What is the maximum Zs for a 25A MCB Type D?

The maximum earth fault loop impedance (Zs) for a 25A MCB Type D is 0.44Ω 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 0.35Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 25A MCB Type D?

The testing limit is 0.35Ω. This is 80% of the tabulated maximum Zs (0.44 × 0.8 = 0.35Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 0.35Ω and 0.44Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

What fault current is needed to trip a 25A MCB Type D within 0.4 seconds?

A 25A MCB Type D requires a minimum fault current (Ia) of 500A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 20 × 25A = 500A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 0.44Ω.

Max Zs for 25A Type D MCB

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

Max Zs (0.4s)

0.44Ω

Testing Limit

0.35Ω

Max Zs (5s)

0.44Ω

Ia (0.4s)

500A

At a Glance

Device: MCB Type D — 25A
Standard: BS EN 60898
Max Zs (0.4s): 0.44Ω
Max Zs (5s): 0.44Ω
Testing Limit (80%): 0.35Ω
Ia (0.4s): 500A

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 25A MCB Type D

Maximum Zs (tabulated) = 0.44Ω (BS 7671 Table 41.3)
Testing limit (80%) = 0.44 × 0.8 = 0.35Ω
Assume Ze = 0.35Ω (typical TN-C-S (PME) supply)
Maximum R1+R2 = 0.35 − 0.35 = —Ω
Using 2.5mm² T&E: R1+R2 = 14.82 mΩ/m
The available R1+R2 margin is insufficient for standard cable — verify the earthing arrangement.

The tight Zs limit for this device means cable runs must be very short. Verify the supply earthing type and consider whether a lower-rated or different device type is more appropriate.

0.4s vs 5s Disconnection — Significant Difference for Type D

At 25A, the max Zs values for Type D are 0.44Ω for both disconnection times.

25A Across Device Types

Device Type	Max Zs (0.4s)	Testing Limit
MCB Type B	1.75Ω	1.4Ω
MCB Type C	0.87Ω	0.7Ω
MCB Type D	0.44Ω	0.35Ω
BS 88 Fuse	1.37Ω	1.1Ω