Cable calculation, steps 11 and 12: Zs and disconnection timeWill it disconnect in 0.4 seconds?

Step 11: Zs and the maximum Zs

In an earth fault, the current doesn't just go round your 27 m of cable. It goes all the way back to the star point. So we add the external part, Ze.

Zs = Ze + (R1 + R2)
   = 0.26 + 0.137
   = 0.397 Ω

Table 41.3: max tabulated Zs, 32 A Type B = 1.37 Ω
0.397 Ω ≤ 1.37 Ω ✓  → automatic disconnection of supply will happen

BS 7671 · Chapter 41 · Table 41.3 · max Zs for circuit-breakers (32 A Type B = 1.37 Ω) · p.75

Tabulated (BS 7671) vs measured (OSG / GN3)

• Why not the OSG's 1.09 Ω? Because this is design. You want to know the circuit disconnects when it's working, at its full operating temperature (70 °C). BS 7671's values are for that.
• The On-Site Guide Table B6 (and GN3) give maximum measured values. They're about 80% of the tabulated ones, for an inspector testing a cold circuit (around 20 °C). For example, a new circuit is tested on a mild April day. The 20% margin allows for the cable warming up in use.
• Compare them: 32 A Type B is 1.37 Ω in BS 7671 and 1.09 Ω in the OSG (Table B6). 6 A Type B is 7.28 Ω in BS 7671, and 80% of that is about 5.82 Ω. Check your copy.

On-Site Guide · Table B6 · max measured Zs (≈ 80% of Table 41.3)

Step 12: disconnection time (Table 41.1)

See Reg 411.3.2.2. Table 41.1 is for final circuits. Two kinds of circuit fall outside it and are allowed longer:

• distribution circuits (feeding another board, e.g. a garage, shed or another building), and
• final circuits above 32 A feeding only fixed equipment, or above 63 A with socket-outlets.

Those get 5 s on TN (or 1 s on TT). Our circuit is a 32 A final circuit with a socket, so we use Table 41.1.

• Choose the voltage band from U₀, the voltage to earth: over 120 V up to 230 V. That's the most commonly used column. Even on three-phase, U₀ is still 230 V, so don't jump to the 400 V column.
• AC, TN system → 0.4 s maximum.

BS 7671 · Reg 411.3.2.2 and Table 41.1 · maximum disconnection times · p.71 (table at the top of p.72)