Transcript Science (done) > Mrs Potts > 11A5 Physics

TITLE: Mains Electricity 2
Objectives – to know:
• What is the casing of a mains socket or plug made
form and why? – grade C
• What is in a mains cable? – grade C
• Why does a 3 pin plug include an earth pin?
Outcomes:
Complete worksheet on VLE
KEY WORDS:
Fuse
current
Circuit
breaker
mains
Earthed
Double
Insulated
Neutral
Live
earth
• Most electrical appliances are
connected to the mains using cable and
a three-pin plug.
• You must know:
– The structure of electrical cables (twocore and three-core).
– The structure and wiring of a three-pin
plug.
Connected to
the ground at
your home
The longest pin on the 3 pin
plug is attached here so when
an appliance with a metal
case is plugged in tit is
automatically earthed.
Pins are made of brass
as it is a good
conductor and does not
rust.
The case is made of plastic as
it is a good insulator and is
shaped so that the wires don’t
touch each other.
Why do some plugs only have two pins?
Some sockets in Europe have two round slots for the pins. Does
this mean that the appliances are not earthed?
Instead of having a pin
for the earth connection,
the plug has a clip on the
side.
This clip corresponds to
a contact point in the
socket and connects the
appliance to earth.
This plug and socket
system is sometimes
referred to as
“Continental European”.
Identifying wrongly-wired plugs
The Fuse
A fuse is a thin piece of wire which will melt when
too much current passes through it.
Current is a flow of electrons (-) and is measured
in amps (A).
Example
A 3A fuse will melt when the current is greater
than 3A.
When a fuse has ‘blown’, the wire inside it has
melted.
The Fuse
When the wire melts, all current is stopped
from reaching the appliance and switches it
off.
The fuse stops a large current from flowing
through the appliance which could cause wires to
overheat, melt or catch fire.
The fuse rating is
the maximum
current that can
pass through it.
• The earth wire and fuse together
protect the wiring of the circuit.
• Candidates should have an
understanding of the link between cable
thickness and fuse value.
• Cables of different thickness are used
for different purposes.
• The cables joining the wall sockets in a
house must be much thicker than those
then going from the walls to the
light fittings.
• The mains cables are thicker as more
current passes along them than along a
cable to a light fitting.
• Due to resistance, if the wire is too
thin, it will get very hot – if the wire
gets dangerously hot a fire may result.
The live wire carries a current
that alternates between a
negative and positive voltage.
The neutral wire completes
the circuit. It is kept at a
zero voltage
by the electricity company.
The earth wire is a
safety wire that is
needed to earth
appliances with
a metal case.
This makes it safer to
touch the appliance if it
develops a fault.
What can go wrong!
The most dangerous thing that can happen is
that the live wire can become loose inside an
appliance and touches the casing.
This makes the casing LIVE!
If you touch the LIVE casing, you could be
electrocuted as you are providing a path for the
electricity to flow.
How does earthing work?
• Appliances with metal cases are usually
earthed.
• Candidates should be aware that some
appliances are double insulated, and
therefore have no earth wire
connection.
What does ‘double insulated’ mean?
Another way of improving the safety of electrical
appliances is to make them double insulated.
Double insulated appliances
have plastic cases, without
any wires connected to the
case.
This means that the case
cannot become live,
because plastic does not
conduct electricity.
So, if one layer of insulation is
damaged the appliance is still
safe for the user.
symbol on double insulated
appliance
What is inside an electrical cable?
A 3 core electrical cable
contains three wires.
copper
These wires are made of copper
because it is a good conductor of
electricity. Each wire is made of thin
strands of copper to keep the cable
flexible.
Each wire is encased in plastic to
stop the wires touching and causing a
short circuit. Plastic is used as it is a
good insulator, as well as being tough
and flexible. The whole cable is
encased in another layer of plastic.
insulating plastic
2 core and 3 core cables
• Normally in appliances where there is no risk of an
electric shock (ie double insulated) a two core cable is
used. When it is plugged into an electrical outlet
current flows into the appliance through one
conductor and back into the outlet through the other.
This completes the circuit and the appliance starts
working.
• In cases where there is a danger of an electric shock
a 3 core cable should be used. The third conductor
connects the metal part of the appliance to earth and
any leakage that may occur due to a fault, will be
safely conducted to ground.
The copper braid
acts as an earth wire
• If an electrical fault causes too great a
current, the circuit is disconnected by a
fuse or a circuit breaker in the live
wire.
• When the current in a fuse wire
exceeds the rating of the fuse it will
melt, breaking the circuit.
• Some circuits are protected by Residual
Current Circuit Breakers (RCCBs).
• RCCBs operate by detecting a
difference in the current between the
live and neutral wires. Knowledge of how
the devices do this is not required.
• Candidates should be aware of the fact
that this device operates much faster
than a fuse.
• RCCB – detects difference between
current in live and neutral (wires)
• fuse – (overheats and) melts
• The difference from the diagram is 100mA
• The value for time for 100mA is 60
milliseconds in the usually no risk and very
fast
• Switches OFF at 60 milliseconds