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)

7.28Ω

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): 7.28Ω
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

Type C MCBs require 10× In for 0.4s disconnection but only 5× In for 5s. At 6A: max Zs = 3.64Ω (0.4s) vs 7.28Ω (5s). For distribution circuits (5s allowed), you have nearly double the impedance budget. Final circuits serving socket outlets must still meet the 0.4s limit.

Disconnection Time	Max Zs	Testing Limit (80%)
0.4 seconds	3.64Ω	2.91Ω
5 seconds	7.28Ω	5.82Ω

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