Mains
Electricity in the UK
The UK
mains supply is about 230 volts AC. It used to be 240V but it has
been brought down in a stage in the transition to 220V (like the rest
of Europe).
Mains voltage can kill if it is not used
safely. See electric shock.
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 plug has:
- 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.
This video shows you practically how to wire a plug:
When connecting an appliance to a 3-pin
plug:
- 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.
Because fuses work on the principal of a large current melting the fuse wire faults can occur in which the current may not be large enough to melt the fuse but enough to seriously harm the user of the electrical appliance. For protection against faults of this nature a circuit-breaker is used in place of a fuse.
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. For more detail click here.
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