What is the maximum Zs for a 40A MCB Type D?

The maximum earth fault loop impedance (Zs) for a 40A MCB Type D is 0.27Ω 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 0.22Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 40A MCB Type D?

The testing limit is 0.22Ω. This is 80% of the tabulated maximum Zs (0.27 × 0.8 = 0.22Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 0.22Ω and 0.27Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

What fault current is needed to trip a 40A MCB Type D within 0.4 seconds?

A 40A MCB Type D requires a minimum fault current (Ia) of 800A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 20 × 40A = 800A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 0.27Ω.

Max Zs for 40A Type D MCB

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

Max Zs (0.4s)

0.27Ω

Testing Limit

0.22Ω

Max Zs (5s)

0.55Ω

Ia (0.4s)

800A

At a Glance

Device: MCB Type D — 40A
Standard: BS EN 60898
Max Zs (0.4s): 0.27Ω
Max Zs (5s): 0.55Ω
Testing Limit (80%): 0.22Ω
Ia (0.4s): 800A

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 40A MCB Type D

Maximum Zs (tabulated) = 0.27Ω (BS 7671 Table 41.3)
Testing limit (80%) = 0.27 × 0.8 = 0.22Ω
Assume Ze = 0.8Ω (typical TN-S supply)
Maximum R1+R2 = 0.22 − 0.8 = —Ω
Using 6mm² T&E: R1+R2 = 6.16 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

Type D MCBs require 20× In for 0.4s but only 10× In for 5s. At 40A: max Zs = 0.27Ω (0.4s) vs 0.55Ω (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	0.27Ω	0.22Ω
5 seconds	0.55Ω	0.44Ω

40A Across Device Types

Device Type	Max Zs (0.4s)	Testing Limit
MCB Type B	1.09Ω	0.87Ω
MCB Type C	0.55Ω	0.44Ω
MCB Type D	0.27Ω	0.22Ω
BS 88 Fuse	0.87Ω	0.7Ω