Mains Electricity » Insulation, Double Insulation and Earthing

Why mains electricity needs safety features

The mains supply in a UK home is about 230 V. That is easily large enough to push a dangerous current through a person. If you touch something connected to the live wire while you are also in contact with the ground, your body becomes a path for the current. Charge flows through you to the earth. This is an electric shock, and it can stop the heart.

So a mains appliance is designed to keep you and the live parts apart, even when something goes wrong inside it. This lesson covers three ways of doing that: insulation, earthing and double insulation. Fuses and circuit breakers, which you met in The Heating Effect, Fuses and Circuit Breakers, work alongside them.

Key terms:

• Electric shock: a current passing through a person's body.
• Insulation: a covering of a material that does not conduct electricity, such as plastic, around the parts that carry a current.
• Neutral wire: the mains wire that completes the circuit by carrying the current back to the supply.
• Earth wire: a safety wire that joins the metal case of an appliance to the earth.
• Earthing: connecting the metal case of an appliance to the earth with an earth wire.
• Double insulation: two layers of insulation between the user and the live parts: the insulation on the wires and a plastic case.

Insulation

Metals such as copper are good conductors, which is why wires are made of them. Plastic is an insulator. It has a very high resistance, so almost no current can pass through it.

Every wire in a mains cable has its own plastic coating. All of the wires are then wrapped together inside a second, outer layer of plastic. Insulation does two jobs:

• It stops a person from touching the bare metal of a wire, so no current can pass through them.
• It stops the wires from touching each other, which would give the current a short path and make it very large.

The same idea is used wherever you put your hands. The case of a plug is made of hard plastic. So are sockets and light switches. You can hold a plug that is connected to the mains quite safely, because there is an insulator between your fingers and the metal inside.

Insulation only protects you while it is in good condition. A cable that is cut, worn or cracked may have bare wire showing. It must be replaced, not covered with tape.

Inside a three-pin plug

A UK mains cable can contain three wires. Each has a different job and its own colour of insulation, so that nobody connects them the wrong way round.

Wire	Colour of insulation	What it does
Live	Brown	Brings the current from the supply. It is at a high voltage, so it is the dangerous one.
Neutral	Blue	Completes the circuit by carrying the current back to the supply. Its voltage is close to 0 V.
Earth	Green and yellow stripes	A safety wire. It carries a current only when there is a fault.

You may be asked to describe or label a plug, so learn this picture of one with its cover off and the cable coming in at the bottom:

1. Three metal pins are fixed in the plastic case: one at the top and two lower down.
2. The earth wire goes to the top pin, which is the longest.
3. The neutral wire goes to the bottom left pin.
4. The live wire goes to the bottom right pin, through the fuse.
5. A cable grip clamps the outer plastic of the cable at the bottom, so the wires cannot be pulled off their pins.

A handy way to remember the two lower wires: blue goes bottom left and brown goes bottom right.

In normal use the current is in the live and neutral wires only. The earth wire just waits.

Earthing a metal case

Some appliances, such as a washing machine or a fridge, have a metal case. Metal conducts, and that creates a danger. Suppose a wire inside works loose and the live wire touches the case.

In the exam you need the full chain of events. Learn it in this order:

1. A fault makes the live wire touch the metal case.
2. The earth wire has a very low resistance, so a large current flows from the live wire, through the case and down the earth wire.
3. This current is larger than the fuse can carry, so the fuse melts (or the circuit breaker trips).
4. This breaks the circuit in the live wire, so the appliance is cut off from the supply.
5. The case is no longer live, so nobody can get an electric shock from it.

Notice that the earth wire and the fuse work as a team. The earth wire alone would not switch anything off. The fuse alone would not melt, because without an earth wire there is no large current. The fuse is in the live wire so that, when it melts, the appliance is disconnected from the live wire.

Double insulation

Many appliances, such as an electric drill or a vacuum cleaner, have a case made completely of plastic. Plastic does not conduct. Even if the live wire comes loose and touches the inside of the case, the outside of the case cannot become live.

These appliances are double insulated. There are two layers of insulation between you and the live parts: the plastic on the wires, and the plastic case. As there is no metal on the outside that could become live, no earth wire is needed. The cable has only two wires, live and neutral. The plug still has its fuse, to protect the cable from overheating.

A double insulated appliance is marked with a symbol: a small square drawn inside a larger square.

Safety feature	What it is	How it protects the user
Insulation	Plastic around wires, plugs and switches	Stops anyone touching the bare live metal
Earthing	A low-resistance wire from a metal case to the earth	A fault gives a large current that melts the fuse, so the case does not stay live
Double insulation	A plastic case as a second layer of insulation	The outside of the case can never become live

Exam-style question

A microwave oven has a metal case. It is connected to the mains supply by a three-pin plug that contains a fuse.

(a) State the name of the wire that is connected to the metal case. [1 mark]

(b) A fault makes the live wire touch the metal case. Explain how the earth wire and the fuse protect a person who then goes to touch the oven. [5 marks]

(c) A food mixer has a plastic case. Its cable has only two wires. Explain why the food mixer does not need an earth wire. [2 marks]