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

The maximum earth fault loop impedance (Zs) for a 125A MCB Type B is 0.35Ω 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 0.28Ω (80% of the tabulated value) to account for conductor temperature rise.

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

The testing limit is 0.28Ω. This is 80% of the tabulated maximum Zs (0.35 × 0.8 = 0.28Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 0.28Ω and 0.35Ω, 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 B within 0.4 seconds?

A 125A MCB Type B requires a minimum fault current (Ia) of 625A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 5 × 125A = 625A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 0.35Ω.

Max Zs for 125A Type B MCB

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

Max Zs (0.4s)

0.35Ω

Testing Limit

0.28Ω

Max Zs (5s)

0.35Ω

Ia (0.4s)

625A

At a Glance

Device: MCB Type B — 125A
Standard: BS EN 60898
Max Zs (0.4s): 0.35Ω
Max Zs (5s): 0.35Ω
Testing Limit (80%): 0.28Ω
Ia (0.4s): 625A

About MCB Type B

Type B MCBs have a magnetic trip range of 3–5× rated current (In). They are the standard choice for domestic and light commercial circuits — lighting, socket outlets, and water heating. The relatively low trip multiplier means higher permissible Zs values compared to Type C or D, making them suitable for circuits with longer cable runs.

Typical Circuits for 125A Type B MCB

Recommended cable sizes for 125A Type B MCB circuits
Circuit	Cable Size	Notes
Main incomer	35mm² SWA or larger	Large domestic or small commercial supply

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

Maximum Zs (tabulated) = 0.35Ω (BS 7671 Table 41.3)
Testing limit (80%) = 0.35 × 0.8 = 0.28Ω
Assume Ze = 0.8Ω (typical TN-S supply)
Maximum R1+R2 = 0.28 − 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 and 5s Disconnection — Same Values for Type B

For Type B MCBs, the magnetic trip operates at 3–5× In regardless of whether the required time is 0.4s or 5s. The max Zs is 0.35Ω for both disconnection times at 125A. This means the same limit applies whether protecting a final circuit (0.4s) or a distribution circuit (5s).

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