What is the maximum Zs for a 20A MCB Type D?

The maximum earth fault loop impedance (Zs) for a 20A MCB Type D is 0.55Ω 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 0.44Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 20A MCB Type D?

The testing limit is 0.44Ω. This is 80% of the tabulated maximum Zs (0.55 × 0.8 = 0.44Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 0.44Ω and 0.55Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

What fault current is needed to trip a 20A MCB Type D within 0.4 seconds?

A 20A MCB Type D requires a minimum fault current (Ia) of 400A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 20 × 20A = 400A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 0.55Ω.

Max Zs for 20A Type D MCB

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

Max Zs (0.4s)

0.55Ω

Testing Limit

0.44Ω

Max Zs (5s)

1.09Ω

Ia (0.4s)

400A

At a Glance

Device: MCB Type D — 20A
Standard: BS EN 60898
Max Zs (0.4s): 0.55Ω
Max Zs (5s): 1.09Ω
Testing Limit (80%): 0.44Ω
Ia (0.4s): 400A

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 20A Type D MCB

Recommended cable sizes for 20A Type D MCB circuits
Circuit	Cable Size	Notes
Welding transformer	4.0mm²	Resistance welding equipment

Worked Example: Maximum Cable Run for 20A MCB Type D

Maximum Zs (tabulated) = 0.55Ω (BS 7671 Table 41.3)
Testing limit (80%) = 0.55 × 0.8 = 0.44Ω
Assume Ze = 0.35Ω (typical TN-C-S (PME) supply)
Maximum R1+R2 = 0.44 − 0.35 = 0.09Ω
Using 2.5mm² T&E: R1+R2 = 14.82 mΩ/m
Maximum cable run = 0.09 ÷ 0.01482 = 6.1m

On a TN-C-S (PME) supply with 2.5mm² cable, the maximum one-way cable run is approximately 6.1m 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 20A: max Zs = 0.55Ω (0.4s) vs 1.09Ω (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	0.55Ω	0.44Ω
5 seconds	1.09Ω	0.87Ω

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