The three-core cable

1. An appliance with a metal case is joined to the mains by a three-core cable, which holds three separately insulated copper cores inside one tough outer sheath.
2. The live wire is insulated in brown. It carries the alternating supply into the appliance, so it is the core that delivers the energy pathway from the mains to the appliance.
3. The neutral wire is insulated in blue. It completes the circuit by carrying the current back out of the appliance towards the supply, and it is joined to earth at the local substation.
4. Charge flows in along one of these two cores and out along the other, swapping over many times each second because the mains supply alternates.
5. The earth wire is insulated in green and yellow stripes. It is a safety wire, connecting the metal case of the appliance to the earth, and it carries no current at all while the appliance is working normally.
6. The striped colouring of the earth core is deliberately unlike the plain colours of the other two, so it cannot be mistaken for a core that carries current in normal use.
7. The cores are copper because copper has a low resistance, while the insulation round each core and the outer sheath are insulators that keep the cores apart and keep them out of reach of the user.

Wiring a plug

1. In a three-pin plug the earth pin is the longest and sits at the top, so it is the first connection made as the plug goes in and the last broken as it comes out.
2. With the earth pin uppermost and the open back of the plug facing the person wiring it, the live terminal is on the right and the neutral terminal is on the left.
3. The brown core goes to the live terminal, which is the terminal joined to the fuse, the blue core goes to the neutral terminal, and the green and yellow core goes to the earth pin.
4. The pins are brass because it conducts well and resists corrosion, while the casing and the pin sleeves are plastic because plastic is an electrical insulator.
5. The cable grip clamps the outer sheath of the cable rather than the individual cores, so that any pull on the cable is taken by the sheath and never reaches the terminals.
6. If the cores themselves took the strain, a tug on the cable could drag the live core out of its terminal and leave a bare live conductor loose inside the plug, where it could touch the case or the other cores.
7. The outer sheath must reach inside the cable grip, and each core is stripped back only far enough for its bare copper to sit fully inside the terminal, so that no bare metal is left exposed.
8. Each terminal screw is tightened firmly, because a loose connection has a higher resistance and so heats up when current passes through it.

How a fuse protects

1. A fuse is a short length of thin wire, usually sealed inside a ceramic cartridge, fitted in the live wire inside the plug.
2. The fuse wire is made thinner than the rest of the circuit so that it has the greatest resistance in the path and heats up the fastest, since the energy dissipated each second in a resistance rises with the square of the current through $P=I^{2}R$.
3. A common fault is the insulation on the live core wearing through inside the appliance, so that the bare live conductor comes into contact with the metal case.
4. From that moment the fault follows a fixed sequence.
   1. The metal case becomes live, because it is now joined directly to the live wire and sits at the same potential as it.
   2. The case is also joined to the earth by the earth wire, so a complete path now exists from the live wire, through the case, along the earth wire, and out to earth.
   3. That path is made of thick copper and has a very low resistance, so a very large current flows along it.
   4. The whole of this large current has to pass through the fuse, because the fuse is in the live wire ahead of the fault.
   5. The thin fuse wire heats up rapidly, reaches its melting point and melts, which is what is meant by the fuse blowing.
   6. A gap opens in the live wire, so the circuit is broken and the current stops.
   7. The appliance is now cut off from the live supply, so the case is no longer live and anyone who touches it is not given a shock.
5. Breaking the circuit also stops the large fault current from overheating the cable and the appliance, which removes a serious fire risk.
6. A blown fuse cannot be reset, so it has to be replaced, and the fault must be traced and repaired first or the replacement will blow as soon as the appliance is switched on.
7. The earth wire by itself would not make the appliance safe, because the fault current would simply keep flowing; it is the fuse melting that disconnects the supply.

Circuit breakers

1. A circuit breaker is an automatic switch that does the same job as a fuse, opening the circuit when the current through it rises above the value it is set for.
2. The current passes through an electromagnet inside the breaker, and a large current makes that electromagnet strong enough to pull a catch and force a pair of contacts apart.
3. Because nothing has to melt, a circuit breaker opens the circuit in a small fraction of a second, while a fuse wire must first heat all the way to its melting point, so the breaker cuts off the supply sooner and less energy is transferred during the fault.
4. A circuit breaker is reset by pressing a switch once the fault has been repaired, so no replacement part is needed, although it costs more than a cartridge fuse.
5. A residual current device works differently again, comparing the current in the live wire with the current in the neutral wire.
6. If current is leaking out of the circuit, for example through a person to earth, the two currents are no longer equal and the device trips, so it responds to a current far too small to melt a fuse and acts quickly enough to protect a person.

Double insulation

1. An appliance with double insulation has two separate layers of insulation between its live parts and anything a user can touch.
2. The first layer is the plastic insulation around the cores and the internal wiring, and the second is an outer casing made entirely of insulating plastic.
3. Because the outer case is an electrical insulator, no part of it can become live even if a live wire works loose inside, so there is nothing that needs connecting to earth.
4. A double-insulated appliance is therefore supplied by a two-core cable carrying only live and neutral, with no earth core, and it is marked with a symbol of one square inside another.
5. A hair dryer, a vacuum cleaner, a plastic-cased electric drill and a food mixer are usually double insulated.
6. An appliance with exposed metal that a user can touch, such as a metal toaster, an electric kettle, a cooker or a washing machine, must be earthed instead, because that metal could otherwise become live.

Switching the live wire

1. The fuse and the appliance's on and off switch are always fitted in the live wire, never in the neutral wire.
2. Opening a switch anywhere in the loop stops the current, so a switch in the neutral wire would still appear to turn the appliance off.
3. A switch in the neutral wire would nonetheless leave every internal part of the appliance connected to the live wire, so those parts would stay at the live potential and could give a shock to anyone who opened the appliance.
4. A switch in the live wire disconnects the appliance from the live side of the supply, which isolates the internal parts.
5. The same argument applies to the fuse. A fuse in the neutral wire would melt and stop the current, but the appliance would still be joined to the live wire, so a faulty case could remain live.
6. A fuse in the live wire cuts the appliance off from the supply as it melts, and that is what makes the case safe to touch afterwards.