Mains Electricity

Van de Graaff Generator

Why are birds not electrocuted on power lines?

Improperly Grounded Appliance with Short  

One milliampere:    tingling sensationTen milliamperes:   nerves and muscles overloaded200 milliamperes:   potentially fatal; heart fibrillation500 -1000 milliamperes:  not necessarily fatal; heart will restart -- One ampere or more:  burn alive

Why are birds not electrocuted on power lines?

Grounded Appliance

                                                                  

     

Ground Plugs Prevent You from becoming the Shortest, Easiest Path

to the Ground!!

Watch this principle in action with a lineman.

Direct current (d.c.) is an electric current that always flows in one direction.

Direct current is produced by cells and batteries.

Electronic circuits such as those in computers and stereos need direct current electricity in order to work.

Direct current cannot be transferred efficiently over large distances.

What is direct current (d.c.)?

Alternating current (a.c.) is an electric current that is constantly changing direction.

Alternating current is produced by most generators and is used in mains electricity.

Motors often work using alternating current. The voltage of alternating current is easily changed with a transformer.

Alternating current can be transferred efficiently over large distances.

What is alternating current (a.c.)?

The difference between alternating current (a.c.) and direct current (d.c.) can be seen using an oscilloscope.

For each current, the oscilloscope trace is a graph showing how the voltage of an electricity supply varies with time.

The voltage of a d.c. supply is steady and always in the same direction.

Comparing a.c. and d.c.

The voltage of an a.c. supply follows a repeated pattern: it rises to a peak, returns to zerochanges direction and so on.

d.c.

timevo

lta

ge a.c.

timevo

lta

ge peak forward

voltage

peak reverse voltage

The frequency of a.c. electricity is the number of complete cycles per second, which is measured in hertz (Hz).

Frequency of alternating current

a.c.

timevo

lta

ge peak forward

voltage

peak reverse voltage

1 complete cycle

The a.c. frequency can be determined from an oscilloscope by counting the number of complete waves per unit time.

If the frequency is increased the number of complete waves shown on the screen increases. For example, if the frequency is doubled the number of waves doubles.

In the UK, the frequency of mains electricity is 50 hertz: this alternating current flows backwards and forwards 50 times per second.

This frequency is the same at any point in the electricity supply system but the voltage varies in different parts of the national grid.

Mains electricity in the UK

The voltage of mains electricity supplied to UK homes is 230 V.

This is an effective voltage which is equal to the voltage of a d.c. supply that would produce the same heating effect. The peak (maximum) voltage is higher than this.

The three wires in electrical cables have different functions.

The colour of the plastic casing tells you what the wire does.

earth

neutral

live

Live, neutral and earth

This colour-coding is a safety feature that makes the wires easy to identify in poor lighting conditions.

What does each wire do?

The neutral wire completes the circuit. It is kept at a zero voltageby the electricity company.

The earth wire is a safety wire that is needed to earth appliances with a metal case.

For example, if the live wire becomes loose and touches the metal case, a very large current flows to earth and blows the fuse. This makes it safe to touch the appliance.

The live wire carries a current that alternates between a negative and positive voltage.

neutral terminal

earth terminal

cable grip

fuse

cable

Parts of a pluglive terminal

A fuse is a safety device that protects an electric cable from overheating so that the insulation does not catch fire.

A plug contains a fuse to prevent the case of a metal appliance from becoming live if a fault develops.

All plugs contain a fuse like the one shown and are simple to change if they ‘blow’ and break the circuit.

What is a fuse?

A fuse is a built-in weak point in a circuit. It contains a thin wire with a higher resistance than normal wire.

thin wire with high resistance

case made of insulating material

terminals

How does a fuse work?

When a large current flows the wire becomes hot. If too much current flows, the wire overheats and melts, which breaks the circuit.

The circuit symbol for a fuse is:

Fuses are labelled with the maximum current that they are designed to allow.

To choose the correct fuse for a device, always choose the nearest one rated above the operating current of the device.

Example:

If a kettle operates with an electrical current of 4.3 A, what fuse should it be fitted with?

You can choose from fuses of 3 A, 5A and 13A.

5 A fuse

Choosing the right fuse