Mains Electricity

Direct and Alternating Currents

There are two different ways that electricity can be supplied. These are direct current (dc) and alternating current(ac).

In dc supplies, the current is always flowing in the same direction. Cells and batteries supply dc current. 

In ac supplies, an alternating current is supplied by the application of alternating voltages. This means that the current is constantly cycling between positive and negative directions. UK mains supply is an ac supply. 

UK mains electricity is provided at 50\text{ Hz}. This means 50 cycles of positive and negative currents per second. 

Electrical Plugs

Most electrical appliances are supplied electricity by three core cables. Each cable is made from a copper wire and a coloured plastic coating. The colours are always the same for each type of wire so that they can be easily identified:

• Live wire (brown) – provides alternating potential difference from the power supply (approximately 230\text{ V}).
• Neutral wire (blue) – completes the circuit back to the power supply. Current flows through the neutral wire but the potential difference is 0\text{ V}. 
• Earth wire (green and yellow)– stops the casing of the electrical appliance from carrying a current. It only carries a current itself when there is a fault in the wiring. 

Electric Shocks and Fires

If you touch the live wire, a large potential difference will be produced between your body and the wire. As a result, a current will flow through you. This will give you an electric shock which may injure or even kill you. 

Remember that even when an appliance is switched off, there is still a potential difference in the live wire and so you can still get an electric shock.

If the live wire and Earth wire touch, this will create a low resistance path through which the current can flow to the Earth. A very large current may flow through this path and this can cause fires.