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

The maximum earth fault loop impedance (Zs) for a 6A MCB Type B is 7.28Ω 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 5.82Ω (80% of the tabulated value) to account for conductor temperature rise.

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

The testing limit is 5.82Ω. This is 80% of the tabulated maximum Zs (7.28 × 0.8 = 5.82Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 5.82Ω and 7.28Ω, 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 B within 0.4 seconds?

A 6A MCB Type B requires a minimum fault current (Ia) of 30A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 5 × 6A = 30A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 7.28Ω.

Max Zs for 6A Type B MCB

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

Max Zs (0.4s)

7.28Ω

Testing Limit

5.82Ω

Max Zs (5s)

7.28Ω

Ia (0.4s)

30A

At a Glance

Device: MCB Type B — 6A
Standard: BS EN 60898
Max Zs (0.4s): 7.28Ω
Max Zs (5s): 7.28Ω
Testing Limit (80%): 5.82Ω
Ia (0.4s): 30A

About MCB Type B

Type B MCBs have a magnetic trip range of 3–5× rated current (In). They are the standard choice for domestic and light commercial circuits — lighting, socket outlets, and water heating. The relatively low trip multiplier means higher permissible Zs values compared to Type C or D, making them suitable for circuits with longer cable runs.

Typical Circuits for 6A Type B MCB

Recommended cable sizes for 6A Type B MCB circuits
Circuit	Cable Size	Notes
Lighting circuit	1.0mm² T&E	Up to 12 points per circuit
Low-power LED lighting	1.0mm² T&E	Common in modern domestic installs

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

Maximum Zs (tabulated) = 7.28Ω (BS 7671 Table 41.3)
Testing limit (80%) = 7.28 × 0.8 = 5.82Ω
Assume Ze = 0.35Ω (typical TN-C-S (PME) supply)
Maximum R1+R2 = 5.82 − 0.35 = 5.47Ω
Using 1mm² T&E: R1+R2 = 36.2 mΩ/m
Maximum cable run = 5.47 ÷ 0.0362 = 151.1m

On a TN-C-S (PME) supply with 1mm² cable, the maximum one-way cable run is approximately 151.1m before Zs exceeds the testing limit. In practice, always measure — parallel earth paths and cable route conditions will affect the actual value.

0.4s and 5s Disconnection — Same Values for Type B

For Type B MCBs, the magnetic trip operates at 3–5× In regardless of whether the required time is 0.4s or 5s. The max Zs is 7.28Ω for both disconnection times at 6A. This means the same limit applies whether protecting a final circuit (0.4s) or a distribution circuit (5s).

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
BS 88 Fuse	8.09Ω	6.47Ω