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)

1.75Ω

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): 1.75Ω
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

Type C MCBs require 10× In for 0.4s disconnection but only 5× In for 5s. At 25A: max Zs = 0.87Ω (0.4s) vs 1.75Ω (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	0.87Ω	0.7Ω
5 seconds	1.75Ω	1.4Ω

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	1.61Ω	1.29Ω