An earthed appliance has a fuse incorrectly fitted in its neutral wire. A live wire touches the case on the supply side of the load. A large fault current flows through the case and earth wire.
Why may the appliance fuse fail to interrupt this fault current?
The fuse has zero resistance because the neutral wire is near earth potential.
The earth wire carries the fault current only after the neutral fuse has melted.
The fault changes the alternating supply into a direct supply across the fuse.
The fault-current path returns through earth without passing through the fuse.
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.