What is the maximum Zs for a 25A MCB Type C?

The maximum earth fault loop impedance (Zs) for a 25A MCB Type C is 0.87Ω 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 0.7Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 25A MCB Type C?

The testing limit is 0.7Ω. This is 80% of the tabulated maximum Zs (0.87 × 0.8 = 0.7Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 0.7Ω and 0.87Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

What fault current is needed to trip a 25A MCB Type C within 0.4 seconds?

A 25A MCB Type C requires a minimum fault current (Ia) of 250A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 10 × 25A = 250A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 0.87Ω.

Max Zs for 25A Type C MCB

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

Max Zs (0.4s)

0.87Ω

Testing Limit

0.7Ω

Max Zs (5s)

0.87Ω

Ia (0.4s)

250A

At a Glance

Device: MCB Type C — 25A
Standard: BS EN 60898
Max Zs (0.4s): 0.87Ω
Max Zs (5s): 0.87Ω
Testing Limit (80%): 0.7Ω
Ia (0.4s): 250A

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 25A Type C MCB

Recommended cable sizes for 25A Type C MCB circuits
Circuit	Cable Size	Notes
Commercial HVAC	4.0mm²	Larger air handling units

Worked Example: Maximum Cable Run for 25A MCB Type C

Maximum Zs (tabulated) = 0.87Ω (BS 7671 Table 41.3)
Testing limit (80%) = 0.87 × 0.8 = 0.7Ω
Assume Ze = 0.35Ω (typical TN-C-S (PME) supply)
Maximum R1+R2 = 0.7 − 0.35 = 0.35Ω
Using 2.5mm² T&E: R1+R2 = 14.82 mΩ/m
Maximum cable run = 0.35 ÷ 0.01482 = 23.6m

On a TN-C-S (PME) supply with 2.5mm² cable, the maximum one-way cable run is approximately 23.6m 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 25A, the max Zs values are the same for both disconnection times: 0.87Ω.

25A Across Device Types

Device Type	Max Zs (0.4s)	Testing Limit
MCB Type B	1.75Ω	1.4Ω
MCB Type C	0.87Ω	0.7Ω
MCB Type D	0.44Ω	0.35Ω
BS 88 Fuse	1.37Ω	1.1Ω