FUSES explaining how fuses work fuses and earthing choosing the

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Presentation transcript:

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

EXPLAINING HOW A FUSE WORKS previous next

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). previous next

battery switch Short circuit fuse lamp previous next

battery switch Short circuit (fault) fuse lamp previous next

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 previous next

Answers Melt Circuit Current Fire

FUSES AND EARTHING previous next

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! previous next

Step by step analysis (with the case earthed) The fault develops (live wire touches the metal case) previous next

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 previous next

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 previous next

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 previous next

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 previous next

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 previous next

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 previous next

CHOOSING THE CORRECT FUSE previous next

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 previous next

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

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 previous next

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 previous next

alternating current (a.c.) Higher level previous next

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) previous next

+ + + _ _ 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) + + + _ _ previous next

+ + + +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) + + + _ _ previous next

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 previous next

Draw these oscilloscope traces into you books F 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

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!

END previous next