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Fuses - breaking capacity, rupture capacity - fuse resistance

Nanren888

Nov 8, 2015
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Was asked one today that I could not answer.
Appreciate any knowledge from any one who knows, has monitoed current when a fuse breaks, or opinion.
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Calculations for the current that a fuse must successfully break.
My friend argues that including the fuse normal resistance in the calculation as a current limiter is inappropriate because as the fuse breaks, the plasma, molten fuse, may have significantly lower resistance. Hence it must break the current possibly equivalent to the circuit without the fuse resistance.
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Seems plausible.
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Anyone know this? Or a standard for such calculation? Or behaviour of current at that point in time?
Thanks in advance.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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There breaking current requirement has more to do with the source than the nominal load.

As one example, you will find HRC fuses in multimeters that are rated to be used with high energy power sources even though the multimeter itself is never going to draw anything like the breaking current of the fuse.
 

Tha fios agaibh

Aug 11, 2014
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Breaking capacity is also known as interrupting rating or IR rating.

Its basically the rating of the maximum amount of instantaneous current available at its source that can be safely interrupted if there was a short circuit.

Even if a fuse has an amp rating much less than the load it can explode like a bomb if it has insufficient IR rating because its not robust enough to handle the amounts of energy available.

This is a extremely basic explanation but the determination takes into account the full load current, the impedance of the circuit, and the available short circuit current available.
 

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Minder

Apr 24, 2015
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Was asked one today that I could not answer.
Appreciate any knowledge from any one who knows, has monitoed current when a fuse breaks, or opinion.
.
Calculations for the current that a fuse must successfully break.
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A little like the question, 'How long is a piece of string'

All fuses have different characteristics.
From delayed action to rectifier fuses, fast sweep through etc.
M.
 

Nanren888

Nov 8, 2015
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Many thanks for the tutorial material on fuses.
I'll try to be more clear.
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When the fault occurs, the current will relate to the total circuit impedance, as taken by the fault current.
If, worst case, the fault is immediately after the fuse, then no intended load impedance is involved.
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As pointed out by *Steve*, this will be dominated by the source impedance.
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As also brought up by Tha fios agaibh, the impedance of the circuit, determines the maximum current.
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Given that, and acknowledging that the question was rather academic in most cases, but should the fuse impedance be included in the impedance of the circuit limiting the maximum fault current (along with the source impedance)?
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A: Yes the fuse impedance is in circuit as it blows.
B: No, the fuse turns to plasma & has possibly a lower impedance as it blows, therefore the fuse's impedance does not limit the maximum current requiring to be broken.
 

Tha fios agaibh

Aug 11, 2014
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Not exactly what I said.
Impedance is a factor but it pales in comparison to the circuit resistance and the available energy at its source.

If you have a bolted short the only significant impedance is in the upstream transformer supplying it, not the fuse itself.
 
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