Electricity in the
home: 1.5 More on the dangers of the live wire,
and earthing appliances for extra safety - fuse calculation
Doc Brown's Physics exam study revision notes: Explaining the dangers of live wire, how to calculate
the correct fuse rating to use, why earthing appliances for extra safety
is important, how do circuit breakers work?
INDEX for physics notes on
electricity in the home
1.5 More on the dangers of the live wire,
and earthing appliances for extra safety
With respect to an external electricity
supply, as with the earth-ground itself, your body is at a p.d. of 0
Unfortunately, this means if you touch the
live wire or anything connected to it e.g. faulty metal casing
touching the live wire, a large p.d. of 240 V occurs
across your body i.e. between you and the 'earth'.
Therefore you are in grave danger of an electric
shock because a current will flow through your body to 'earth' -
unfortunately, the fluids in your body contain enough ions for quite
efficient conduction with a p.d. of 230 V.
The electrical current flow
will give you an electric shock, which can be sufficient
to kill you.
Even if the appliance is 'switched off' there is still a danger of an electric
shock because the live wire is still at high pd (eg 240 V).
- if a heating element or metal casing of an appliance is
faulty and they come into contact
(i) Wearing electrically insulating rubber boots may offer some
protection, but is that what you normally wear!?
(ii) Water is a poor conductor, but
with a high potential difference it can conduct. You should also
remember from your chemistry (electrolysis)
that ions from salts increase the electrical conductivity of
water and you have salt ions in your blood, cells and nervous
(iii) It is the function of fuse
to protect you and the appliance from current surges (next
When things go wrong!
function of a fuse and how to calculate the fuse
rating for an appliance
If a heating element or metal casing of an
appliance is faulty and they come into contact, if you are
touching the appliance when it is switched on, then you can be electrocuted as the current will flow through to earth (the ground).
BUT, you should be saved by an earth connection
from the case to the ground and a fuse fitted in the plug or a
circuit breaker (next section).
How do the earth wire and fuse work
in the circuit of an appliance
In any household or
industrial circuit, you can get sudden surge (increase) in
The current surge maybe due to a fault, but
switching appliances on and off can trigger a sensitive circuit
breaker, but shouldn't blow a fuse.
A current surge due to a fault
can lead to overheating, damaging the appliance or even causing
KEY: Live wire (brown),
neutral wire (blue)
and the earth wire (yellow/green)
and switch of the plug and socket.
An appliance is fitted with
an earth wire and a fuse in the live wire - and
before the appliance's ON/OFF switch.
From the diagram above where the
appliance could be an electric toaster or a kettle:
Appliance in safe condition, earth wire connected, fuse intact, no
faults and switched off.
in safe condition, earth wire connected, fuse intact, no faults and
switched on and working safely.
3. Heating element broken
(maybe from corrosion) and touching the metal casing, switched off, but
4. The appliance is
switched on and the current flows through the casing and down to the
ground through the earth wire, in doing so, the heat generated in the
fuse wire, melts it, breaking the circuit and making it safe.
So, if a fault develops and the
live wire comes into contact with the metal case, then,
as long as the metal case is 'earthed' (connected to the earth
wire) the current surge flows harmlessly from the live wire,
through the case and down the earth wire to earth.
The current surge should melt
the fuse, as long as the correct fuse rating is used, and the
current surge is over the fuse rating (in amperes).
This is why the fuse must
be connected in the live wire before the appliances heating
Once the fuse has melted the
circuit is broken and the live wire supply is cut off.
isolates the whole appliance so you cannot get an electric shock
from touching the case.
Fuses rely on an 'overheating'
effect to protect an appliance from damage (e.g. so it might be
repaired) and ourselves from electrocution from a high voltage
current running through our body to earth.
If the temperature of a resistor
becomes too high from a current surge causing overheating, the
resistance increases and so does the heat transfer to an external thermal energy store
around the wire.
This may interfere with the
working of an appliance due to the increase in temperature of a
The temperature might rise
sufficiently to melt the wire in a circuit component and the break
in the circuit stops the 'device' working.
This is how a fuse works
fault develops and too much current flows, a fuse wire melts from
this overheating effect, this breaks the circuit and makes it safe.
The larger the current in the
appliance the thicker the wire should be to minimise the
resistance and overheating. Generally speaking the fuse rating
increases with increase in cable thickness.
(i) As well as
appliances, the ring main circuits to the 'plug in' sockets
and lighting are protected with fuses in the same way.
(ii) You can
protect circuits with
There are several types
of circuit breakers e.g. some work off the magnetic effect
of a solenoid so that a current surge produces a magnetic
field strong enough to make a magnet open two contacts to
break the circuit.
Circuit breakers are
safer than normal household fuses.
A wire does not melt, but
the circuit is broken by a fast 'switching off'
action - faster than a fuse melts.
They also have the
advantage of being reset, which is less trouble than
fitting a replacement fuse. They are however, more
expensive, but safer!
to calculate the correct fuse to fit?
Fuse ratings and how you choose the safest
fuse to use?
For domestic appliances in
the home the most common fuse ratings in the UK are 3A, 5A, 8A,
The fuse should have a
rating of close to, but, just above the maximum safe current that
will run through an appliance.
If a fault develops, and the
current rises a few amps above expected, the fuse must melt and
break the circuit making it safe.
This means you have to work
out the current flowing from the power rating of the appliance
from the formula ...
power (W) = current (A) x potential difference
P = IV
Example 1. A 2kW electric fire works
of a 230 V ac mains supply of electricity.
Calculate the current flowing in the
appliance and suggest a suitable fuse rating.
2kW = 2000 W
P = IV, I = P/V = 2000/230 =
would do, but its likely that in this case the appliance
would be fitted with a 13A fuse.
Obviously, you choose the nearest fuse rating
from what is available.
Example 2. What fuse would you choose
to put in the plug of a 700 W electric iron working off 230V
I = P/V = 700/230 =
would be best, but
a 5A would be acceptable.
To protect you from electric
shock, all appliances with metal cases should be earthed
i.e. the metal case is connected to the earth wire, using three core cable as previously described.
An earthed conductor can
never become live.
A metal casing is obviously
an electrical conductor, but if the appliance has plastic casing
(electrical insulator) with no external electrically
conducting metal parts that can be
touched, it is said to double insulated.
(The internal wiring is
insulated apart from heating elements!)
This means the appliance doesn't need an earth wire
and so is only connected with two
core cable - live and neutral wires only - which are all that is
required to power an appliance.
INDEX of ELECTRICITY Notes 1. Electricity in the home
Keywords, phrases and learning objectives for
Be aware and able to describe and explain the
dangers of live wire if a fault develops in an appliance.
Know how to calculate the correct fuse rating to
use in a plug.
Know, explain and describe why earthing appliances
for extra safety is important.
Know what a circuit breaker is and how it works.
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