A fuse is a deliberate weak link in the live wire

1. The fuse wire is deliberately thinner than the rest of the cable, so it has a higher resistance and heats up faster than anything else in the circuit.
2. If the current climbs past the rated value the fuse wire reaches its melting point, breaks, and cuts the current off.
3. Because the fuse sits in the live wire, breaking it disconnects the appliance from the live supply completely.
4. A blown fuse has to be replaced, and it must never be swapped for a higher-rated one or bridged with a piece of wire.
5. A fuse takes a moment to melt, so it protects the cable from a sustained overload rather than from a brief surge.

Choosing a fuse rating: work out the current, then round up

1. Find the current the appliance draws from $I=\frac{P}{V}$, using its power rating and $230V$ for the mains.
2. Choose the next standard fuse above that value, from the usual set of $3A$, $5A$ and $13A$.
3. A $700W$ television draws $700÷230=3.0A$, so it takes a $5A$ fuse.
4. A $3000W$ kettle draws $3000÷230\approx 13A$, so it needs a $13A$ fuse and a thicker flex.
5. A fuse rated too low blows every time the appliance is switched on, and one rated too high lets a dangerous current flow before it acts.

A trip switch does the same job faster, and resets

1. The current supplying the circuit passes through a coil, and a larger current produces a stronger magnetic field.
2. Above the set current the electromagnet becomes strong enough to pull a catch, and the switch snaps open.
3. It acts in a fraction of a second, far faster than any fuse wire can melt, so it protects people as well as cables.
4. It is reset by hand rather than replaced, so there is nothing to buy and nothing that can be fitted with the wrong rating.
5. The trip switches in a consumer unit are chosen the same way as fuses, matching the rating to the circuit each one protects.

Earthing and double insulation are two answers to the same problem

1. If a live wire works loose inside an appliance and touches a metal case, the case becomes live and anyone touching it gets a shock.
2. Earthing connects that metal case to the ground through the earth wire.
   1. A fault then sends a very large current straight to earth through the low-resistance earth wire.
   2. That surge immediately blows the fuse or trips the switch, cutting the supply off.
   3. The case is therefore never left sitting at a dangerous voltage waiting for someone to touch it.
3. Double insulation takes the other route: the outer case is made of a non-conducting plastic, or the working parts are sealed inside a second layer of insulation.
   1. A live wire touching the inside of a plastic case cannot make the outside live, so no earth wire is needed and the plug carries only two wires.
   2. Such an appliance carries a double-square symbol, and its fuse then protects the circuit and the cabling rather than the user.
4. A kettle with a metal body is earthed, while a plastic-cased hairdryer or a phone charger is usually double insulated.