Mains electricity
Mains electricity refers to the energy that is brought into people's homes.
The energy input for households is ac as the National Grid can only use ac input. This means that the direction of the current and voltage changes 100 times per second. The direction of a direct current (dc) voltage, such as that produced by a cell or a battery, does not change.
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In the diagrams below, A shows an ac voltage, and B shows a dc voltage.
A
B
Live, neutral and earth mains wires
The UK mains supply is connected to appliances in the home by using a plug. The plug contains three wires ‐ the live, neutral and earth wires.
In a plug, the live wire (brown) and the neutral wire (blue) are the two wires that form the complete circuit with a household appliance. The earth wire (green and yellow) does not normally form part of the circuit and is included as a safety wire. It is connected to 'earth', which is usually a large metal spike buried under a house.
The potential difference values between these wires is different.
| Wires | Voltage between them (V) |
| Live and neutral | 230 |
| Live and earth | 230 |
| Neutral and earth | 0 |
| Wires | Live and neutral |
|---|---|
| Voltage between them (V) | 230 |
| Wires | Live and earth |
|---|---|
| Voltage between them (V) | 230 |
| Wires | Neutral and earth |
|---|---|
| Voltage between them (V) | 0 |
Safety features in plugs and appliances
There are three main safety features that are found in plugs and appliances to ensure that no harm comes to the user.
- The fuse ‐ a fuse is designed to melt if a current flows through it that is too large. For example, if a fuse has a rating of 3 A then it will melt if the current flowing through it exceeds 3 A. This is because if the current gets too high, too much heat could be produced which would cause the appliance to catch fire. Once the fuse melts, the circuit is broken and no current can flow.
- Double insulation ‐ devices which contain metal parts are surrounded with a casing that is made from a plastic insulation. This stops the user from coming into contact with any of the metal parts that conduct current. Coming into contact with these areas could cause electrocution.
- The earth wire - this is designed so that a large current passes through the fuse in the event of the live wire loosening and touching a metal part of the appliance. If there were no earth wire, and a live wire were to loosen and touch a metal part of the appliance, someone touching the appliance would be electrocuted, as a high current would pass through the body to the ground (Earth).