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

The maximum earth fault loop impedance (Zs) for a 6A MCB Type C is 3.64Ω 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 2.91Ω (80% of the tabulated value) to account for conductor temperature rise.

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

The testing limit is 2.91Ω. This is 80% of the tabulated maximum Zs (3.64 × 0.8 = 2.91Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 2.91Ω and 3.64Ω, 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 C within 0.4 seconds?

A 6A MCB Type C requires a minimum fault current (Ia) of 60A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 10 × 6A = 60A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 3.64Ω.

Max Zs for 6A Type C MCB

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

Max Zs (0.4s)

3.64Ω

Testing Limit

2.91Ω

Max Zs (5s)

3.64Ω

Ia (0.4s)

60A

At a Glance

Device: MCB Type C — 6A
Standard: BS EN 60898
Max Zs (0.4s): 3.64Ω
Max Zs (5s): 3.64Ω
Testing Limit (80%): 2.91Ω
Ia (0.4s): 60A

About MCB Type C

Type C MCBs trip magnetically at 5–10× In. They are designed for circuits with moderate inrush current — motor-driven equipment, air conditioning, and discharge lighting. The higher trip multiplier means lower maximum Zs values, so circuit impedance must be kept tighter than with Type B devices.

Typical Circuits for 6A Type C MCB

Recommended cable sizes for 6A Type C MCB circuits
Circuit	Cable Size	Notes
Small motor circuit	1.0mm²	Fan motors, small pumps with high inrush

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

Maximum Zs (tabulated) = 3.64Ω (BS 7671 Table 41.3)
Testing limit (80%) = 3.64 × 0.8 = 2.91Ω
Assume Ze = 0.35Ω (typical TN-C-S (PME) supply)
Maximum R1+R2 = 2.91 − 0.35 = 2.56Ω
Using 1mm² T&E: R1+R2 = 36.2 mΩ/m
Maximum cable run = 2.56 ÷ 0.0362 = 70.7m

On a TN-C-S (PME) supply with 1mm² cable, the maximum one-way cable run is approximately 70.7m 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 — Different Values for Type C

At 6A, the max Zs values are the same for both disconnection times: 3.64Ω.

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