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

The maximum earth fault loop impedance (Zs) for a 20A MCB Type C is 1.09Ω 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 0.87Ω (80% of the tabulated value) to account for conductor temperature rise.

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

The testing limit is 0.87Ω. This is 80% of the tabulated maximum Zs (1.09 × 0.8 = 0.87Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 0.87Ω and 1.09Ω, 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 C within 0.4 seconds?

A 20A MCB Type C requires a minimum fault current (Ia) of 200A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 10 × 20A = 200A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 1.09Ω.

Max Zs for 20A Type C MCB

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

Max Zs (0.4s)

1.09Ω

Testing Limit

0.87Ω

Max Zs (5s)

2.19Ω

Ia (0.4s)

200A

At a Glance

Device: MCB Type C — 20A
Standard: BS EN 60898
Max Zs (0.4s): 1.09Ω
Max Zs (5s): 2.19Ω
Testing Limit (80%): 0.87Ω
Ia (0.4s): 200A

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 20A Type C MCB

Recommended cable sizes for 20A Type C MCB circuits
Circuit	Cable Size	Notes
Motor circuit (medium)	2.5–4.0mm²	Pump motors, compressors

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

Maximum Zs (tabulated) = 1.09Ω (BS 7671 Table 41.3)
Testing limit (80%) = 1.09 × 0.8 = 0.87Ω
Assume Ze = 0.35Ω (typical TN-C-S (PME) supply)
Maximum R1+R2 = 0.87 − 0.35 = 0.52Ω
Using 2.5mm² T&E: R1+R2 = 14.82 mΩ/m
Maximum cable run = 0.52 ÷ 0.01482 = 35.1m

On a TN-C-S (PME) supply with 2.5mm² cable, the maximum one-way cable run is approximately 35.1m 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 20A: max Zs = 1.09Ω (0.4s) vs 2.19Ω (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	1.09Ω	0.87Ω
5 seconds	2.19Ω	1.75Ω

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