Electricity travels from its source of supply to the electrical appliances and fittings within our homes by means of the 'live' copper wire (a conductor). Conductors are identified by the colour of the insulating cover.
It should be noted that the insulating material colour was changed in March 2004, so that where normal household 2 core and earth cable up to this date would have been: red for live, changed to brown, and the black neutral changed to blue. The earth core remained a bare conductor and is covered with the green and yellow sleeving where exposed, making new cable colours the same as UK flex.
Having fed and operated the fitting or appliance, electricity passes back to the source of supply along the 'neutral'conductor.
Electricity can only flow when a circuit is completed, that is, when a fitting or 
appliance is switched on and in use. If this circuit is in anyway broken, the electrical flow stops and the appliance stops working.
To demonstrate this, shown here on the left, is a simple complete circuit using a battery as the source of the supply, the wire connections as the conductors and the light bulb the appliance. The bulb glows, because the circuit is complete, achieved by a connection between one of the terminals of the battery to the bulb and a second from the bulb returning to the other battery terminal.
If there is only a single wire connection present between the battery terminal and the bulb as shown on the right, the bulb would not glow, the circuit is not complete.
On - Off Switch
If any part of the wire connectors in the complete circuit are disconnected, the bulb would stop glowing, however it would glow again if the wire was re-joined. This is what happens when a switch is introduced into a circuit, the light is controlled by switching between a complete and an incomplete circuit.
Where mains electricity is concerned, a circuit work in the same way, however, for safety reasons, the wiring should be such that the switch breaks the 'live' feed to the appliance so that when switched off, the appliance is completely dead because of not receiving a supply. If the neutral was used for this purpose, the appliance would not work, but it would still be 'live'
For more advanced electrical circuits or for information on Printed Circuit Board Assembly check out Sloan Electronics, the UK’s leading PCB Assembly and PCB Design website
