What is the maximum Zs for a 20A MCB Type B?

The maximum earth fault loop impedance (Zs) for a 20A MCB Type B is 2.19Ω 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 1.75Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 20A MCB Type B?

The testing limit is 1.75Ω. This is 80% of the tabulated maximum Zs (2.19 × 0.8 = 1.75Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 1.75Ω and 2.19Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

What fault current is needed to trip a 20A MCB Type B within 0.4 seconds?

A 20A MCB Type B requires a minimum fault current (Ia) of 100A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 5 × 20A = 100A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 2.19Ω.

Max Zs for 20A Type B MCB

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

Max Zs (0.4s)

2.19Ω

Testing Limit

1.75Ω

Max Zs (5s)

2.19Ω

Ia (0.4s)

100A

At a Glance

Device: MCB Type B — 20A
Standard: BS EN 60898
Max Zs (0.4s): 2.19Ω
Max Zs (5s): 2.19Ω
Testing Limit (80%): 1.75Ω
Ia (0.4s): 100A

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 20A Type B MCB

Recommended cable sizes for 20A Type B MCB circuits
Circuit	Cable Size	Notes
Radial socket circuit	2.5–4.0mm² T&E	Larger floor area
Water heater	2.5mm² T&E	Up to 4.6kW

Worked Example: Maximum Cable Run for 20A MCB Type B

Maximum Zs (tabulated) = 2.19Ω (BS 7671 Table 41.3)
Testing limit (80%) = 2.19 × 0.8 = 1.75Ω
Assume Ze = 0.35Ω (typical TN-C-S (PME) supply)
Maximum R1+R2 = 1.75 − 0.35 = 1.4Ω
Using 2.5mm² T&E: R1+R2 = 14.82 mΩ/m
Maximum cable run = 1.4 ÷ 0.01482 = 94.5m

On a TN-C-S (PME) supply with 2.5mm² cable, the maximum one-way cable run is approximately 94.5m before Zs exceeds the testing limit. In practice, always measure — parallel earth paths and cable route conditions will affect the actual value.

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 2.19Ω for both disconnection times at 20A. This means the same limit applies whether protecting a final circuit (0.4s) or a distribution circuit (5s).

20A Across Device Types

Device Type	Max Zs (0.4s)	Testing Limit
MCB Type B	2.19Ω	1.75Ω
MCB Type C	1.09Ω	0.87Ω
MCB Type D	0.55Ω	0.44Ω
BS 88 Fuse	1.84Ω	1.47Ω
BS 1361 Fuse	2.42Ω	1.94Ω
BS 3036 Semi-enclosed Fuse	2.14Ω	1.71Ω