What is the maximum Zs for a 20A BS 3036 Semi-enclosed Fuse?

The maximum earth fault loop impedance (Zs) for a 20A BS 3036 Semi-enclosed Fuse is 2.14Ω 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 1.71Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 20A BS 3036 Semi-enclosed Fuse?

The testing limit is 1.71Ω. This is 80% of the tabulated maximum Zs (2.14 × 0.8 = 1.71Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 1.71Ω and 2.14Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

Can I still use BS 3036 Semi-enclosed Fuse fuses in new installations?

BS 3036 rewireable fuses are not recommended for new installations. If retained, apply the 0.725 cable sizing correction factor (Regulation 433.1.1). The less precise operating characteristics mean less reliable disconnection times compared to MCBs or HRC fuses.

Max Zs for 20A BS 3036 Fuse

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

Max Zs (0.4s)

2.14Ω

Testing Limit

1.71Ω

Max Zs (5s)

4.14Ω

Ia (0.4s)

102A

At a Glance

Device: BS 3036 Semi-enclosed Fuse — 20A
Standard: BS 3036
Max Zs (0.4s): 2.14Ω
Max Zs (5s): 4.14Ω
Testing Limit (80%): 1.71Ω
Ia (0.4s): 102A

About BS 3036 Semi-enclosed Fuse

BS 3036 semi-enclosed (rewireable) fuses are the oldest type still encountered. They require a 0.725 correction factor when sizing cables (Regulation 433.1.1) due to their less precise operating characteristics. New installations should use MCBs or BS 88 fuses instead.

Typical Circuits for 20A BS 3036 Fuse

Recommended cable sizes for 20A BS 3036 Fuse circuits
Circuit	Cable Size	Notes
Socket radial	2.5mm²	Older domestic installation

Worked Example: Maximum Cable Run for 20A BS 3036 Semi-enclosed Fuse

Maximum Zs (tabulated) = 2.14Ω (BS 7671 Table 41.4)
Testing limit (80%) = 2.14 × 0.8 = 1.71Ω
Assume Ze = 0.35Ω (typical TN-C-S (PME) supply)
Maximum R1+R2 = 1.71 − 0.35 = 1.36Ω
Using 2.5mm² T&E: R1+R2 = 14.82 mΩ/m
Maximum cable run = 1.36 ÷ 0.01482 = 91.8m

On a TN-C-S (PME) supply with 2.5mm² cable, the maximum one-way cable run is approximately 91.8m 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 20A: max Zs = 2.14Ω (0.4s) vs 4.14Ω (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	2.14Ω	1.71Ω
5 seconds	4.14Ω	3.31Ω

20A Across Device Types

Device Type	Max Zs (0.4s)	Testing Limit
MCB Type B	2.19Ω	1.75Ω
MCB Type C	1.09Ω	0.87Ω
MCB Type D	0.55Ω	0.44Ω
BS 88 Fuse	1.84Ω	1.47Ω
BS 1361 Fuse	2.42Ω	1.94Ω
BS 3036 Semi-enclosed Fuse	2.14Ω	1.71Ω