Maximum Zs Values BS 7671: Complete Table Guide

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Maximum Zs values are the highest earth fault loop impedance values at which a protective device (MCB, fuse, or RCBO) will still disconnect the supply within the required time in the event of an earth fault. In a TN system, Regulation 411.3.2.2 and Table 41.1 require 0.4 seconds for final circuits rated up to 63 A with socket-outlets or up to 32 A supplying only fixed connected equipment, and Regulation 411.3.2.3 permits 5 seconds for distribution circuits and for any final circuit outside that scope. Tables 41.2 to 41.4 are the TN tables and are calculated to ensure those times are achieved. TT systems are different — Table 41.1 requires 0.2 seconds and Regulation 411.3.2.4 permits 1 second for a distribution circuit — and on TT the disconnecting device is normally an RCD.

The calculation is straightforward: the protective device has a time-current characteristic that defines the minimum current at which it will trip within the required time. BS 7671 applies a voltage factor Cmin of 0.95 to the nominal 230 V supply to account for voltage tolerance (per Appendix 3), giving an effective voltage of 218.5 V. Using Ohm's law, the maximum Zs equals this effective voltage divided by the minimum trip current. For example, a Type B 32 A MCB trips magnetically at 5 times its rated current (160 A), giving a maximum Zs of 218.5/160 = 1.37 ohms.

If the actual earth fault loop impedance (Zs) exceeds the maximum permitted value, the fault current will be insufficient to trip the protective device within the required time. This means that in the event of an earth fault, metalwork could remain live at a dangerous voltage for longer than the permitted duration, creating a risk of lethal electric shock.

Type B MCBs are the most commonly used protective devices in domestic and light commercial installations. They trip magnetically at 3 to 5 times their rated current. The BS 7671 maximum Zs values are calculated using the worst case (5 times rated current) to ensure compliance even at the upper end of the device tolerance.

The rightmost column shows the corrected values (tabulated x 0.8) that your measured Zs at ambient temperature should not exceed. These are the values you compare your MFT readings against on site.

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Select the MCB type (B, C, or D) and rating in Elec-Mate, and the app instantly shows both the tabulated and corrected (x 0.8) maximum Zs values.

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Type C MCBs have a higher magnetic trip point than Type B — they trip at 5 to 10 times their rated current (compared to 3 to 5 times for Type B). This higher trip threshold makes them suitable for circuits with high inrush currents (motors, discharge lighting, transformers) but results in lower maximum Zs values because more fault current is needed to achieve rapid disconnection.

Notice how much lower the Type C values are compared to Type B at the same rating. A C32 has a corrected maximum of only 0.54 ohms, compared to 1.10 ohms for a B32. This is why Type C MCBs should only be used where the inrush current characteristics of the load genuinely require them — using Type C unnecessarily on a lighting or socket circuit significantly reduces the available Zs margin and may cause the circuit to fail.

BS 3036 rewirable fuses (also known as semi-enclosed fuses) are the older type of fuse that uses replaceable fuse wire. They are still found in many existing installations in the UK, particularly in properties that have not been rewired. The maximum Zs values for BS 3036 fuses are specified in BS 7671 Table 41.2.

These are the current Table 41.2 figures. If you are working from older notes you may recognise a different set — 10.35 Ω for a 5 A, 3.26 Ω for a 15 A and so on. Those are the pre-Cmin values and they are no longer correct: NOTE 1 to Table 41.2 records that the tabulated impedances were determined using a Cmin of 0.95, which tightens every figure in the table. Working to the old numbers will pass a circuit that the current edition fails.

BS 3036 fuses have less predictable operating characteristics than MCBs because the fuse wire can deteriorate over time, oxidise, or be replaced with the wrong gauge by someone who is not qualified. That is a good reason to treat a measured value close to the limit with suspicion, and the 0.8 temperature correction still applies when testing at ambient.

Free to use, no sign-up and no email needed. Choose the device — MCB or RCBO on a B, C or D curve, BS 3036 or BS 88 fuse, or an RCD — and the lookup returns the tabulated maximum Zs from Tables 41.2 to 41.5 alongside the 0.8-corrected figure you compare your ambient reading against. Switch the disconnection time between 0.4 s and 5 s, or type a designation such as B32 or C20 straight into the quick device box.

Working the other way round, the compliance mode takes a measured Zs and lists every protective device that reading would satisfy — useful when you are deciding whether a circuit needs a smaller rating or a different curve.

NOTE 2 to Tables 41.2 to 41.4 sets out the temperature basis, and it is worth reading carefully because it is not simply “everything at 70 degrees”. The tabulated values should not be exceeded when the line conductors are at the maximum permitted operating temperature given in Table 52.2 — 70 degrees Celsius for 70 °C thermoplastic (PVC), which covers the vast majority of domestic installations — and the circuit protective conductors are at the assumed initial temperature given in Tables 54.2 to 54.5. When you measure Zs on site with your multifunction tester, the conductors are at ambient temperature — typically between 10 and 25 degrees Celsius — and NOTE 2 directs you to Appendix 3 to adjust the reading.

As cables carry current during normal operation, they heat up. Copper conductor resistance increases by approximately 0.4% per degree Celsius. Between ambient temperature (20 degrees Celsius) and maximum operating temperature (70 degrees Celsius), this is an increase of approximately 20%. The impedance of the earth fault loop therefore increases by the same proportion when the cables are at their operating temperature.

The 0.8 factor is not something the trade invented — it is published in BS 7671 itself. Appendix 3 states that where impedance measurements are made at ambient temperature, the requirements of Regulation 411.4.4 or 411.5.4 are considered to be met when Zs(measured) ≤ 0.8 × U0 × Cmin / Ia, and defines 0.8 as “a factor to take into account the increase of resistance of the conductors with the increase of temperature due to load current”. Appendix 3 also notes that this is one method of correcting for temperature difference and that other methods are not precluded. Elec-Mate applies the 0.8 correction automatically when validating Zs measurements.

Using the maximum Zs tables is straightforward once you understand the process. For each circuit in the installation, follow these steps:

You should also verify that the measured Zs is consistent with the calculated value of Ze + (R1+R2). If the measured Zs is significantly higher than the calculated value, there may be a high-resistance connection in the earth path that requires investigation.

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If the measured Zs exceeds the corrected maximum (tabulated x 0.8), the circuit does not comply and action must be taken. The specific action depends on the circumstances and the extent to which the maximum is exceeded.

On an EICR, a Zs value exceeding the maximum permitted value is recorded as an observation. The classification depends on the severity: C2 (potentially dangerous) if no RCD protection is present, or C3 (improvement recommended) if RCD protection is providing adequate disconnection despite the high Zs.

BS 7671 provides maximum Zs values for 5-second disconnection. Under Regulation 411.3.2.3 that applies in a TN system to distribution circuits and to any circuit not covered by Regulation 411.3.2.2 — so it also picks up large final circuits above 63 A with socket-outlets or above 32 A supplying only fixed equipment. But there is a catch that trips up a lot of people, and it is worth being precise about.

The practical upshot: if you are looking for a more lenient 5-second figure for a Type B or Type C MCB, there isn't one, and any table offering you a higher number is wrong. In a typical domestic installation it is mainly the circuit feeding a sub-distribution board that uses 5-second values, along with any final circuit large enough to fall outside Regulation 411.3.2.2. Everything supplying socket outlets and lighting uses 0.4 seconds. If in doubt, use the 0.4-second values — they are never wrong, only conservative.

Elec-Mate removes the need to carry BS 7671 tables on site. The app provides an instant Zs lookup calculator — select the protective device type (B, C, D MCB, BS 3036 fuse, BS 88 fuse) and rating, and the app shows both the tabulated maximum Zs and the corrected value (tabulated x 0.8) immediately.

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Schedule of tests with auto-Zs-validation

Enter your measured Zs values into the schedule of test results and Elec-Mate validates every reading against the BS 7671 maximum for the specific…

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The auto-validation works across all test values — not just Zs. Insulation resistance is checked against the 1 MΩ minimum of Table 64, RCD trip times are checked against the single AC test at IΔn — 300 ms maximum for a general non-delay RCD, 130 to 500 ms for a Type S — and R1+R2 values are cross-referenced with Zs. Voice-to-test-results lets you speak values while testing — no clipboards, no double-handling of data.

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Voice to test results — speak Zs values hands-free

On site with your MFT in hand? Just speak: 'Ring 1, Zs 0.89 ohms.' Elec-Mate fills in the schedule and validates automatically.

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