Transcript Fuses - iGCSE Science Courses

PHYSICS – Dangers of electricity
LEARNING
OBJECTIVES
Core
• State the hazards of: – damaged
insulation – overheating of cables –
damp conditions
• State that a fuse protects a circuit
• Explain the use of fuses and circuit
breakers and choose appropriate fuse
ratings and circuit-breaker settings
• Explain the benefits of earthing metal
cases
Supplement
In the UK,
domestic
electricity is
supplied at a
voltage of
230V (+/- 6%)
http://ownersrepny.com/
Mains
electricity can
be dangerous –
possible
hazards?
Old, frayed wiring might
mean strands of wire with a
higher resistance at one
point. A heating effect
may be enough to melt the
insulation and cause a fire.
In the UK,
domestic
electricity is
supplied at a
voltage of
230V (+/- 6%)
http://ownersrepny.com/
Mains
electricity can
be dangerous –
possible
hazards?
Old, frayed wiring might
mean strands of wire with a
higher resistance at one
point. A heating effect
may be enough to melt the
insulation and cause a fire.
Extension leads may
overheat if used when
coiled up. The current
flowing will warm the wire,
but the tight bundle means
that the heat has less
room to escape.
In the UK,
domestic
electricity is
supplied at a
voltage of
230V (+/- 6%)
http://ownersrepny.com/
Mains
electricity can
be dangerous –
possible
hazards?
Old, frayed wiring might
mean strands of wire with a
higher resistance at one
point. A heating effect
may be enough to melt the
insulation and cause a fire.
Extension leads may
overheat if used when
coiled up. The current
flowing will warm the wire,
but the tight bundle means
that the heat has less
room to escape.
In the UK,
domestic
electricity is
supplied at a
voltage of
230V (+/- 6%)
http://ownersrepny.com/
Mains
electricity can
be dangerous –
possible
hazards?
If water leaks into a plug
or socket there is a risk of
someone getting
electrocuted as water will
conduct the current.
Old, frayed wiring might
mean strands of wire with a
higher resistance at one
point. A heating effect
may be enough to melt the
insulation and cause a fire.
Extension leads may
overheat if used when
coiled up. The current
flowing will warm the wire,
but the tight bundle means
that the heat has less
room to escape.
In the UK,
domestic
electricity is
supplied at a
voltage of
230V (+/- 6%)
http://ownersrepny.com/
Mains
electricity can
be dangerous –
possible
hazards?
If water leaks into a plug
or socket there is a risk of
someone getting
electrocuted as water will
conduct the current.
When using a lawnmower of
electric hedgetrimmer
there is always the danger
of cutting through the
cable. A plug-in RCD
should always be used to
avoid electrocution.
Mains electricity
Mains electricity
Plastic insulated casing.
Mains electricity
Plastic insulated casing.
Neutral wire
(blue)
Mains electricity
Plastic insulated casing.
Neutral wire
(blue)
Earth wire
(yellow and
green). Stops
metal appliances
becoming live if
there is a fault.
Mains electricity
Plastic insulated casing.
Neutral wire
(blue)
Live wire (brown) – goes
alternately negative and
positive, making current flow
backwards and forwards
through the circuit.
Earth wire
(yellow and
green). Stops
metal appliances
becoming live if
there is a fault.
Mains electricity
Plastic insulated casing.
Neutral wire
(blue)
Fuse – always
connected to the
live wire. If the
current gets too
high the fuse will
‘blow’ and break
the circuit.
Live wire (brown) – goes
alternately negative and
positive, making current flow
backwards and forwards
through the circuit.
Earth wire
(yellow and
green). Stops
metal appliances
becoming live if
there is a fault.
Mains electricity
Plastic insulated casing.
Cable grip – keeps
the wires inside
the plug securely.
Fuse – always
connected to the
live wire. If the
current gets too
high the fuse will
‘blow’ and break
the circuit.
Neutral wire
(blue)
Live wire (brown) – goes
alternately negative and
positive, making current flow
backwards and forwards
through the circuit.
Earth wire
(yellow and
green). Stops
metal appliances
becoming live if
there is a fault.
Mains electricity
Plastic insulated casing.
Plastic wire outer
covering
Cable grip – keeps
the wires inside
the plug securely.
Fuse – always
connected to the
live wire. If the
current gets too
high the fuse will
‘blow’ and break
the circuit.
Neutral wire
(blue)
Live wire (brown) – goes
alternately negative and
positive, making current flow
backwards and forwards
through the circuit.
Earth wire
(yellow and
green). Stops
metal appliances
becoming live if
there is a fault.
Mains electricity
In some countries the plug may have
only two pins (live and neutral). The
earth connection is made by two
metal contacts at the edge
Mains electricity
In some countries the plug may have
only two pins (live and neutral). The
earth connection is made by two
metal contacts at the edge
Some appliances, for example radios,
do not have an earth wire. This is
because their outer case is made of
plastic rather than metal.
The plastic acts as an extra layer of
insulation around the wires.
Mains electricity
Mains current is 230V AC (in the
UK). AC means alternating current.
The current flows backwards and
forwards 50 times per second. We
say that it has a frequency of 50
hertz (50Hz).
Some appliances, for example radios,
do not have an earth wire. This is
because their outer case is made of
plastic rather than metal.
The plastic acts as an extra layer of
insulation around the wires.
Mains electricity
Fuses
Mains electricity
Fuses
How does the fuse and earth wire
working together prevent shocks?
Mains electricity
1. If a fault develops
(eg. Live touches the
earthed metal case)
then a big current
flows in through the
live, through the case
and out down the earth
wire
Fuses
Mains electricity
1. If a fault develops
(eg. Live touches the
earthed metal case)
then a big current
flows in through the
live, through the case
and out down the
earth wire
2. This surge in current
blows the fuse (or
trips the circuit
breaker), which cuts
off the live supply
Fuses
Mains electricity
Fuses
1. If a fault develops
(eg. Live touches the
earthed metal case)
then a big current
flows in through the
live, through the case
and out down the
earth wire
2. This surge in current
blows the fuse (or
trips the circuit
breaker), which cuts
off the live supply
3. This isolates the
whole appliance,
making it impossible to
get an electric shock
from the case. It also
prevents fire risk from
the heating effect of a
large current.
Mains electricity
Fuses
1. If a fault develops
(eg. Live touches the
earthed metal case)
then a big current
flows in through the
live, through the case
and out down the
earth wire
4. Fuses should be
rated as near as
possible but just
higher than the normal
operating current.
2. This surge in current
blows the fuse (or
trips the circuit
breaker), which cuts
off the live supply
3. This isolates the
whole appliance,
making it impossible to
get an electric shock
from the case. It also
prevents fire risk from
the heating effect of a
large current.
Mains electricity
What
fuse to
use?
Fuses
Mains electricity
Fuses
What
fuse to
use?
3A, 5A, 13A
Mains electricity
Fuses
What
fuse to
use?
3A, 5A, 13A
WARNING: if the
wrong fuse is used then
if a fault develops, the
circuit might overheat
and catch fire without
the fuse blowing.
Mains electricity
Fuses
What
fuse to
use?
3A, 5A, 13A
WARNING: if the
wrong fuse is used then
if a fault develops, the
circuit might overheat
and catch fire without
the fuse blowing.
To work out the fuse required we
need to know the power rating of an
appliance and the voltage rating. We
can then work out the current used by
the appliance using this equation:
I = P/V
Mains electricity
Fuses
What
fuse to
use?
3A, 5A, 13A
Eg. Hair dryer.
Power rating =
1.1kW
Voltage rating
= 230V
Mains electricity
Fuses
What
fuse to
use?
3A, 5A, 13A
I = P/V
I = 1100 / 230
Eg. Hair dryer.
Power rating =
1.1kW
Voltage rating
= 230V
I = 4.8A
Use the fuse rated a little
higher than the normal current.
In this case, use a 5A fuse.
Mains electricity
Fuses
What
fuse to
use?
3A, 5A, 13A
Eg. Television.
Power rating =
150W
Voltage rating
= 230V
Mains electricity
Fuses
What
fuse to
use?
3A, 5A, 13A
I = P/V
I = 150 / 230
Eg. Television.
Power rating =
150W
Voltage rating
= 230V
I = 0.7A
Use the fuse rated a little
higher than the normal current.
In this case, use a 3A fuse.
Mains electricity
The fuse contains a piece of
wire that melts easily. If the
current going through the fuse
is too great, the wire heats up
until it melts and breaks the
circuit. The fuse then has to
be replaced.
Fuses
Mains electricity
The fuse contains a piece of
wire that melts easily. If the
current going through the fuse
is too great, the wire heats up
until it melts and breaks the
circuit. The fuse then has to
be replaced.
Fuses
A circuit breaker does the
same job as a fuse, but it
works in a different way.
Circuit breakers are automatic
switches that ‘trip’ (turn off)
when the current rises above a
specific value. The circuit
breaker can be reset by
pressing a button (no need to
replace).
http://wiki.diyfaq.org.uk/index.php?title=MCB
LEARNING
OBJECTIVES
Core
• State the hazards of: – damaged
insulation – overheating of cables –
damp conditions
• State that a fuse protects a circuit
• Explain the use of fuses and circuit
breakers and choose appropriate fuse
ratings and circuit-breaker settings
• Explain the benefits of earthing metal
cases
Supplement
PHYSICS – Dangers of electricity