What is the maximum Zs for a 10A MCB Type C?

The maximum earth fault loop impedance (Zs) for a 10A MCB Type C is 2.19Ω 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 1.75Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 10A MCB Type C?

The testing limit is 1.75Ω. This is 80% of the tabulated maximum Zs (2.19 × 0.8 = 1.75Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 1.75Ω and 2.19Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

What fault current is needed to trip a 10A MCB Type C within 0.4 seconds?

A 10A MCB Type C requires a minimum fault current (Ia) of 100A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 10 × 10A = 100A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 2.19Ω.

Max Zs for 10A Type C MCB

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

Max Zs (0.4s)

2.19Ω

Testing Limit

1.75Ω

Max Zs (5s)

2.19Ω

Ia (0.4s)

100A

At a Glance

Device: MCB Type C — 10A
Standard: BS EN 60898
Max Zs (0.4s): 2.19Ω
Max Zs (5s): 2.19Ω
Testing Limit (80%): 1.75Ω
Ia (0.4s): 100A

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 10A Type C MCB

Recommended cable sizes for 10A Type C MCB circuits
Circuit	Cable Size	Notes
Fluorescent lighting	1.5mm²	Discharge lighting with high inrush

Worked Example: Maximum Cable Run for 10A MCB Type C

Maximum Zs (tabulated) = 2.19Ω (BS 7671 Table 41.3)
Testing limit (80%) = 2.19 × 0.8 = 1.75Ω
Assume Ze = 0.35Ω (typical TN-C-S (PME) supply)
Maximum R1+R2 = 1.75 − 0.35 = 1.4Ω
Using 1.5mm² T&E: R1+R2 = 24.2 mΩ/m
Maximum cable run = 1.4 ÷ 0.0242 = 57.9m

On a TN-C-S (PME) supply with 1.5mm² cable, the maximum one-way cable run is approximately 57.9m 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 10A, the max Zs values are the same for both disconnection times: 2.19Ω.

10A Across Device Types

Device Type	Max Zs (0.4s)	Testing Limit
MCB Type B	4.37Ω	3.5Ω
MCB Type C	2.19Ω	1.75Ω
MCB Type D	1.09Ω	0.87Ω
BS 88 Fuse	4.85Ω	3.88Ω