What is the maximum Zs for a 125A BS 88 Fuse?

The maximum earth fault loop impedance (Zs) for a 125A BS 88 Fuse is 0.27Ω for 0.4s disconnection time, as listed in BS 7671 Table 41.4 of BS 7671:2018+A2:2022. For on-site testing, the measured value must not exceed 0.22Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 125A BS 88 Fuse?

The testing limit is 0.22Ω. This is 80% of the tabulated maximum Zs (0.27 × 0.8 = 0.22Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 0.22Ω and 0.27Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

Can I still use BS 88 Fuse fuses in new installations?

BS 88 fuses remain appropriate for new installations, particularly in distribution boards and industrial settings. They offer very high breaking capacity and are well-suited to protecting sub-mains and main incomers.

Max Zs for 125A BS 88 Fuse

BS 7671 Table 41.4 — BS 7671:2018+A2:2022

Max Zs (0.4s)

0.27Ω

Testing Limit

0.22Ω

Max Zs (5s)

0.54Ω

Ia (0.4s)

809A

At a Glance

Device: BS 88 Fuse — 125A
Standard: BS 88
Max Zs (0.4s): 0.27Ω
Max Zs (5s): 0.54Ω
Testing Limit (80%): 0.22Ω
Ia (0.4s): 809A

About BS 88 Fuse

BS 88 fuses (also called HRC or HBC fuses) are high breaking capacity industrial fuses. They protect distribution board sub-circuits and main incomers. Their time/current characteristics differ from MCBs — disconnection time depends on the precise fault current level, not a fixed multiplier.

Worked Example: Maximum Cable Run for 125A BS 88 Fuse

Maximum Zs (tabulated) = 0.27Ω (BS 7671 Table 41.4)
Testing limit (80%) = 0.27 × 0.8 = 0.22Ω
Assume Ze = 0.8Ω (typical TN-S supply)
Maximum R1+R2 = 0.22 − 0.8 = —Ω
Using 25mm² T&E: R1+R2 = 1.47 mΩ/m
The available R1+R2 margin is insufficient for standard cable — verify the earthing arrangement.

The tight Zs limit for this device means cable runs must be very short. Verify the supply earthing type and consider whether a lower-rated or different device type is more appropriate.

0.4s vs 5s Disconnection Times

Fuse characteristics differ from MCBs — disconnection time depends on the time/current curve. At 125A: max Zs = 0.27Ω (0.4s) vs 0.54Ω (5s). The 5s value is higher because a lower fault current will still melt the fuse element given more time. Use the 0.4s value for final circuits and 5s for distribution circuits only.

Disconnection Time	Max Zs	Testing Limit (80%)
0.4 seconds	0.27Ω	0.22Ω
5 seconds	0.54Ω	0.43Ω

125A Across Device Types

Device Type	Max Zs (0.4s)	Testing Limit
MCB Type B	0.35Ω	0.28Ω
MCB Type C	0.17Ω	0.14Ω
MCB Type D	0.09Ω	0.07Ω
BS 88 Fuse	0.27Ω	0.22Ω