What is the maximum Zs for a 16A MCB Type D?

The maximum earth fault loop impedance (Zs) for a 16A MCB Type D is 0.68Ω for 0.4s disconnection time, as listed in BS 7671 Table 41.3 of BS 7671:2018+A2:2022. For on-site testing, the measured value must not exceed 0.54Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 16A MCB Type D?

The testing limit is 0.54Ω. This is 80% of the tabulated maximum Zs (0.68 × 0.8 = 0.54Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 0.54Ω and 0.68Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

What fault current is needed to trip a 16A MCB Type D within 0.4 seconds?

A 16A MCB Type D requires a minimum fault current (Ia) of 320A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 20 × 16A = 320A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 0.68Ω.

Max Zs for 16A Type D MCB

BS 7671 Table 41.3 — BS 7671:2018+A2:2022

Max Zs (0.4s)

0.68Ω

Testing Limit

0.54Ω

Max Zs (5s)

1.37Ω

Ia (0.4s)

320A

At a Glance

Device: MCB Type D — 16A
Standard: BS EN 60898
Max Zs (0.4s): 0.68Ω
Max Zs (5s): 1.37Ω
Testing Limit (80%): 0.54Ω
Ia (0.4s): 320A

About MCB Type D

Type D MCBs trip magnetically at 10–20× In. They handle the highest inrush currents — transformers, welding equipment, and X-ray machines. The very high trip multiplier produces very low max Zs values, which severely limits cable run lengths. Type D devices are rarely suitable for final circuits with 0.4s disconnection requirements.

Typical Circuits for 16A Type D MCB

Recommended cable sizes for 16A Type D MCB circuits
Circuit	Cable Size	Notes
Transformer primary	2.5mm²	Control transformer with magnetising inrush

Worked Example: Maximum Cable Run for 16A MCB Type D

Maximum Zs (tabulated) = 0.68Ω (BS 7671 Table 41.3)
Testing limit (80%) = 0.68 × 0.8 = 0.54Ω
Assume Ze = 0.35Ω (typical TN-C-S (PME) supply)
Maximum R1+R2 = 0.54 − 0.35 = 0.19Ω
Using 2.5mm² T&E: R1+R2 = 14.82 mΩ/m
Maximum cable run = 0.19 ÷ 0.01482 = 12.8m

On a TN-C-S (PME) supply with 2.5mm² cable, the maximum one-way cable run is approximately 12.8m 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 — Significant Difference for Type D

Type D MCBs require 20× In for 0.4s but only 10× In for 5s. At 16A: max Zs = 0.68Ω (0.4s) vs 1.37Ω (5s). The 5s value is exactly double. This is why Type D MCBs are mainly used on distribution circuits where 5s disconnection is acceptable.

Disconnection Time	Max Zs	Testing Limit (80%)
0.4 seconds	0.68Ω	0.54Ω
5 seconds	1.37Ω	1.1Ω

16A Across Device Types

Device Type	Max Zs (0.4s)	Testing Limit
MCB Type B	2.73Ω	2.18Ω
MCB Type C	1.37Ω	1.1Ω
MCB Type D	0.68Ω	0.54Ω
BS 88 Fuse	2.42Ω	1.94Ω