P2: Mains Electricity

Direct current (d.c.) = an electric current which flows in one direction

Alternating current (a.c.) = an electric current which changes direction of flow continuously

Cells and batteries supply direct current whereas mains electricity is an alternating current (a.c.) supply.

The UK mains supply is 230 volts with a frequency of 50 cycles per second (50 hertz).

Most electrical appliances are connected to the mains using cable and a three pin plug.

The inner cores of the wires are made of copper because it is a good conductor.

The outer layers are made of flexible plastic which is a good insulator.

The pins of a plug are made from brass because it is a good conductor.

Note: You must know the structure of a three pin plug and the correct wiring of a three pin plug.

If an electrical fault causes too great a current the circuit should be switched off by a fuse or a circuit breaker.

A circuit breaker is a safety device which automatically breaks an electric circuit of it become overloaded. It depends on an electromagnet which separates a pair of contacts when the current becomes too high.

A circuit breaker:

• current becomes too high
• strength of electromagnet increases
• pair of contacts are pulled apart
• circuit is broken
• cable or appliance is protected

A fuse is a short, thin piece of wire with a low melting point.

When the current passing through it exceeds the current rating of the fuse, the fuse wire gets hot and melts or breaks; breaking the circuit.

The current rating of the fuse must be just above the normal working current for the safety system to work properly.

With a Fuse:

• Current larger than current rating of fuse
• fuse wire melts
• circuit is broken
• no current flows and the cable or appliance is protected

All electrical appliances with outer metal cases must be earthed.The outer case of an appliance is connected to the earth pin in the plug through the earth wire.

The earth wire and fuse together protect the appliance and the user.

If a fault in the appliance connects the live wire to the case, the case will become live.

The current will then transfer to the earth through the earth wire as this offers least resistance.

This overload of current will cause the fuse to melt, or the circuit breaker to trip.