What is the maximum Zs for a 6A MCB Type D?

The maximum earth fault loop impedance (Zs) for a 6A MCB Type D is 1.82Ω 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 1.46Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 6A MCB Type D?

The testing limit is 1.46Ω. This is 80% of the tabulated maximum Zs (1.82 × 0.8 = 1.46Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 1.46Ω and 1.82Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

What fault current is needed to trip a 6A MCB Type D within 0.4 seconds?

A 6A MCB Type D requires a minimum fault current (Ia) of 120A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 20 × 6A = 120A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 1.82Ω.

Max Zs for 6A Type D MCB

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

Max Zs (0.4s)

1.82Ω

Testing Limit

1.46Ω

Max Zs (5s)

3.64Ω

Ia (0.4s)

120A

At a Glance

Device: MCB Type D — 6A
Standard: BS EN 60898
Max Zs (0.4s): 1.82Ω
Max Zs (5s): 3.64Ω
Testing Limit (80%): 1.46Ω
Ia (0.4s): 120A

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.

Worked Example: Maximum Cable Run for 6A MCB Type D

Maximum Zs (tabulated) = 1.82Ω (BS 7671 Table 41.3)
Testing limit (80%) = 1.82 × 0.8 = 1.46Ω
Assume Ze = 0.35Ω (typical TN-C-S (PME) supply)
Maximum R1+R2 = 1.46 − 0.35 = 1.11Ω
Using 1mm² T&E: R1+R2 = 36.2 mΩ/m
Maximum cable run = 1.11 ÷ 0.0362 = 30.7m

On a TN-C-S (PME) supply with 1mm² cable, the maximum one-way cable run is approximately 30.7m 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 6A: max Zs = 1.82Ω (0.4s) vs 3.64Ω (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	1.82Ω	1.46Ω
5 seconds	3.64Ω	2.91Ω

6A Across Device Types

Device Type	Max Zs (0.4s)	Testing Limit
MCB Type B	7.28Ω	5.82Ω
MCB Type C	3.64Ω	2.91Ω
MCB Type D	1.82Ω	1.46Ω