This document discusses the dangers of electricity in homes. It states that damaged insulation, overheating cables, and damp conditions pose fire and electrocution hazards. Fuses and circuit breakers are used to protect circuits from high currents by interrupting the flow. Fuses contain a wire that melts when overheated, while circuit breakers automatically trip the current. Proper fuse or circuit breaker ratings should be selected based on an appliance's power and voltage ratings to prevent overheating without blowing the fuse. Earthing metal appliance cases prevents electric shock if a fault occurs.

1. PHYSICS – Dangers of electricity

2. 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

3. http://ownersrepny.com/
In the UK,
domestic
electricity is
supplied at a
voltage of
230V (+/- 6%)
Mains
electricity can
be dangerous –
possible
hazards?

4. http://ownersrepny.com/
In the UK,
domestic
electricity is
supplied at a
voltage of
230V (+/- 6%)
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.

5. http://ownersrepny.com/
In the UK,
domestic
electricity is
supplied at a
voltage of
230V (+/- 6%)
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.

6. http://ownersrepny.com/
In the UK,
domestic
electricity is
supplied at a
voltage of
230V (+/- 6%)
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.
If water leaks into a plug
or socket there is a risk of
someone getting
electrocuted as water will
conduct the current.

7. http://ownersrepny.com/
In the UK,
domestic
electricity is
supplied at a
voltage of
230V (+/- 6%)
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.
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.

8. Mains electricity

9. Mains electricity
Plastic insulated casing.

10. Mains electricity
Plastic insulated casing.
Neutral wire
(blue)

11. Mains electricity
Plastic insulated casing.
Neutral wire
(blue)
Earth wire
(yellow and
green). Stops
metal appliances
becoming live if
there is a fault.

12. Mains electricity
Plastic insulated casing.
Neutral wire
(blue)
Earth wire
(yellow and
green). Stops
metal appliances
becoming live if
there is a fault.
Live wire (brown) – goes
alternately negative and
positive, making current flow
backwards and forwards
through the circuit.

13. Mains electricity
Plastic insulated casing.
Neutral wire
(blue)
Earth wire
(yellow and
green). Stops
metal appliances
becoming live if
there is a fault.
Live wire (brown) – goes
alternately negative and
positive, making current flow
backwards and forwards
through the circuit.
Fuse – always
connected to the
live wire. If the
current gets too
high the fuse will
‘blow’ and break
the circuit.

14. Mains electricity
Plastic insulated casing.
Neutral wire
(blue)
Earth wire
(yellow and
green). Stops
metal appliances
becoming live if
there is a fault.
Live wire (brown) – goes
alternately negative and
positive, making current flow
backwards and forwards
through the circuit.
Fuse – always
connected to the
live wire. If the
current gets too
high the fuse will
‘blow’ and break
the circuit.
Cable grip – keeps
the wires inside
the plug securely.

15. Mains electricity
Plastic insulated casing.
Neutral wire
(blue)
Earth wire
(yellow and
green). Stops
metal appliances
becoming live if
there is a fault.
Live wire (brown) – goes
alternately negative and
positive, making current flow
backwards and forwards
through the circuit.
Fuse – always
connected to the
live wire. If the
current gets too
high the fuse will
‘blow’ and break
the circuit.
Cable grip – keeps
the wires inside
the plug securely.
Plastic wire outer
covering

16. 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

17. 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.

18. 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.

19. Mains electricity
Fuses

20. Mains electricity
Fuses
How does the fuse and earth wire
working together prevent shocks?

21. 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

22. 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

23. 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.

24. 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.
4. Fuses should be
rated as near as
possible but just
higher than the normal
operating current.

25. Mains electricity Fuses
What
fuse to
use?

26. Mains electricity Fuses
What
fuse to
use?
3A, 5A, 13A

27. 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.

28. 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

29. Mains electricity Fuses
What
fuse to
use?
3A, 5A, 13A
Eg. Hair dryer.
Power rating =
1.1kW
Voltage rating
= 230V

30. Mains electricity Fuses
What
fuse to
use?
3A, 5A, 13A
Eg. Hair dryer.
Power rating =
1.1kW
Voltage rating
= 230V
I = P/V
I = 1100 / 230
I = 4.8A
Use the fuse rated a little
higher than the normal current.
In this case, use a 5A fuse.

31. Mains electricity Fuses
What
fuse to
use?
3A, 5A, 13A
Eg. Television.
Power rating =
150W
Voltage rating
= 230V

32. Mains electricity Fuses
What
fuse to
use?
3A, 5A, 13A
Eg. Television.
Power rating =
150W
Voltage rating
= 230V
I = P/V
I = 150 / 230
I = 0.7A
Use the fuse rated a little
higher than the normal current.
In this case, use a 3A fuse.

33. Mains electricity Fuses
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.

34. Mains electricity Fuses
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.
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

35. 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

36. PHYSICS – Dangers of electricity