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Physics Question

Postby val101 » Tue Apr 01, 2008 5:16 pm

Hi all

I’m doing an assignment for physics class and its to do with electrics. The question is:

Which one of the following would cause an unsafe situation?

a. A 6 amp fuse in a 60 amp circuit
b. A 60 amp fuse in a 6 amp circuit
c. A 60 amp wire fuse in a 80 amp circuit
d. A 6 amp cartridge fuse in a 60 amp wire fuse way

I do not just want the answer but an explanation would be helpful in helping me understand why it is the answer, many thanks.
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Simply Build It

Postby ericmark » Tue Apr 01, 2008 8:25 pm

Fuse. A device which, by the melting of one or more of its specially designed and proportioned components, opens the circuit in which it is inserted by breaking the current when this exceeds a given value for a sufficient time. The fuse comprises all the parts that form the complete device.
The idea is to have some easy renewable bit of cable which will burn out before the slightly heavier main cable switches etc. on the same circuit. Thus saving the main wiring and switches from damage. So the sacrificial fuse will always be smaller in rated amps than any other part of the circuit.
As the description says the fuse melts and the casing which holds this fuse must allow it to melt without damage to itself. A 80 amp fuse carrier will be able to handle more heat than a 8 amp fuse carrier and if one was to put a 80 amp fuse in a 8 amp fuse carrier it may not be able to get rid of the heat quick enough and the carrier could melt as well as the fuse. So carriers and fuses are designed so you can fit a small fuse in large carrier but not the other way around.
Cartridge fuses contain the fusible link within a casing so it leaves no residue on the carrier which with a fuse link wire could build up and allow some current to track. Therefore, they can be shorter than a fuse wire. So a fuse wire can be replaced with cartridge fuse but not other way around. In reality this can’t normally be done as a porcelain lump on the fuse carrier increases the length of the fuse wire and this lump stops cartridge fuses being fitted.

Postby sparx » Tue Apr 01, 2008 8:56 pm

Hi, if you haven't worked it out from Erics excellent explaination the book answer is b)
All the others have the fuse as the 'weakest-link' in the circuit except b), in which the 6A rated circuit wiring would burn out before the 60A fuse new there was a problem,
regards SPARX
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Postby ericmark » Wed Apr 02, 2008 2:39 pm

He did say he didn't want the answer! Will you work out my Mesh-current and nodal analysis questions for me? Show me how to use determinants? (For any one else reading this we are talking about level 5 maths used by electrical engineers). I never though of asking questions here when doing my degree!

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