What is the maximum Zs for a 15A BS 1361 Fuse?

The maximum earth fault loop impedance (Zs) for a 15A BS 1361 Fuse is 3.12Ω 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 2.5Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 15A BS 1361 Fuse?

The testing limit is 2.5Ω. This is 80% of the tabulated maximum Zs (3.12 × 0.8 = 2.5Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 2.5Ω and 3.12Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

Can I still use BS 1361 Fuse fuses in new installations?

BS 1361 cartridge fuses may be used in existing installations but are rarely specified in new work. Modern consumer units use MCBs to BS EN 60898, which offer more consistent tripping characteristics and resettability.

Max Zs for 15A BS 1361 Fuse

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

Max Zs (0.4s)

3.12Ω

Testing Limit

2.5Ω

Max Zs (5s)

6.2Ω

Ia (0.4s)

70A

At a Glance

Device: BS 1361 Fuse — 15A
Standard: BS 1361
Max Zs (0.4s): 3.12Ω
Max Zs (5s): 6.2Ω
Testing Limit (80%): 2.5Ω
Ia (0.4s): 70A

About BS 1361 Fuse

BS 1361 cartridge fuses were commonly used in domestic consumer units before MCBs became standard. They are still found in older installations and are acceptable for continued use. Their max Zs values are generally more generous than equivalent MCB ratings.

Typical Circuits for 15A BS 1361 Fuse

Recommended cable sizes for 15A BS 1361 Fuse circuits
Circuit	Cable Size	Notes
Immersion heater	2.5mm²	Dedicated water heating circuit

Worked Example: Maximum Cable Run for 15A BS 1361 Fuse

Maximum Zs (tabulated) = 3.12Ω (BS 7671 Table 41.4)
Testing limit (80%) = 3.12 × 0.8 = 2.5Ω
Assume Ze = 0.35Ω (typical TN-C-S (PME) supply)
Maximum R1+R2 = 2.5 − 0.35 = 2.15Ω
Using 1.5mm² T&E: R1+R2 = 24.2 mΩ/m
Maximum cable run = 2.15 ÷ 0.0242 = 88.8m

On a TN-C-S (PME) supply with 1.5mm² cable, the maximum one-way cable run is approximately 88.8m before Zs exceeds the testing limit. In practice, always measure — parallel earth paths and cable route conditions will affect the actual value.

0.4s vs 5s Disconnection Times

Fuse characteristics differ from MCBs — disconnection time depends on the time/current curve. At 15A: max Zs = 3.12Ω (0.4s) vs 6.2Ω (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	3.12Ω	2.5Ω
5 seconds	6.2Ω	4.96Ω

15A Across Device Types

Device Type	Max Zs (0.4s)	Testing Limit
BS 1361 Fuse	3.12Ω	2.5Ω
BS 3036 Semi-enclosed Fuse	2.76Ω	2.21Ω