Wiring a Plug
A plug has three wires. The Earth wire is green and yellow and connects to the top pin. It is the first wire that will go into the plug socket. It connects the device to the ground with low resistance wiring so if there is an electrical fault the charge will flow through that rather than the person plugging it in! It is green and yellow so that colour blind people can recognise it.
The Live wire is brown and connects to the right pin via the fuse. The live wire carries current to the device.
The Neutral wire is blue and connects to the left pin. It carries current away from the device and completes the circuit.
It is important not to have exposed wires even inside the plug! The wires are double insulated: outside the plug they have the white plastic outer insulation which is a poor conductor.
All the wires are held in place by the cable grip.
The fuse will blow if too much current flows through, stopping the device and preventing fires and electric shocks.
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If too much current flows through a circuit, the circuit can become hot and could start a fire. Fuses are designed to melt before that happens: if too much current flows they will melt and break the circuit. In order to use the device again, you have to replace the fuse.
Fuses have values rated in Amps- this is the upper limit of current it will allow through. You want to use a fuse that has a higher current rating than the device but not too high or it won't melt in time!
Fuses are irritating because you have to replace them each time they blow. A circuit breaker also stops the circuit if there is too much current going through it but is resetable.
An RCCB, residual current circuit breaker, works like this. Current flows through the two contacts and around the coil. If too much current flows around the coil, it will increase the strength of the electromagnet. This magnet will attract the contact and pull them apart, breaking the circuit and stopping the current from flowing. The contacts can be pushed back together using the reset button.
Earthing and Insulation
If a device is 'earthed' or 'grounded' it has a wire that connects it directly to the ground. This is a very low resistance wire that any charge that builds up on the device will flow through rather than giving someone a shock or a spark causing a fire.
Most electrical devices will have cases that are earthed, especially metal ones. These devices will have three core cables: live, neutral and earth wires. Devices that are double insulated, have two layers of insulation, do not need to be earthed as charge does not build up on them so there devices only have two core cables: live and neutral wires.