Earthing is the act of connecting the metal casing of the appliance to earth via a wired connection to the bare ground. Earthing wires are usually have a green and yellow bands around them. Why do you need earthing? Consider this scenario:
- The live wire is frayed and touched the metal casing of the appliance. (Another phrase for this is: The metal casing of the appliance becomes live.)
- The appliance do not have an earthing cable.
- YOU touched the metal casing of the appliance.
- Your body would have completed the circuit $\rightarrow$ electricity will pass through your body.
- The current is not high enough to trigger the breaking of fuse in the power plug.
- You get electrocuted.
If you have earthing (connecting the metal casing to the ground), the current will have two paths to take to complete the circuit:
- The low resistance earthing wire
- Your body (Note that your body have very high electrical resistance (orders of magnitude larger) when compared to the earthing wire)
From the theory for current flows in parallel circuits, the current through the low resistance earthing wire will be orders of magnitude higher than the current flowing through your body (which will be negligible). In effect, the situation can be summarised as “the current flowing through the low resistance earthing wire instead of your body”.
Hence, the earthing will divert the current into the earth by providing an alternate path to the large current flow via the earth wire, rather than through the user’s body.
There are some appliances which do NOT have an earth wire. They have another way to protect the user: double insulation. Double insulation protects the user of the appliance from an electrical shock by preventing any possibility of the external casing becoming live (the live wire can not touch the casing even if wires inside become loose), thus eliminating the need for an earth connection. The two layers of insulation are:
- First insulation: Insulating electrical cable from the internal component of the appliance.
- Second insulation: Insulating internal metal part which could become live from the external casing.
Note: If the external casing is plastic, there’s no way the external casing can become live.
You can identify which appliances have an earth wire by checking the mains plug. If it is a 3-pin plug (all three pins are made of metal), it would have an earthing wire.