on the diagram to see an enlarged version)
Electricity usually comes to our homes (from the power stations via
the National Grid) via an underground cable carrying two wires - the
live wire (usually red) and the neutral wire (usually black) - earthing'
is done at the home!!
The neutral is earthed
at the local sub-station and so there is no p.d. between it and the
earth. The live wire is 230 V A.C. and oscillates between +330 V and
-330 V .
on the diagram to see an enlarged version - suitable for printing)
N.B. You should be
able to sketch the above diagram!!
Appliances are always connected in parallel with the supply via a socket. This means
that each socket is wired to receive the full 230 V mains p.d. and will not diminish
appliances you use. (See notes on parallel and series circuits).
Switches and fuses are always inserted into the live wire. If they
were in the neutral, lamp and power sockets would be 'live' when the switches
were 'off' or a fuse
was blown. A shock could then occur when touching the element of an electric
fire that was switched off!
A staircase circuit is controlled by two two-way switches. This
enables the user to switch a single lamp 'on' or 'off from two vantage
The ring main circuit
has live and neutral wires each running in complete rings around the
house (there is usually one ring upstairs and another downstairs).
Another ring of wire
connected to earth at the house (by connection to a metal water pipe or
an earth connection on the supply cable) also runs around the house. (See
Sockets can then be
easily connected in parallel across these wires at desired points without
A fuse of 30 A is
included in the consumer unit as the wires using in the house must not
carry more than 30 A at any one time otherwise the wires would become
too hot and a fire could start.
The advantages of having the wires in a ring are:-
ease of location
of new sockets (or change of position of old ones).
the current is split into two (two halves of ring can each carry
current) therefore thinner wire can be used.
one cable supplies many outlets.
each outlet is standard (13 A - 230 V A.C.) - therefore making
purchase and sale of appliances universal within Britain
The lighting circuit
is on a separate set of parallel circuits not a ring andthere is no earth
wire in the lighting circuit.It has to carry a much smaller load and is
therefore only connected to a 5 A fuse on the consumer unit. The wires
on the lighting' circuit can therefore be much thinner than those in the
socket circuit. It is important when changing a fuae in the consumer unit
to ensure that the correct fuse-wire is used,
to remember about the lighting circuit:
not on a ring.
smaller fuse rating (must not be overloaded by plugging hairdriers
etc into the light sockets).
no earth connection, it could therefore be potentially dangerous
to connect a metal cased kettle into a light socket.
High current appliances
such as electric cookers and immersion heaters are wired direct to the
consumer unit, taking their power directly from the supply and
not from the ring main. These draw so much current that they need high
value fuses of their own.
IN EXAMINATION QUESTIONS IT IS UNLIKELY THAT YOU WILL BE ASKED TO DRAW
A RING MAIN CIRCUIT - BUT YOU MAY WELL BE ASKED TO EXPLAIN FEATURES IN
A GIVEN ONE, OR COMPLETE A PARTIALLY DRAWN ONE. THE ABOVE NOTES SHOULD
HELP YOU TO DO THIS. DON'T JUST LEARN THE DIAGRAM - MAKE SURE YOU UNDERSTAND
- January 2003