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)

0.35Ω

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): 0.35Ω
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

Type C MCBs require 10× In for 0.4s disconnection but only 5× In for 5s. At 125A: max Zs = 0.17Ω (0.4s) vs 0.35Ω (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	0.17Ω	0.14Ω
5 seconds	0.35Ω	0.28Ω

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	0.27Ω	0.22Ω