Download Mains Electricity

Mains Electricity
Van de Graaff Generator
Why are birds not electrocuted on
power lines?
Improperly Grounded Appliance with Short
Why are birds not electrocuted on power lines?
One milliampere: tingling sensation
Ten milliamperes: nerves and muscles overloaded
200 milliamperes: potentially fatal; heart fibrillation
500 -1000 milliamperes: not necessarily fatal; heart will restart -- One ampere or more: burn
alive
Grounded Appliance
Ground Plugs Prevent You from
becoming the Shortest, Easiest Path to the Ground!! 
Watch this principle in action with a lineman.
What is direct current
(d.c.)?
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 alternating current (a.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.
Comparing a.c. and d.c.
d.c.
time
voltage
voltage
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.
peak forward
voltage
a.c.
time
peak reverse
voltage
The voltage of a d.c. supply
is steady and always in the
same direction.
The voltage of an a.c. supply
follows a repeated pattern: it
rises to a peak, returns to zero
changes direction and so on.
Frequency of alternating current
voltage
The frequency of a.c. electricity is the number of complete
cycles per second, which is measured in hertz (Hz).
1 complete cycle
peak forward
voltage
a.c.
time
peak reverse
voltage
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.
Mains electricity in the UK
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.
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.
Live, neutral and earth
The three wires in electrical cables have different functions.
The colour of the plastic casing tells you what the wire does.
live
earth
neutral
This colour-coding is a safety feature that makes the
wires easy to identify in poor lighting conditions.
What does each wire do?
The live wire carries a current
that alternates between a
negative and positive voltage.
The neutral wire completes the
circuit. It is kept at a zero voltage
by 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.
Parts of a plug
earth
terminal
live
terminal
fuse
neutral
terminal
cable
grip
cable
What is a fuse?
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.
How does a fuse work?
A fuse is a built-in weak point in a circuit. It contains a thin
wire with a higher resistance than normal wire.
terminals
thin wire with
high resistance
case made of
insulating material
When a large current flows the wire becomes hot. If too
much current flows, the wire overheats and melts, which
breaks the circuit.
Choosing the right fuse
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? 5 A fuse
You can choose from fuses of 3 A, 5A and 13A.