A metal-cased sound mixing desk is connected to the mains supply by a three-core cable. It has a 3 A fuse in the live wire.
At time 0 ms, damaged insulation allows the live conductor to touch the metal case. Sensors record the magnitudes of the currents in the three wires.
| Time relative to the fault in ms\mathrm{ms}ms | Current in live wire in A\mathrm{A}A | Current in neutral wire in A\mathrm{A}A | Current in earth wire in A\mathrm{A}A |
|---|---|---|---|
| -20 | 2.0 | 2.0 | 0.0 |
| +10 | 50 | 2.0 | 48 |
| +40 | 0.0 | 0.0 | 0.0 |
At +40 ms, the potential difference between the metal case and earth is 0 V.
Use the readings to explain why the fuse must be fitted in the live wire rather than the neutral wire.
81 exam-style questions on Edexcel GCSE Physics 10.4 Mains electricity, covering 10.4.1 Direct and alternating current, 10.4.2 Live, neutral and earth wires; safety, 10.4.3 Mains wire potential differences and dangers, and 10.4.4 Power ratings and energy use. Each one has a worked solution and a mark scheme showing where the marks go.