What is the maximum Zs for a 125A MCB Type C?

The maximum earth fault loop impedance (Zs) for a 125A MCB Type C is 0.17Ω 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 0.14Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 125A MCB Type C?

The testing limit is 0.14Ω. This is 80% of the tabulated maximum Zs (0.17 × 0.8 = 0.14Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 0.14Ω and 0.17Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

What fault current is needed to trip a 125A MCB Type C within 0.4 seconds?

A 125A MCB Type C requires a minimum fault current (Ia) of 1250A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 10 × 125A = 1250A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 0.17Ω.

Max Zs for 125A Type C MCB

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

Max Zs (0.4s)

0.17Ω

Testing Limit

0.14Ω

Max Zs (5s)

0.17Ω

Ia (0.4s)

1250A

At a Glance

Device: MCB Type C — 125A
Standard: BS EN 60898
Max Zs (0.4s): 0.17Ω
Max Zs (5s): 0.17Ω
Testing Limit (80%): 0.14Ω
Ia (0.4s): 1250A

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.

Worked Example: Maximum Cable Run for 125A MCB Type C

Maximum Zs (tabulated) = 0.17Ω (BS 7671 Table 41.3)
Testing limit (80%) = 0.17 × 0.8 = 0.14Ω
Assume Ze = 0.8Ω (typical TN-S supply)
Maximum R1+R2 = 0.14 − 0.8 = —Ω
Using 25mm² T&E: R1+R2 = 1.47 mΩ/m
The available R1+R2 margin is insufficient for standard cable — verify the earthing arrangement.

The tight Zs limit for this device means cable runs must be very short. Verify the supply earthing type and consider whether a lower-rated or different device type is more appropriate.

0.4s vs 5s Disconnection — Different Values for Type C

At 125A, the max Zs values are the same for both disconnection times: 0.17Ω.

125A Across Device Types

Device Type	Max Zs (0.4s)	Testing Limit
MCB Type B	0.35Ω	0.28Ω
MCB Type C	0.17Ω	0.14Ω
MCB Type D	0.09Ω	0.07Ω
BS 88 Fuse	0.17Ω	0.14Ω