# Does a fuse blow at the same amperage regardless of voltage?



## booksix (Aug 26, 2008)

May sound like a dumb question, but, for example, will a 600 amp/300v fuse blow at 600 amp/144v? Also, I've seen a few ac/dc fuses... amperage still stay the same?


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## Frank (Dec 6, 2008)

In general, for the conditions you mention, I believe the answer is "yes." You can't run a low voltage fuse at higher voltages though. If it lasts it may blow spectacularly at lower current than wanted. Generally a fuse will blow at rated voltage after a specified time period which shortens as current rises. Manufacturers spec sheets can give you this information.


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## Qer (May 7, 2008)

booksix said:


> May sound like a dumb question, but, for example, will a 600 amp/300v fuse blow at 600 amp/144v?


No problem. An ordinary mechanical fuse blows (or rather melts) when the temperature in it goes too high and the temperature is a result of the current, not the voltage. However, you should never use a fuse for a higher voltage than it's rated for since it can arc if the voltage is too high, but using a 300 Volt fuse for 144 Volt is just peachy.

And no, it's not a dumb question, it's a very sane question. After all, you only have one life and there's no slot for new coins...



booksix said:


> Also, I've seen a few ac/dc fuses... amperage still stay the same?


Now it's getting tricky in here. 

An ordinary fuse, like a car fuse, that's all mechanical doesn't care about if it's AC or DC, it's still the same amount of heat that's needed before the metal melts, HOWEVER the voltage rating for AC and DC might differ since DC has bigger problems with arcing. But if a fuse is rated, say, 48 Volt DC, 48 Volt AC won't be a problem (but the opposite might be).

Other kind of fuses, like the ones that looks like switches and that can be reseted (don't know what they're called in English, sorry) might not work well with DC if they're made for AC and the opposite, so in those cases you must make sure there's a correct rating for the type of current you're going to use!

Additional questions? Just fire away and I (or someone else) will answer.


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## CPLTECH (Sep 14, 2007)

If I might add some to Qer’s correct answer…

In an industrial environment, it was explained to me that the voltage rating has to do with the possible aftermath of an over-current condition found inside the fuse casing. Therefore the insulated case must withstand the arcing as it melts without busting open, thus you will see differences between an automotive fuse and the type used in a microwave oven. Some are surrounded by sand, some are in a fiber tube, and some are totally visible inside a glass tube, etc. Yes, I have used 32V auto fuses in a 120V circuits and felt it was safe to do so under the conditions.
Perhaps this can help you decide what is acceptable under your circumstances.


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## Frank (Dec 6, 2008)

Another factor to consider is speed rating, usually "fast" or "slow." Fast blows are generally used to protect sensitive equipment whereas slow-blows are more insensitive to momentary surges. When I connected the heater in my EV conversion (120 volt pack) I only had an automotive fuse on hand (rated 32 VDC.) It blew in a heartbeat (20A recommended and this fuse was a 30A.) A 125 VDC fast blow didn't work due to a start up surge when first turning the thing on (I don't have any sort of inrush protection.) A 125 VDC slow blow will work but not with a freshly charged pack (pack sits at ~130 or so. I would have thought it'd work...) I now run a 250 VDC 30A fuse and can use the heater under any conditions. (BTW, it's connected with #10 wire; always remember to size the fuse to protect your wiring!)


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## booksix (Aug 26, 2008)

Thanks, got it... so an 800 amp fuse rated up to 600v would be fine for an 800 amp, 144 system...


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## Qer (May 7, 2008)

booksix said:


> Thanks, got it... so an 800 amp fuse rated up to 600v would be fine for an 800 amp, 144 system...


Provided you've taken into account all the stuff we've told you (environment, AC/DC, fast/slow etc) you should be home free, yes.


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## TX_Dj (Jul 25, 2008)

And so long as you're not confusing a fuse with a circuit breaker. One does not want to use an AC breaker with DC power.


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## booksix (Aug 26, 2008)

No, yeah, I knew that one. I most mix breakers.

One other question: any reason not to buy a used fuse?



Edit: also, do I want a slow blow for my traction system?


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## rfengineers (Jun 2, 2008)

booksix said:


> One other question: any reason not to buy a used fuse?


As long as the fuse is still shorted, I don't see any reason not to use a used fuse.



booksix said:


> also, do I want a slow blow for my traction system?


I would think so, you want something that can survive brief periods of over-current. A fast-acting fuse would not be a good choice for a load that draws a large start-up current, like a motor.


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## booksix (Aug 26, 2008)

Ok, that's what I was thinking (on both Q's). Thanks!


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