3-Pin Plug


The UK mains supply is about 230 volts AC (RMS). (It used to be 240V but it has been brought down in the first stage of the transition to making it 220V like the rest of Europe).

The voltage is not a steady 230V. It varies sinusoidally with time at a frequency of 50Hz.

for a diagram of voltage against time for mains electricity - you should know this diagram.

Mains voltage can kill if it is not used safely.

to find out about the dangers.

The Mains Supply is brought into the house via the main fuse box and is distributed around the house using cabling under the floorboards and within walls. Sockets are fitted so that the house occupiers can access the electricity supply easily and safely.

Most electrical appliances are connected to the Mains using cable and a 3-pin plug. The 3-pin plug can then be plugged into the socket and make a connection with the Mains Supply.

A typical cable comprises of:

two or three inner cores of copper, because copper is a good conductor;

outer layers of flexible plastic, because plastic is a good insulator.

A plug has:

a plastic or rubber case, because plastic and rubber are good insulators;

connector pins made from brass, because brass is a good conductor;

a fuse (3A, 5A and 13A are the most common values);

an earth pin;

a cable (or cord) grip to hold the cable firmly in place and stop a user pulling wires free from their correct connections. The other end of the cable is connected to the electrical circuit inside the appliance.

This video shows you practically how to wire a plug:

Please avoid calling current 'ampage' as the lady in the clip does... it should not be used in and swering examination questions. Use 'current' and 'potential difference' rather than ampage and voltage.


When connecting an appliance to a 3-pin plug:

the blue wire is connected to the neutral terminal; The neutral terminal stays at a voltage close to zero (it tends to vary around zero at 50Hz with max and min close to zero volts) with respect to earth. It is 'earthed' at the electricity sub-station.

the brown wire is connected via a fuse to the live terminal; The live terminal of the mains supply alternates sinusoidally between a positive and negative voltage with respect to the neutral terminal.

the green/yellow wire (when fitted) is connected to the earth terminal;

the cable should be secured in the plug by the cable grip;

a fuse of the current value (rating) should be in place.


When provided with appropriate diagrams you should be able to:

recognise errors in the wiring of a mains (3-pin) plug;

recognise dangerous practice in the use of mains electricity.

Try the 'wiring a plug' crossword