| Rating | Max Zs at 0°C | Max Zs (measured) |
|---|
BS 7671 sets a maximum earth fault loop impedance (Zs) for every protective device rating — exceed it and the device may not disconnect fast enough to prevent injury or fire. These tables give the limit for any MCB, RCBO or fuse type at a glance.
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. A lower Zs means a higher fault current flows, which trips the device faster.
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, and it's the figure to check a real test result against, not the raw table value.
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, such as motors and fluorescent lighting. Type D devices trip at 10–20× rated current and are reserved for specialist high-inrush equipment such as welders and X-ray machines. Fuse characteristics differ again — BS 88 industrial fuses disconnect faster than BS 3036 rewirable fuses, which is reflected in their lower maximum Zs values.
Maximum earth fault loop impedance values (Table 41.3 for MCBs/RCBOs, Table 41.4 for fuses) are sourced from BS 7671:2018+A2:2022 (18th Edition Amendment 2), the IET Wiring Regulations — the current UK standard for electrical installations. These tables are for reference only; always verify against the full regulations and confirm with a qualified electrician before certifying an installation.