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    Max Zs tables

    Ω
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    MCB Type B — maximum Zs (Ω)
    Rating Max Zs at 0°C Max Zs (measured)
    Source: BS 7671:2018+A2:2022 (18th Edition Amendment 2). Table 41.3 (MCBs), Table 41.4 (fuses). Values apply to 230V single-phase TN systems. "Max Zs at 0°C" is the table value; "Max Zs (measured)" is the 0.8× corrected value for site measurements at ambient temperature.
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    The earth fault loop impedance (Zs) of a circuit determines how quickly the protective device disconnects in the event of a fault. BS 7671 requires that Zs does not exceed the maximum value for the device — if it does, the device may not trip quickly enough to prevent injury or fire.

    The table values are taken from BS 7671 at 0°C conductor temperature. When measuring on site at ambient temperature, conductors are warmer and therefore have higher resistance — the measured Zs must not exceed 80% of the table value to allow for this. This is the "Max Zs (measured)" column.

    Type B MCBs trip at 3–5× rated current and are standard for most domestic and light commercial circuits. Type C devices trip at 5–10× rated current and suit loads with higher starting currents. Type D devices trip at 10–20× rated current and are reserved for specialist equipment. Fuse characteristics differ — BS 88 industrial fuses are faster than BS 3036 rewirable fuses, which is reflected in their lower maximum Zs values.