Mains voltage can kill if it is not used safely.

Click here for a diagram of voltage against time for mains electricity - you should know this diagram.

House wiring in the UK

This little 'vidclip' explains the wiring of a 3-pin plug and the earthing of appliances.

The 3-pin Plug

Most electrical appliances are connected to the mains using cable and a 3-pin plug (see diagram below).

A typical cable comprises of:

• two or three inner cores of copper, because copper is a good conductor;
• outer layers of flexible plastic, because plastic is a good insulator.

• a plastic or rubber case, because plastic and rubber are good insulators;
• connector pins made from brass, because brass is a good conductor;
• a fuse (3A, 5A and 13A are the most common values);
• an earth pin;
• a cable (or cord) grip to hold the cable firmly in place and stop a user pulling wires free from their correct connections.

When connecting an appliance to a 3-pin plug:

• the blue wire is connected to the neutral terminal; The neutral terminal stays at a voltage close to zero with respect to earth. It is 'earthed' at the electricity sub-station.
• the brown wire is connected via a fuse to the live terminal; The live terminal of the mains supply alternates sinusoidally between a positive and negative voltage with respect to the neutral terminal.
• the green/yellow wire (when fitted) is connected to the earth terminal;
• the cable should be secured in the plug by the cable grip;
• a fuse of the current value (rating) should be in place.

When provided with appropriate diagrams you should be able to:

• recognise errors in the wiring of a mains (3-pin) plug;
• recognise dangerous practice in the use of mains electricity.

Try the 'wiring a plug' crossword

The fuse in a plug should always be the same as the one recommended by the manufacturer of the appliance. A simple calculation can find out the most suitable fuse.

P = IV

for domestic fuses V = 230V (UK mains voltage)

P (the maximum power of the appliance) is marked on the appliance casing in watts

rearranging the equation we have :

I = P/V

so it is easy to calculate the maximum current that the appliance is designed to have flowing through it.

A fuse is a safety device. It protects an electrical circuit from the effect of excessive current. It is basically a strip of metal that will melt when a given current passes through it, creating a gap in the circuit and thereby stopping current flow.

Fuses are designed so that the casing:

- prevents fire hazard should the metal strip melt
- is easily and quickly slotted into the circuit.

Fuses are produced

- in bulk to cut costs
- in standard values
- in standard sized cases

If a fault in an electrical circuit or an appliance causes too great a current to flow (e.g. when a short circuit occus), the circuit is switched off by a fuse or a circuit breaker (you need to be able to explain how each of these works!).

The fuse should be chosen to have a value higher than, but as close as possible to, the current through the appliance when it is working normally.

When the current through a fuse wire exceeds the current rating of the fuse the wire becomes hot and will (eventually) melt breaking the circuit and switching off the current.

Many appliances are 'fused' in the circuitry and also in the plug. This happens when there is delicate circuitry that would be expensive to replace if it was 'overloaded' and 'burnt out'. Very small value fuses can be incorporated in this way. Simple appliances will just be protected by a fuse in the plug.

The Electric Lamp: hyperlink to a page devoted to this.

Earthing of Metal-cased Appliances

Appliances with metal cases are usually earthed. They have one end of a green/yellow wire fixed and connected to the metal case of the appliance, the other end of this wire is in the cable that goes to the plug. If a fault in the appliance connects the case to the live wire, and the supply is switched on, a very large current flows to earth and overloads the fuse. This 'blows' (melts and breaks the circuit) making the appliance safe to touch.

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 or 50 hertz (Hz) which means that it changes direction and back again 50 times each second.

Cells and batteries supply a current which always flows in the same direction. This is called a direct current (d.c.).

Candidates should be able to compare the voltages of d.c. supplies and the frequencies and peak voltages of a.c. supplies from diagrams of oscilloscope traces.

Power Ratings of Common Appliances

The more current that has to flow through an appliance the greater its power rating. Motors and heaters need large currents to operate. The more powerful these components of an appliance need to be, the larger the power rating will be and the larger the fuse value will need to be. This is the link between power rating and function. The homework activity instructions below help you to verify this.

Homework Activity