In domestic devices, energy is transferred from the batteries or ac mains to the energy of the motors and heating devices.
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.
Each part of the plug has a function.
|Features of a plug||Function|
|Outer 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 wires do not become loose|
|Live wire||Copper wire coated with brown plastic - this wire connects to the alternating potential difference pushing the current in the circuit|
|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 - this wire is connected to a voltage close to zero, to ensure the live voltage always has a difference in potential to make the push for the current|
|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 (also a zero voltage connection) 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 live current passing through the case will follow this path to the ground instead of passing through a person. However, this would generate a very large current, leading to intense heating which could start a fire, so a fuse or a circuit breaker is also included in the circuit.
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. If there is a fault that causes the casing of the device to become live, a large current will flow through the live wire and low-resistance earth wire. This high current will cause the fuse to melt.
Once the fuse has melted, the circuit is broken and no more current flows through the device. This means the case of the device is no longer live and there is no more risk of electrocution. A circuit breaker can serve the same function as a fuse but can be reset without the need for replacement if it trips.
The fuse or circuit breaker must be connected in the live wire side of a domestic circuit to ensure that it keeps high voltage from reaching the user, or surroundings, if a fault develops.