IP The plug © Oxford University Press 2011 The plug

IP The plug © Oxford University Press 2011  Electrical devices are connected to the mains supply by the use of a plug and a socket.  A standard UK plug has 3 pins: live, neutral and earth.  Other countries have different style plugs with different orientations of pins. Most notably, a lot of other countries do not have an earth pin and therefore only have 2 pins.  The cables connecting the devices to the plug are either 3-core or 2-core depending on whether there is a need for an earth cable. Copper is used as it is a good conductor and this is surrounded by a flexible plastic sleeve to insulate it.

IP The plug © Oxford University Press 2011  The materials making up a plug are specifically chosen to help with safety. The pins are made from brass which is a hard metal that is a good conductor. The casing is made from hard plastic that is robust to bumps and knocks and acts as an insulator. Did you know that all UK sockets have a special piece of guard plastic that ensures that the earth pin has to enter the socket first in order to allow the other two pins to enter? This is why the earth pin is slightly longer than the other two.

IP The plug © Oxford University Press 2011  It is very important to correctly wire a plug to ensure safety.  The live wire (brown) must always be connected to the pin with the fuse on the right.  The neutral wire (blue) must always be connected to the left-hand pin.  The earth wire (green and yellow) must always be connected to the top pin.  The wires should always be prepared with the correct tools to ensure the plastic sleeves are not damaged and that each wire is the correct length. The cable clamp must always be in place around the cable sleeve not the individual wires.

IP The plug © Oxford University Press 2011  All UK plugs have fuses connected to the live wire. It is a safety device which breaks the circuit if the device draws too much current from the supply.  The fuse is a piece of wire which has a certain current rating. If a current higher than its rating passes through it then it heats up and melts, breaking the circuit.  A 13 A fuse will allow a current up to 13 A to flow through the plug. A 3 A fuse will only allow a current up to 3 A to flow.

IP The plug © Oxford University Press 2011  The earth cable is another safety device. It is connected to the metal exterior of devices. If there is a fault inside the device that would allow the live wire to make a connection to the metal case, the case would become live and be very dangerous if someone touched it. The earth cable means that in this situation the case cannot become live as the current passes down the earth cable and then melts the fuse.  These devices are either made from plastic and have no external metal case or they are ‘double insulated’ which means that all their electrical components are surrounded by an insulator inside the device so the metal case could never become live. Why do some devices only have a 2-core cable with no connection to earth?