Mains Electricity

ALTERNATING CURRENT

Cells and batteries supply current that passes round the circuit in one direction.  This is called direct current or dc.  The current from the mains supply passes in one direction, then reverses and passes in the other direction.  This is called alternating current, or ac.

The frequency of the UK mains supply is 50Hz.  This means it changes direction 50 times a second.  The voltage of the mains supply is 230V.  The live wire of the mains supply alternates between a positive and a negative potential with respect to the neutral wire.  The neutral wire stays at 0V.  The live wire alternates between peak voltages of +325V and -325V.  In terms of electrical power, this is equivalent to a direct potential difference of 230V. 

The frequency of the supply can be determined from an oscilloscope trace using the equation:

f = 1 / t, where f is the frequency in Hz and t is the time for one cycle in one seconds.

CABLES AND PLUGS

Most electrical appliances are connected to the sockets of the mains supply using cable and a three-pin plug.  The outer cover of the three-pin plug is made of plastic or rubber because they are both good electrical insulators.  The pins of the plug are made of brass because it is a good electrical conductor, it is hard and will not rust or oxidise.  The earth wire is connected to the longest pin.  It is important that the cable grip is fastened tightly over the cable.  There should be no bare wires showing inside the plug and the correct cable must be connected firmly to the terminal of the correct pin.  The brown wire is connected to the live pin.  The blue wire is connected to the neutral pin.  The green and yellow wire in  a three-core cable is connected to the earth pin.  A two-core cable does not have an earth wire.

Brown wire - live wire, blue wire - neutral wire, yellow and green wire - earth wire

Appliances with metal cases must be earthed - the case is attached to the earth wire in the cable.  Appliances with plastic cases do not need to be earthed.  They are double insulated and connected to the supply with a two-core cable containing just a live wire and a neutral wire.  Appliances with metal cases need to be earthed so that the case does not become live and electrocute anyone who touches it.

Cables of different thickness are used for different purposes.  The more current to be carried, the thicker the cable needs…