The job was a simple garage conversion. That’s what Tom told his wife, anyway. But for an electrician, “simple” is a trap.
Adiabatic equation. The one that stops you dying. [ S = \frac{\sqrt{I^2 \times t}}{k} ] He measured the earth fault loop impedance (Zs) at the board: 0.35Ω. A 48A load meant a 230A fault current. The 32A Type B MCB would trip in 0.1 seconds. Copper k factor = 115. [ S = \frac{\sqrt{230^2 \times 0.1}}{115} = \frac{72.7}{115} = 0.63\text{mm}^2 ] His 16mm² earth was massively overkill. But if he’d used a cheap 1.5mm? Zap. No second chances. cable calculations bs7671
He circled the final design: 16mm² twin and earth. 50A Type C RCBO. Earthing via TN-C-S, but only after verifying the DNO’s maximum Ze. The job was a simple garage conversion
Ashworth looked at the dense figures: Ib ≤ In ≤ Iz , the volt drop, the adiabatic. He signed the order. Adiabatic equation