IGCSE Physics / Mains electricity
Fuses and earthing
Edexcel International GCSE Physics (4PH1), chapter 6: Mains electricity, specification point 2.2. Free lesson.
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The live, neutral and earth wires have different jobs in a mains plug. A fuse and an earth wire help prevent electric shocks by breaking the circuit if a fault causes a large current.
What the wires in a plug do
A plug has three wires. The live wire carries the high voltage from the mains to the appliance, while the neutral wire completes the circuit by carrying current back to the supply. The earth wire is a safety wire. Normally, no current flows through it. Live wire: brown. Neutral wire: blue. Earth wire: green and yellow. It is used for appliances with metal cases.
How the rest of the plug keeps you safe
The fuse is joined to the live pin, so it is in the live wire. The cable grip clamps the outer cable so the wires cannot be pulled out of their pins. The plastic case is an electrical insulator, so it is safe to hold. The cable grip holds the cable firmly in place. The plastic case does not conduct electricity.
How a fuse breaks the circuit
A fuse is a short, thin piece of wire inside a small case. It is connected in series with the appliance, on the live side. Current warms the fuse wire, but only a current higher than its rating, the current it is designed to carry, makes it hot enough to melt. Once it melts, the circuit is broken and current stops flowing. The fuse must be in the live wire, never in the neutral or earth wire. When it melts, it cuts the appliance off from the high voltage.
How earthing protects a metal case
If a loose live wire touches the metal case of a kettle, the case could become live. The earth wire provides a very low-resistance path to earth. This allows a large current to flow, which is far above the fuse's rating. The fuse melts and breaks the circuit, cutting off the live wire so the case is not left live. Live wire touches the metal case: a fault occurs. The earth wire connects the case to earth. A large current flows to earth through the low-resistance path. The current melts the fuse, breaking the circuit. The case is no longer live, reducing the risk of electric shock.
Why the fuse goes in the live wire
The live wire carries the high voltage. If the fuse melts in that wire, the appliance is disconnected from the high voltage. This means no part of it is left at a high voltage that could give someone a shock.