The a.c. supply of
electricity to the home, wiring, insulation, and plugs
The electricity supply to your home is a.c.
(alternating current) where the current constant reversing direction
e.g. in the UK, an oscillation of 50 Hz (50 cycles/second) is normal (in
other countries it may be up to 60 Hz).
Reminder: A d.c. supply only flows in one
direction (from + to -) and is often at a much lower potential
difference e.g. the p.d. of batteries or cells is usually in the
range 1.5 V to 24 V.
The a.c. supply to the ring mains circuits in
your house originates from the
National Grid system.
Alternating currents are produced from alternating voltages in which
the positive and negative terminals of the potential difference keep alternating
(+ <=> -).
CRO traces illustrating the difference between AC and DC
The a.c. mains supply in the UK is usually around
230-240 V with a frequency of 50 Hz (50 hertz or 50 cycles/second).
It can vary slightly from country to country e.g. some supply
systems work on 60 Hz.
Other devices will use a d.c. (direct current)
supply from cells or batteries, in which the current only flows in one direction
e.g. torch batteries.
A d.c. current is produced by a direct voltage -
potential difference (p.d.) and is either positive or negative, but NOT
You can convert an ac current into
a dc current using a diode.
Many electrical appliances in the home are connected
to the ring mains circuit with three core cables fitting into a
The plug (pictures below) is inserted into a
socket which is directly
connected to the a.c. mains supply.
sockets have their own switches connected to the live wire of the
ring mains circuit in a house.
A switch must be connected in the live, so the
wire is NOT live if the switch is on OFF.
enables the circuit to be broken and isolate any appliance if there
is a risk of electric shock.
The cables consist of a copper wire core and sheathed in an
plastic covering, each of which is colour coded to clearly indicate its
function (annotated image below).
The colour coding is kept the same for all appliances
so that you know exactly which wire is which!
If wired incorrectly you may blow the fuse or
have an accident - potentially fatal electrocution (see earth wire),
so make sure you know which is which and how to wire a plug safely
irrespective of any GCSE physics exam!
The function of each of the three wires in a three core cable.
- brown colour insulation
The live wire provides the alternating
current potential difference with a p.d. of +/- ~230-240 V.
It is the live wire that carries the high
The appliance switch must always be in the
live wire, otherwise the circuit would always be live!
The live wire carries the p.d. directly from the mains supply
and this 'live' wire must never be touched if the circuit is
switched on for obvious reasons!
In fact you should never touch or
manipulate any wire, especially the live wire, if the circuit is
If you touch a live wire, a large
potential difference is produced across your body and a
surge of current passes through your body. The subsequent
electric shock can injure you and can be fatal.
A short circuit of a faulty appliance
or anywhere in a circuit, can cause a fire from the energy
release - electrical energy to the thermal energy store of
the wire and surroundings.
For an appliance, the sequence of
wiring in the live wire is:
plug ==> fuse ==> switch ==> heating element
The action of a
fuse or circuit
breaker protects you from harm and minimises fire risk.
- blue colour insulation
The neutral wire completes the circuit
to the appliance and carries away the current.
The neutral wire provides the return path to the
local electricity sub-station (transformer).
The neutral wire is earthed, so that it is as
close to being an earth potential of 0 V.
This allows the current to flow
in through the live wire (maximum p.d. of ~230 to 240 V) and out
through the neutral wire (minimum p.d. ~0 V).
The p.d. between the live wire and neutral
wire is ~230-240 V for the mains electricity supply.
wire - green + yellow coloured bands insulation
The earth wire has a safety function to
protect the wiring and YOU!
It doesn't normally carry a current and its
p.d. should be 0 V.
The p.d. between the live wire and the earth
wire is ~230-240 V.
There is no p.d. between the neutral wire and
earth wire, both are at a p.d. of 0 V.
The earth wire is connected to the metal casing of
an appliance and carries the current away safely if a fault develops in the
If a fault develops and the live wire touches
any conducting part of the appliance, the current will run to
earth through the earth wire and NOT through you if you touch the appliances.
This also might, and should, blow the fuse
because of the surge in current, so the circuit is broken and made
See also Part 5.
on the dangers of the live wire, fuses and earthing appliances for extra
danger of the live wire
The danger of electrocution - safety
function of earth wire
Under normal conditions your body has a p.d.
of 0 V with respect to the ground ('earth').
Unfortunately, if you touch the live wire
with the circuit switched on, a potential difference is produced
across your body and the current flows through you to the ground -
In other words, you will experience
electrocution - potential injury from an electric shock, and, if the
current is large enough, it may kill you!
It doesn't matter whether the appliance is
switched on or not, if the plug is in the socket, there is a
connection to the live wire which always has a p.d. of ~230-240
If there is any low resistance connection
between the live wire and earth wire a sudden huge current
can flow to earth, which is dangerous.
This is the cause of many house fires due to
a faulty connection where lots of heat is produced.
For more on electrical safety see
live wire and fuses notes.
INDEX of ELECTRICITY Notes 1. Electricity in the home
Keywords, phrases and learning objectives for the wiring of a plug
You should know how to wire a plug is wired.
Know a plastic sheath electrically insulates the
copper wire and know the colours of the live, neutral and earth wires.
Know for electrical safety in the home, be able to
explain the function of
the live wire, earth wire and neural wire, with or without a diagrams;
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