What is the maximum Zs for a 50A MCB Type C?

The maximum earth fault loop impedance (Zs) for a 50A MCB Type C is 0.44Ω 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 0.35Ω (80% of the tabulated value) to account for conductor temperature rise.

What is the testing limit for a 50A MCB Type C?

The testing limit is 0.35Ω. This is 80% of the tabulated maximum Zs (0.44 × 0.8 = 0.35Ω). When you measure Zs on site at ambient temperature, your reading must be at or below this value. If it falls between 0.35Ω and 0.44Ω, you can perform a more precise temperature correction calculation per IET Guidance Note 3.

What fault current is needed to trip a 50A MCB Type C within 0.4 seconds?

A 50A MCB Type C requires a minimum fault current (Ia) of 500A to guarantee tripping within 0.4 seconds. This is derived from the magnetic trip characteristic: 10 × 50A = 500A. The circuit must be capable of delivering at least this current under fault conditions, which is why max Zs is limited to 0.44Ω.

Max Zs for 50A Type C MCB

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

Max Zs (0.4s)

0.44Ω

Testing Limit

0.35Ω

Max Zs (5s)

0.87Ω

Ia (0.4s)

500A

At a Glance

Device: MCB Type C — 50A
Standard: BS EN 60898
Max Zs (0.4s): 0.44Ω
Max Zs (5s): 0.87Ω
Testing Limit (80%): 0.35Ω
Ia (0.4s): 500A

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 50A Type C MCB

Recommended cable sizes for 50A Type C MCB circuits
Circuit	Cable Size	Notes
Large industrial motor	10mm²	Heavier workshop or factory plant

Worked Example: Maximum Cable Run for 50A MCB Type C

Maximum Zs (tabulated) = 0.44Ω (BS 7671 Table 41.3)
Testing limit (80%) = 0.44 × 0.8 = 0.35Ω
Assume Ze = 0.8Ω (typical TN-S supply)
Maximum R1+R2 = 0.35 − 0.8 = —Ω
Using 10mm² T&E: R1+R2 = 3.66 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 50A: max Zs = 0.44Ω (0.4s) vs 0.87Ω (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	0.44Ω	0.35Ω
5 seconds	0.87Ω	0.7Ω

50A Across Device Types

Device Type	Max Zs (0.4s)	Testing Limit
MCB Type B	0.87Ω	0.7Ω
MCB Type C	0.44Ω	0.35Ω
MCB Type D	0.22Ω	0.18Ω
BS 88 Fuse	0.68Ω	0.54Ω