The three-pin plug
Structure and wiring a three-pin plug
A cable and a three-pin plug connect most electrical appliances to the mains. The cable carries the three wires, live, neutral and earth, or "twin" and "earth". Insulating material encloses the cable and all the plug except the pins.
In a three-pin plug, the wires are colour coded:
- Earth wire is yellow and green, connected to the top pin
- Neutral wire is blue, connected to the left pin.
- Live wire is brown, connected through the fuse to the right pin.
- The fuse joins the brown wire and the right pin.
When the fuse blows, it issolates the live wire and no current flows.
Appliances with plastic outer casing and no touchable metal parts use twin-core cable, with live and neutral wires only. They are double-insulated appliances: they have a fuse and the casing is an insulator. Examples are lawn mowers and hair dryers.
Safety at Home
Fuses and Earth wires
An electrical fault may cause a surge in the current that could damage an appliance and injure or kill someone. Fuses and circuit breakers are designed to break the circuit instantly and cut off the current.
The fuse in a plug joins the live wire to its pin. A current that exceeds the fuse rating will heat and melt the fuse wire.
In circuit breakers, an eletromagnet or electronic mechanism immediately breaks the circuit when there is a hazardous increase in current. Circuit breakers are uses in fuse boxes.
Appliances with metal casings have an earth wire. If the live wire touches the metal casing, a large current flows from the live wire through the earth wire. The surge in current melts the fuse and breaks the circuit.
Calculating the correct fuse rating
The rate at which energy is transformed is measure in joules per seconds, also know as watts, 1 J/s = 1 watt. The rate of energy transfer is called power:
Power (watts, W) = energy transformed (joules, J) / time taken (seconds, s)
The power rating is the amount of energy an electrical appliance uses per second. The power rating determines the fuse
Power = p.d x current so Current = power / p.d
A blender has a power rating of 1000W. What fuse does it require?
Current = power / p.d so 1000W / 230V = 4.3 A
A 3A fuse would melt when the blender is switched on, and a 13A fuse would not melt if a fault occured. The correct fuse is 5A.