(click 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 .

The earth wire is connected to earth (0V) locally to the supply..

(click on the diagram to see an enlarged version - suitable for printing)

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 the more appliances you use. (See notes on parallel and series circuits).

Switches and fuses

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 points.

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
diagram).

Sockets can then be easily connected in parallel across these wires at desired points without much trouble.

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

This 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,

Things 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

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 IT !!

LOJ - January 2003