fuse needs in the order of 200 A to blow within 0.4 s, which is why its 0.4 s maximum is only about 1.04 Ω, lower than a 32 A Type B breaker despite the similar rating.

BS 1361 cartridge fuses, the ones in older consumer units and cut-outs, have been superseded by BS 88-3 and share its values. BS 88-2 gG fuses, common in industrial switchfuses, have their own columns. Because fuse figures come from curves rather than a formula, read them from Table 41.2 or 41.4 in your edition rather than calculating them, and apply the same 0.8 factor to a cold measurement. Where a rewirable fuse protects a socket circuit and the measured Zs is over, the usual EICR answer is not a longer cable but a change of device, since a BS 3036 fuse provides none of the 30 mA additional protection that Regulation 411.3.3 requires for socket-outlets anyway.

When does Table 41.5 for RCDs apply?

Table 41.5 gives the maximum Zs where an RCD provides fault protection: 1667 Ω for a 30 mA device, 500 Ω for 100 mA, 167 Ω for 300 mA and 100 Ω for 500 mA, all from 50 V ÷ IΔn. Two situations use it.

The first is TT. Regulation 411.5.3 requires RA × IΔn ≤ 50 V, where RA is the sum of the earth electrode resistance and the cpc. A 30 mA RCD gives the theoretical 1667 Ω, but the note to Table 41.5 says an electrode resistance above 200 Ω may not be stable, so 200 Ω is the practical ceiling and the actual electrode value goes on the certificate. Method and coding are in Earth Electrode Testing on TT Systems.

The second is a TN circuit whose circuit-breaker cannot meet Table 41.3. Regulation 411.4.5 allows the disconnection requirement of 411.4.4 to be satisfied by an RCD, and the On-Site Guide's Table 7.1(ii) of maximum circuit lengths has a "with RCD" column for exactly this reason. NICEIC's position, given in a Voltimum Q&A on 3 October 2013 under the 17th Edition numbering and unchanged in substance since, is that recording the Table 41.5 figure is legitimate where an RCD or RCBO provides ADS, with three cautions. A Zs under 1667 Ω proves only that the RCD will clear an earth fault, not that the circuit complies with the rest of BS 7671. The line-neutral loop impedance must still be low enough for the overcurrent device to clear a short circuit before the cable reaches its limit temperature (Regulation 434.5.2), and an RCD does nothing for a line-neutral fault. And on TN-S or TN-C-S, a Zs well above the Table 41.3 value is more often a high-resistance joint than a long cable, and a high-resistance joint is a fire risk that an RCD will not address. Record which table you used and why in the observations.

What is a typical maximum Ze for TN-S, TN-C-S and TT supplies?

BS 7671 does not set a maximum Ze. It limits the whole loop, Zs, and leaves the supply impedance to the Electricity Safety, Quality and Continuity Regulations and the distributor. The figures electricians quote come from Energy Networks Association guidance on what a DNO will normally declare for a new supply:

A measured Ze above the declared figure is not itself a BS 7671 failure, but it eats into the Zs budget of every circuit on the board and usually points to a supply earth problem that belongs with the DNO, not the installation. On a TN-C-S supply with Ze at 0.60 Ω, a 32 A Type B radial has only 0.50 Ω of R1+R2 to play with against the 80 per cent figure, roughly 25 m of 2.5/1.5 mm² at about 19.5 mΩ per metre cold, so an outbuilding circuit that would have been fine on a 0.20 Ω supply now fails. The three earthing arrangements and their Ze are explained in Earthing Arrangements: TN-S, TN-C-S and TT.

What about MCCBs and devices not in the tables?

Tables 41.2 to 41.4 cover fuses and BS EN 60898 and 61009 breakers up to 125 A. For MCCBs to BS EN 60947-2 and anything else, Appendix 3 says to use manufacturer data wherever possible, and IET Wiring Matters issue 94 (March 2023) sets out the method. Read Ia from the device's time-current curve by drawing a horizontal line at the required disconnection time (0.4 s or 5 s) across to the curve and dropping a vertical to the current axis, then apply 218.5 ÷ Ia. Where the curve does not already include manufacturing tolerance, add it; the article's example uses 20 per cent on the magnetic setting.

For an adjustable MCCB the settings matter. Ir is the long-time thermal setting, typically 0.8 to 1.0 × In, so a 160 A frame set to 0.8 is a 128 A device. Isd (sometimes Im) is the short-time magnetic setting, typically 1.5 to 10 × Ir; at 5 × Ir that is 640 A, and the 0.4 s Zs follows from that figure with tolerance applied. The 5 s figure is read from the thermal part of the curve, in the article's example 4.2 × Ir = 538 A. The same article shows how much manufacturer data can help: a 50 A Type D breaker gives 0.22 Ω for 0.4 s and 0.44 Ω for 5 s from the generic Appendix 3 curve, but one manufacturer's published curve gave 1.04 Ω for 5 s, more than double. Check that the manufacturer has applied Cmin; data sheets written before 2015 will not have.

Can Zs be verified by calculation instead of measuring?

Yes. Regulation 643.7.3 allows Zs to be determined either by direct measurement or by measuring Ze at the origin and adding the circuit's R1+R2 from the dead continuity test described in Continuity Testing: the R1+R2 Method. Use the calculated route where a live loop test is not practicable, for example a circuit that cannot be energised on the day or a sensitive RCD that trips even in no-trip mode.

Worked example: TN-C-S, 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 by the 80 per cent method, or 0.28 + (0.42 × 1.2) = 0.78 Ω against 1.37 Ω by the correction method. Both pass comfortably, and a live reading at the same socket should land within a few hundredths of 0.70 Ω.

The calculation also sanity-checks a live reading that looks wrong. A measured Zs lower than Ze is not a better circuit; it is a parallel earth path through bonded pipework, SWA armour or a steel containment system, and the cpc on its own may be worse than the reading suggests. A measured Zs well above Ze + (R1+R2) means a joint that passed the low-current continuity test but opens up under the loop test current, which is the classic signature of a loose cpc terminal or a corroded junction box. A good instrument makes the arithmetic easy: the Megger MFT-X1, for instance, resolves loop impedance to 0.001 Ω and runs a three-wire non-trip loop test in around seven seconds on a quiet circuit (test-meter.co.uk listing, checked 17 October 2026), which is quick enough to take a live reading at every circuit end rather than relying on calculation alone.

How do you code a high Zs on an EICR?

Electrical Safety First's Best Practice Guide 4 (Issue 7.4, August 2026) sets the principle: a C2 is for a deficiency that is potentially dangerous, and a circuit whose protective device cannot disconnect an earth fault in time is exactly that. Where there is no RCD providing fault protection and the measured Zs exceeds the device maximum, the usual code is C2. Where a 30 mA RCD covers the circuit, has passed its test under Regulation 643.8, and Table 41.5 is satisfied, the circuit has fault protection and most inspectors record the reading with a C3 or a note explaining which table applies, having first ruled out a high-resistance joint. A very high or open-circuit reading, meaning no effective earth on exposed metalwork, is C1 if that metalwork could become live in normal use and C2 otherwise. A reading over the 80 per cent figure but under the raw table value with no obvious cause is worth an observation and, if the correction method does not rescue it, a C3. Write the numbers into the observation: measured Zs, the limit used, Ze and R1+R2. The coding framework is in EICR Observation Codes Explained.

How Certio helps

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Written by the Certio team. Last checked 17 October 2026.