Using
Electricity
Prepared in Jan 1999
Second editing in March 2000
Learning objectives
At the end of this unit you should be able to :
1. show understanding of the use of fuses and fuse
rating.
2. give the meaning of the term live, neutral and
earth.
3. give the reasons for switches and fuses in the live
leads.
4. show understanding of the use of ring circuit for
household.
Electric
Electric cables
cables
– the cable for lighting consists of live wire and
neutral wire.
– Live wire (brown colour)
neutral wire (blue colour)
live wire
Electric lighting
neutral wire
(continue on next slide)
Electric
Electric cables
cables
– its cable consists of three types of wires,
namely live wire (brown colour), neutral
wire (blue colour) and earth wire (yellow /
green).
Live wire
Electric Appliance
Neutral wire
Earth wire
Quiz
Name them.
Fuse
• Fuse is a safety device and can prevent
the cable from overheating if more
current passes through it.
• It is always installed on the live wire.
• It is designed to cut off the current if the
appliance passes more current than it is
Fuse
supposed to.
• Fuse rating:
– 1A, 3A, 5A, 10A, 13A,
– 15A, 20A, etc
Circuit
Circuit Breakers
Breakers
Switches
Switches
• Switch always connects to the live wire.
(Why?)
• These switches are designed for different
purposes. Where it is the best choice?
Fuse
Fuse plugs
plugs
• Practice wiring a
plug, taking care not
to damage the
insulation or the wire
by cutting too deeply.
• Ensure that the cordgrip screws are firm.
• Fit a fuse of correct
value.
earth
neutral
live
GCE O-Level
Past Examination Paper
Science (Physics)
All rights go to University of Cambridge Examinations Syndicate and other sources
November 1989
18. Which one of the following should not be
connected directly to an a.c. supply?
A
B
C
D
electric fire
electric kettle
moving-coil meter
power drill
C
Nov 1991
16. A plug connected to a table lamp contains a 3A
fuse.
Why is the fuse needed?
A
B
C
D
to make it easier for the current to flow
to increase the resistance of the circuit
to reduce the voltage across the lamp
to protect the wiring from overheating
D
O’ level Physics
Nov 1995
17. The earth wire of an electric appliance should
be connected to the
A
B
C
D
fuse
metal case
ON/OFF switch
plastic handle
B
GCE O
Nov 1996
17. In an a.c. (household) electrical circuit, why
should the switch for any device (such as an
electric kettle) always be connected to the ‘live’
lead ?
A no current ever flows in the neutral lead
of the device.
B the device will be shorted if the switch is
in the earth lead.
C the device can never be switched off if the
switch is in the neutral lead.
D the device can only be isolated (made safe)
if the switch is in the live lead.
D
Nov 1998
16. A plug connected to a table lamp contains a 3A fuse.
Why is the fuse needed?
A
B
C
D
to increase the resistance of the circuit
to make it easier for the current to flow
to protect the wiring from over heating
to reduce the voltage across the lamp
C
Nov 1997
8. (a) Explain why the metal case of electrical
equipment which is operated from the mains
supply should be earthed. ( Your answer should
[4]
mention the need for a fuse.)
When there is a electrical fault, the leakage
electricity will then diverge to the ground by
the earth wire if the earth wire is connected
to the metal casing. This process will be
minimized the danger given by the electric
shock.
Fuse is needed to control the quantity flow of
electricity from the source. It is served as
safety device.
(continue in next slide)
(Cont. …) Q 8
Nov 1997
8(b) Explain why thin wire should not be used in
the electrical system of an office building, even
though this would be cheap.
Thin wire has high resistance and will then
convert a lot electric energy into heat. If the
input current is too high, it might cause the wire
overheated and might lead to fire.
(Cont. …) Q. 2
Nov 1990
2.(b)(i) The cable used to connect the kettle to the
mains supply has three wires. Describe to
what part of the kettle each of these wires is
[5]
connected.
Live wire connects to input of heating
element. Neutral wire connects to output
of heating element. Earth wire connects to
metal casing of the kettle.
(continue in next slide)
(Cont. …) Q. 2
Nov 1990
2(b)(ii) In which wire should a fuse be placed?
Explain your answer by describing what
happens when a fault develops which causes
[3]
the fuse to ‘blow’.
Fuse must placed in live wire.
When exceed electric current flows from
the source to the equipment through live
wire. If the value of ampere of electricity
is much higher than the limitation of the
fuse, fuse will melt and stop the electricity
supply. It serves as safety devise.
(continue in next slide)