Maximum Zs Values BS 7671: Complete Table Guide

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. BS 7671 specifies maximum disconnection times of 0.4 seconds for final circuits and 5 seconds for distribution circuits. The maximum Zs values in the BS 7671 tables are calculated to ensure these disconnection times are achieved.

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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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.

BS 3036 fuses have less predictable operating characteristics than MCBs because the fuse wire can deteriorate over time, oxidise, or even be replaced with the wrong rating by someone who is not qualified. The values in Table 41.2 already include a built-in correction factor of 0.725 to account for this variability. The 0.8 temperature correction should still be applied on top of these values when testing at ambient temperature.

The maximum Zs values in BS 7671 tables are specified at the maximum conductor operating temperature — 70 degrees Celsius for PVC-insulated cables (which covers the vast majority of domestic installations). When you measure Zs on site with your multifunction tester, the conductors are at ambient temperature — typically between 10 and 25 degrees Celsius.

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 a design guideline recommended by the IET rather than a regulation within BS 7671 itself. However, it is widely adopted, expected by verifying bodies, and considered best practice. 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 also provides maximum Zs values for 5-second disconnection times. These apply to distribution circuits — circuits that supply other distribution boards but do not directly supply current-using equipment or socket outlets. The 5-second values are higher (more lenient) because the risk of electric shock is lower on distribution circuits.

In a typical domestic installation, only the circuit feeding a sub-distribution board (if present) would use the 5-second values. All circuits supplying socket outlets, lighting, cookers, showers, and other current-using equipment must use the 0.4-second values. If in doubt, use the 0.4-second values — they are more conservative and always compliant.

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.

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, RCD trip times are checked against the 40 ms and 300 ms limits, 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.

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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