A plug connects a device to the mains electricity supply. The cable between the device and the three-pin plug contains three copper wires that are coated with plastic.
The insulation of the wires is colour-coded to ensure correct connection.
|Earth||Green with yellow stripe|
Each part of the plug has a function.
|Features of a plug||Function|
|Outer cable insulation||All three wires in the cable are bundled together and there is extra plastic insulation wrapped round them all for safety.|
|Cable grip||This holds the cable tightly in place so that the cable cannot be pulled out of the plug.|
|Live wire||Copper wire coated with brown plastic along which the current enters the device. This is the wire that is at 230 V and is connected to the fuse.|
|Fuse||A glass or ceramic canister containing a thin wire that melts if the current gets too high.|
|Neutral wire||Copper wire coated with blue plastic that also connects to the cable in the wall and completes the circuit.|
|Earth wire||Copper wire coated in striped plastic that provides a path for current to flow from the case of the device to the ground if there is a fault.|
Without the earth wire, if a fault occurs and the live wire becomes loose, there is a danger that it will touch the case. The next person who uses the appliance could get electrocuted.
The earth wire is therefore connected to the case and is attached to a metal plate or water pipe underground. As the wire is made of copper, the earth wire provides a low resistance path to the ground. In the event of a fault, the large live current passing through the case earth wire will follow this path to the ground instead of passing through a person, and will blow the fuse rendering the appliance safe.
Some appliances, such as vacuum cleaners and electric drills, do not have an earth wire. This is because they have plastic casings, or they have been designed so that the live wire cannot touch the casing. As a result, the casing cannot give an electric shock, even if the wires inside become loose.
These appliances have double insulation and carry this symbol:
A fuse provides a built-in fail-safe to the electrical circuit for a device. The fuse contains a thin wire that will melt if the current gets too high. There may be a fault in the appliance that causes too much current flow. There may be a fault in the wiring that could cause the casing of the device to become live. In both cases a large current will flow through the low-resistance earth wire.
Once the fuse has melted, the circuit is broken and no more current flows through the device. This means the circuit or the case of the device is no longer live and there is no more risk of electrocution.
Fuses in plugs are made in standard ratings. The most common are 3 A, 5 A and 13 A. The fuse should be rated at a slightly higher current than the device needs:
Cars also have fuses. An electrical fault in a car could start a fire, so all the circuits have to be protected by fuses.
To decide which size fuse to use, use the power rating of the appliance and the value of the mains electricity voltage, usually 230 V.
Then from power = voltage × current or \(P = V I\)
current = power ÷ voltage or \(I = P \div V\)
A kettle has a power of 2.2 kW. What size fuse should be used in the plug?
Power (P) = 2.2 kW or 2,200 W
\[I = P \div V\]
\[I = 2,200 \div 230 = 9.6 \: A\]
A 3 A or 5 A rated fuse would blow under normal circumstances, so a 13 A fuse must be used.