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Old 8th November 2005, 02:03 AM   #1
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Default Fuses

I always wanted to know: fuses blow based on the current that is being drawn through them

==> why do fuses have voltage ratings?
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Old 8th November 2005, 02:29 PM   #2
anatech is offline anatech  Canada
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Hi rtarbell,
That's an easy one to answer. While the fuse is blowing, it has to interrupt the current flow. There is sometimes an arc to quench. The applied voltage, if too high, may continue to arc. At the very least, the current is not interrupted in the time specified for that fuse.

Ever price out 600V fuses? Ouch!

-Chris
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Old 8th November 2005, 02:54 PM   #3
dnsey is offline dnsey  United Kingdom
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BTW, it's not always appreciated that most fuses are rated at their maximum continuous current - the rupture current is often appreciably higher.
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Old 8th November 2005, 02:59 PM   #4
anatech is offline anatech  Canada
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Hi dnsey,
Correct. They are rated at the current that they will blow at in X amount of time. It is a temperature vs time thing. There are also time rating at 200% of current and others. The length of time to blow will surprise you.

-Chris
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Old 8th November 2005, 03:06 PM   #5
mzzj is offline mzzj  Finland
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Quote:
Originally posted by anatech
Hi dnsey,
The length of time to blow will surprise you.

-Chris
Especially slow-blow "household" fuses, they can take 2-3x nominal current for half an hour.

------
One rating more for fuses is max amperage breaking capacity, if you use normal small glass tube in 300v circuit with short-circuit current of 15kA it wont behave as fuse anymore..
No need to care about this normally, exept when buying those !#%"# expensive fuses for your Fluke.
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Old 8th November 2005, 03:07 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
Yes that time delay on rupturing is very fortunate.
It allows the output transistors enough time to protect the fuses and save having to replace them too often.

More sensibly use T fuses on the mains side to prevent spurious blowing at switch on, F fuses after the smoothing caps and do not fuse the speaker leads.
__________________
regards Andrew T.
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Old 8th November 2005, 03:10 PM   #7
mzzj is offline mzzj  Finland
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Quote:
Originally posted by AndrewT
Hi,
Yes that time delay on rupturing is very fortunate.
It allows the output transistors enough time to protect the fuses and save having to replace them too often.
Old joke about output transistors protecting fuses...
Or buy ultrafast semiconductor fuses, tough they may cost more than your output transistors
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Old 8th November 2005, 03:14 PM   #8
anatech is offline anatech  Canada
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Quote:
Or buy ultrafast semiconductor fuses, tough they may cost more than your output transistors
Music peak detectors.

I feel the transistors short to ensure the fuses blow. That way you can't sue the fuse manufacturer!

-Chris
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