What is mains electricity and how can it be used safely?

Mains electricity is useful but can be very dangerous. It is important to know how to use it safely.

to recognise errors in the wiring of a three-pin plug
to recognise dangerous practice in the use of mains electricity
to compare potential differences of d.c. supplies and the peak potential differences of a.c. supplies from diagrams of oscilloscope traces

What is Alternating Current?
Cells and batteries supply current which always passes in the same direction. This is called direct current (d.c.).
An alternating current (a.c.) is one which is constantly changing direction. Mains electricity is an a.c. supply.

In the UK it has a frequency of 50 cycles per second (50 hertz). UK mains supply is about 230 volts.
 

The 3 Pin Plug
Most electrical appliances are connected to the mains using cable and a three-pin plug.

The structure of electrical cable.
The structure of a three-pin plug.
Correct wiring of a three-pin plug.

	Remember the names of the 3 wires and their colours: Live = Brown Neutral = Blue Earth = Green and Yellow The live wire is connected to the fuse The wires are made of copper (good conductor) while the plug and insulation is made of plastic.

Fuses, Circuit Breakers and Earth
If an electrical fault causes too great a current the circuit should be switched off by a fuse or a circuit breaker.
When the current in a fuse wire exceeds the rating of the fuse it will melt, breaking the circuit.

Appliances with metal cases are usually earthed.
The earth wire and fuse together protect the appliance and the user.
 

Higher Tier Stuff

HT " The live terminal of the mains supply alternates between positive and negative potential with respect to the neutral terminal.   HT " The neutral stays at a potential close to zero with respect to earth.   The live wire is "pushing" and "pulling" the electricity round the circuit. The neutral wire is just completing the circuit. The bulb is actually flashing but with a 50Hz supply it is doing it so fast that it looks lit all the time.

HT " to determine the period and hence the frequency of a supply from diagrams of oscilloscope traces.   The horizontal squares tell us how long one cycle takes. Count the number of divisions and multiply by the ms/div (the Time base setting) and this will tell you the period of the signal. The frequency is one over this. Note that in this example 20ms = 0.02s