Earthing

If the outer casing of an appliance is made of metal, then it can be made safe by 'earthing'. The live and neutral wires from the plug are connected to the cicuitry of the appliance (in the diagram below - that is the motor circuit) and the earth wire from the plug is fixed to the metal case.

When functioning properly the earth wire does nothing. It is at 0V and if someone touches the case they will therefore not get a shock. The live and neutral wires are connected to the motor - they therefore come into play when the machine is switched on.

If something goes wrong inside the appliance and the live wire touches the metal case, then the earth wire acts like a neutral wire and completes the circuit for the electricity. A very large current suddenly flows because the metal case has little resistance - the current has a path of low resistance to earth. This large current blows the fuse in the plug and disconnects the appliance from the power supply.

Let us consider what would happen if the appliance was not earthed.

When functioning properly the live and neutral wires are connected to the motor - they therefore come into play when the machine is switched on. It would function just like the earthed machine.

If the live wire comes loose the motor will no longer have a p.d. across it and will not operate - it will switch off. If it happens to touch the metal casing the case becomes 'live'. Therefore if someone touches the machine (perhaps to check why it had 'switched off') s/he will have one hand varying at mains voltage and her/his feet will be at 0V. S/he will therefore get a 'shock'.

This will only happen if the live wire touches the case. If the neutral wire touched the case instead then the case would not be 'live' but at 0V - anyone touching it would therefore not get electrocuted - but the diconnection of the neutral wire from the motor would stop the appliance operating.

Earthing a metal cased appliance is therefore a safety measure.

On the continent earthing is not common. Most countries in Europe have two pin plugs (no third 'earthing' pin - so therefore no earthing wire). The way that they ensure metal cased appliances are safe is to double insulate them. The appliance is designed in such a way that the electrical parts should never come into contact with the outer casing of the device.

In the UK some appliances are double insulated too. They often have plastic cases. Common double insulated appliances are hair dryers, radios and cassette players.

A wet double insulated appliance is exceptionally dangerous - water is a good conductor of electricity and will easily reach the live electrical components within the case. Any human user touching the casing will then receive an electric shock. An additional problem is that the resistance of the human body is greatly redulced when it is wet. Therefore when your hands are wet and you touch an electric source a greater current will flow through you.

For this reason, do not operate a mains radio, hairdryer or double insulated appliance in any wet or steamy area - especially a bathroom.