Old glass AGC fuses - what does voltage mean?

jerry_njJuly 26, 2010

I purchased a kit of AGC fuses as I needed a couple 10 amp 250 volt fuses. The kit contains 10 of each size 5 amp and 10 amp are 250 volt, then there are 15, 20, 25 and 30 amp rated at 32 volt.

Besides the obvious, what does the voltage rating mean in terms of the physical fuse? I mean all have the same external dimensions - the only apparent difference is the size of the "wire" inside, the fuse.

How would one get in trouble putting one of the 32 volt fuses in a 115 or 230 vac fuse holder?

I can post on the electrical forum too, but have started here as my past experience associates these fuses mostlly with older car electrical system. Yes, I know they are 12 vdc systems, unless we go way back to the 6 vdc systems.

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john_g

It has to do with the way the fuse is designed to open circuit when an overload occurs. As the fuse fails, there is a spark and current keeps flowing until the gap between the broken fuse becomes large enough that the voltage applied to the circuit can not keep the gap ionized and so current stops flowing. A 250v fuse when it blows has more space in-between the failed ends of the fuse than a 20v one would need.

    Bookmark   July 26, 2010 at 8:39PM
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jerry_nj

Thanks, I had some other input elsewhere that expressed the view that the amount of metal melted in the tube was too great in the larger amp fuses to interrupt the higher voltage, e.g., 250 volts. Not sure that this is a different mechanism, but it seems in either case the higher voltage rating is made more difficult as the "diameter" of break wire increases. That said, I'm sure it doesn't jump down all the way from 250 volts to 32 volts when the fuse wire grows from 10A to 15A, but the "standards" were set for just these two levels.

I suppose too that the idea is if you are going to "switch" or "fuse" high energy circuits, such as more 250 volts at 15 A (3,750 watts or Volt Amps) then get something more substantial that a little glass fuse.

    Bookmark   July 26, 2010 at 9:37PM
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jemdandy

AC - Alternating current (current oscillates from plus to minus, for example, in a circuit supplied by a transformer or an alternator without rectification)

DC - Direct current (current flows one way, for example, in a battery powered circuit.)

John_G is correct. The voltage rating on a fuse indicates its ability to clear a circuit in a safe and uneventful manner, and within a prescribed time specified by fuse standards under short circuit conditions. The maximum current availability of the supply circuit also affects clearing performance. Therefore, a fuse rated at 250Vac for use in a circuit up to 10,000 amp capability can be applied in an AC circuit up to and including 250Vac, and with short circuit current less than 10,000 amp.

The VDC rating may be less. DC circuits are harder to clear than AC circuits because in AC circuits, the current goes through zero twice per voltage cycle. This zero crossing aids in opening the circuit.

Never apply a fuse in a circuit where the supply voltage is more than the rating on the fuse.

The VAC and VDC ratings are usually different. Watch that difference. A fuse may be marked with only the AC voltage; More information is required to discover its DC rating.

    Bookmark   July 26, 2010 at 10:26PM
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jerry_nj

Thanks the fuse pack I purchased shows:

5A & 10A
IR 10,000A, 125Vac; IR 200A, 250 Vac

15A, 20A, 25A, & 30A show:
IR 1000A 32 Vac.

    Bookmark   July 27, 2010 at 9:41AM
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