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Wiring a Mains Plug - Diagram
Most of the appliances in your house that run off mains electricity will have a mains plug. In this flashcard we have a mains plug which has had its back taken off so we can see how it is wired inside.
There are some parts here that you need to be able to identify.
1. The Live Wire
The live wire carries 230 volts of electrical voltage and it's the source of the electrical energy for the appliance. Think of the live wire as the positive terminal of a power supply.
2. The Neutral Wire
In order to make a complete circuit, we need to have a return path from the live wire. That return path is made via the neutral wire which is similar to the negative terminal of a power supply.
3. The Earth Wire
All appliances with a metal casing should have an earth wire. The earth wire prevents the risk of electric shock that can be caused by the live wire touching the metal casing, perhaps due to it becoming frayed or dislodged.
4. The Fuse
The fuse will melt if the current becomes too high, which can be caused by an electrical fault. A melted fuse will cut off the live supply and so make the appliance safe.
5. The Outer Insulation
In a mains cable there are two layers of insulation - the inner insulation made of flexible plastic for each of the wires (brown for live, blue for neutral and yellow/green for the earth), and then around those the outer insulation (usually white or black plastic)
6. The Cable Grip
This grips the outer insulation and prevents the wires from being pulled and dislodged from their connections inside the mains plug.
Types of Mains Cable
There are two types of mains cable: three-core and two-core cable. Three-core cable has an earth wire (green/yellow) as well as the live and neutral wires. The earth wire is needed for appliances which have a metal casing.
The two-core cable only has the live and the neutral wires but no earth wire. This is for appliances that do not have a metal casing, which are called 'double insulated' appliances - usually made of plastic.
Common mains plug wiring faults
Some common faults when wiring a plug include:
- An insulation break in the live wire could cause a short circuit if the exposed copper wires touch another connection.
- If a cable is trapped by the body of the plug when the back is screwed on, there is a danger that the cable's insulation could split, exposing the wire.
- Frayed wire ends that are not connected securely into the connection terminals could cause a short circuit by touching another terminal.
- The cable grip should grip the outer insulation, not the individual wires' insulation.
Finally, why are half of the live and neutral pins black? This is a safety feature. When you push a plug halfway into a wall socket, the plug pins can just make contact with the socket connections. In this position it is possible for little fingers (e.g. curious toddlers) to touch the exposed brass pin which is now 'live'. This could result in an electric shock. Covering the exposed pins in a layer of black plastic insulation will prevent this from happening because the insulation doesn't conduct electricity.