1. FUSES explaining how fuses work fuses and earthing choosing the
correct fuse
alternating
current(H)
resources

2. EXPLAINING HOW
A FUSE WORKS
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3. We should draw a fuse like this to show that the fuse is a thin
How does a fuse work?
We should draw a fuse like this
to show that the fuse is a thin
strand of wire which melts easily
if too much current flows ( a fault).
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4. battery
switch
Short circuit
fuse
lamp
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5. battery
switch
Short circuit
(fault)
fuse
lamp
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6. Too much current flowing causes the fuse wire to
HOW FUSES WORK
Too much current flowing causes the fuse wire to
This makes a gap and breaks the _________
so that no _________ flows.
The job of a fuse is to protect the device it is
connected to and to prevent electrical _______
from developing.
fault occurs
large current flows
heats and melts the fuse
breaks the circuit
stops the current flowing
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7. Answers
Melt
Circuit
Current
Fire

8. FUSES AND
EARTHING
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9. 4. OUCH! Step by step analysis for no earth
The fault develops (live wire touches the metal case)
2. The metal case becomes LIVE
3. A teacher wants to make a cup of coffee
4. OUCH!
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10. Step by step analysis (with the case earthed)
The fault develops (live wire touches the metal case)
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11. Step by step analysis (with the case earthed)
The fault develops (live wire touches the metal case)
2. Earth wire provides a low resistance path for current
so a large current flows
3. The fuse melts
and breaks the circuit
4. Electric current stops flowing and the live wire is now
safe
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12. Step by step analysis (with the case earthed)
The fault develops (live wire touches the metal case)
2. Earth wire provides a low resistance path for current
so a large current flows
3. The fuse melts and breaks the circuit
4. Electric current stops flowing and the live wire is now
safe
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13. Kettles with a plastic case don’t need to be earthed
Metal wire touches
the metal case
Case becomes LIVE
(230volts, a.c.)
This could cause
a fatal shock
Kettles with a plastic case
don’t need to be earthed
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14. Good earthing provides an easy path We call this easy path a
for the current
We call this easy path a
low resistance path
The low resistance
draws a big current
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15. It is vital that the fuse is the weakest link in the circuit
The large current
flows through the fuse
Large current will
cause heating
It is vital that the fuse is the
weakest link in the circuit
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16. The fuse melts and makes
a gap in the circuit
The current
stops flowing
The metal case is
not live any more
The fuse has protected
the kettle and us
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17. CHOOSING
THE
CORRECT FUSE
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18. Choosing the correct fuse Find out the current rating for the device
Choose the fuse which is just above this current
If the fuse is the same as the current rating you
must choose the next fuse up
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19. Choosing the correct fuse
Fuses available 2, 3, 5, 8, 10 and 13Amp
Device
Current rating
Correct fuse
lamp
0.25A
kettle
10A TV
0.5A
drill 4A

20. Choosing the correct fuse Fuses available 2, 3, 5, 8, 10 and 13Amp
Device
Current rating
Correct fuse
steam iron 8A
Dyson 6A
fan 3A
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21. Choosing the correct fuse Fuses available 2, 3, 5, 8, 10 and 13Amp
Device
Current rating
Correct fuse
lamp
0.25A 2A
kettle
10A
13A TV
0.5A
drill 4A 5A
steam iron 8A
Dyson 6A
fan 3A
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22. alternating
current (a.c.)
Higher level
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23. The current changes direction (a.c.)
The live wire goes above and below the axis
The neutral remains at 0Volts all the time
p.d.
(volt)
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24. + + + _ _ The live pd (voltage) goes above and below the axis
The pd (voltage) changes between + and – values
This only happens for the live wire
p.d.
(volt) + + + _ _
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25. + + + +230Volts +230Volts +230Volts -230Volts -230Volts 0Volts _ _
The mains voltage is 230Volts ac
Only the live wire alternates (changes)
The neutral wire stays at 0volts
+230Volts
+230Volts
+230Volts
-230Volts
-230Volts
0Volts
p.d.
(volt) + + + _ _
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26. Mains electricity is alternating current a.c.
Mains potential difference is 230volts
50 cycles per second (frequency is 50Hertz)
Normal current is about 10 Amps
If too much current flows the fuse melts
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27. Draw these oscilloscope traces into you booksF
Draw these oscilloscope traces into you books
Match the trace letter traces to the correct number
1 earth voltage
3 Small ac supply
5 1.5V battery
2 3Volt battery
4 large ac supply
6 flat battery

28. ANSWERS! 2 3Volt battery 1 earth voltage A D 6 flat battery
5 1.5V battery E B
4 large ac supply
3 Small ac supply F C
ANSWERS!

29. END
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