Challenge: Determine If This Circuit Breaker Provides Fault Protection Within 0.4s

The question is simple: under an earth fault condition at the far end of this circuit, will the fault current be high enough to trip the MCB within 0.4 seconds?

Per BS 7671:2018, Regulation 411.3.2.2, the maximum disconnection time for a final circuit not exceeding 32 A in a TN system is 0.4 seconds.

Step 1: Calculate the circuit conductor resistances (R₁ and R₂).

From BS 7671, Table 4D1B, Column 3 (resistance at conductor operating temperature for thermoplastic insulation, 70°C):

• 6 mm² copper: R₁/m = 3.70 mΩ/m
• 2.5 mm² copper: R₂/m = 9.22 mΩ/m

For the 25 m cable run:

R1 = 3.70 × 25 / 1000 = 0.0925 Ω

R2 = 9.22 × 25 / 1000 = 0.2305 Ω

Step 2: Calculate total earth fault loop impedance (Z_s).

Zs = Ze + R1 + R2

Zs = 0.35 + 0.0925 + 0.2305

Zs = 0.673 Ω

Step 3: Calculate prospective earth fault current (I_f).

If = Uo / Zs

If = 230 / 0.673

If = 341.8 A

Step 4: Check against the MCB requirements.

A Type C MCB has a magnetic (instantaneous) trip range of 5–10 times its rated current (I_n). For a 32 A Type C MCB:

• Minimum instantaneous trip current: 5 × 32 = 160 A
• Guaranteed instantaneous trip current: 10 × 32 = 320 A

Our fault current of 341.8 A exceeds 320 A, so the MCB will trip instantaneously (within approximately 10 ms). But wait — we should also check against the published maximum Z_s from BS 7671, Table 41.3.

From BS 7671:2018, Table 41.3, the maximum Z_s for a 32 A Type C MCB for 0.4-second disconnection is:

Z_s(max) = 0.73 Ω

Our calculated Z_s = 0.673 Ω.

0.673 Ω < 0.73 Ω — PASS

The circuit complies with the 0.4-second disconnection requirement. But look at the margin: only 0.057 Ω separating compliance from failure. That is an 8% margin.

In practice, this margin is tight for several reasons:

• Z_e varies. The supply authority’s declared Z_e is a maximum value, but actual Z_e at the installation can vary with network loading and switching. A Z_e of 0.40 Ω (within the supply authority’s declared range for TN-S) would push Z_s to 0.723 Ω — still passing but with only 1% margin.
• Temperature affects resistance. The Table 4D1B values are at 70°C operating temperature. If the conductors are hotter (overloaded circuit, high ambient), resistance increases and Z_s increases.
• Connections add impedance. Every junction box, terminal, and connector adds a small impedance. Over a 25 m run with multiple connection points, this can add 0.01–0.05 Ω.

A measured Z_s at commissioning that exceeds 0.73 Ω would fail the installation test. With a calculated margin this tight, there is a real risk of failing on test.

If Z_s exceeded 0.73 Ω — or if you want a more comfortable margin — four remediation options exist:

1. Increase the CPC size. Replacing the 2.5 mm² CPC with a 4 mm² CPC reduces R₂ from 0.2305 Ω to 0.1438 Ω (Table 4D1B: 5.75 mΩ/m × 25 m). New Z_s = 0.35 + 0.0925 + 0.1438 = 0.586 Ω. This provides a 20% margin and is the simplest fix if the conduit can accommodate the larger CPC.
2. Use a Type B MCB instead of Type C. A 32 A Type B MCB has a maximum Z_s of 1.15 Ω for 0.4-second disconnection (Table 41.3). Our Z_s of 0.673 Ω would have a 41% margin. However, Type B may nuisance-trip on motor starting currents in a commercial kitchen (compressor motors for refrigeration, dishwasher pumps). Assess the load profile before changing MCB type.
3. Shorten the cable run. Not always possible, but if the distribution board can be relocated closer to the load, R₁ and R₂ decrease proportionally. A 20 m run instead of 25 m gives Z_s = 0.608 Ω.
4. Add supplementary RCD protection. Per BS 7671, Regulation 411.4.9, where fault protection by overcurrent device alone cannot be achieved, a 30 mA RCD provides supplementary protection. The RCD will trip at earth fault currents as low as 30 mA within 40 ms, ensuring disconnection regardless of Z_s. Note: this is supplementary protection — the overcurrent device must still provide overload and short-circuit protection. The RCD addresses the earth fault disconnection time only.

Standards referenced: BS 7671:2018+A2, Regulation 411.3.2.2, Table 41.3, Table 4D1B. MCB standard: BS EN 60898-1.