Author Topic: EQUIVALENCE OF AC AND DC FUSES  (Read 3838 times)

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jack11

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EQUIVALENCE OF AC AND DC FUSES
« on: July 19, 2016, 01:12:22 PM »
I need some 10A fuses to use in 120 or 240 VAC circuits, found them surprisingly high-priced on ebay.
But, I have a pile of automotive 10A fuses (for DC circuits), rated I believe at 32 VDC.

Looks like I should be able to use these DC fuses in AC circuits, no ???

1. the RMS AC amps rating should be equivalent to the constant DC amps rating (equivalent heating effects to burn the fuse on over-current)?

2. I still can't see how the fuse's voltage rating matters here? A fuse is for over-current protection. The voltage drop across the fuse is minuscule. Which voltage are we talking about?

Anyone can shed more light on this.

Note this post is different from my EQUIVALENCE OF AC AND DC CIRCUIT BREAKERS post from some time ago.

DamonHD

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Re: EQUIVALENCE OF AC AND DC FUSES
« Reply #1 on: July 19, 2016, 03:31:46 PM »
Avoiding arcing when the fuse is blowing/blown will be a factor in voltage rating.

So no, I don't think that using one rated far below the voltage you intend to deploy it for is a good idea.

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joestue

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Re: EQUIVALENCE OF AC AND DC FUSES
« Reply #2 on: July 19, 2016, 04:34:28 PM »
3ag fuses and fuse holders are usually cheap.. unless you want military grade ones, which i have a few of and they are so much more solid, water proof, and won't melt if you hold the soldering iron on the tab too long.

automotive dc fuses are more like short pieces of wire than they are a fuse, they would probably reliably break a 120vac circuit, but i don't think i would use them on a 240v circuit. the reason why i say that is often there is only 1/16th inch gap between relay contacts that are rated for 277vac. and that's about how wide the gap is on automotive fuses.
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oztules

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Re: EQUIVALENCE OF AC AND DC FUSES
« Reply #3 on: July 20, 2016, 03:36:41 AM »
Providing the higher voltage is AC then I see no problem.

It is very difficult to keep the arc going with AC as the voltage drops through zero every half cycle.. and so quenches the arc very quickly.

.............oztules



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jack11

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Re: EQUIVALENCE OF AC AND DC FUSES
« Reply #4 on: July 20, 2016, 01:45:31 PM »
OK, thank you all for good comments, I can see the reasons now.

I believe I'll do some testing in 120 and 240 VAC circuits to see what happens.

dnix71

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Re: EQUIVALENCE OF AC AND DC FUSES
« Reply #5 on: July 20, 2016, 09:14:03 PM »
I assume you are looking for the standard long glass appliance fuses. Those are rated 240v even though the usual appliance that uses them is 120v. There are fast and slow blow types. I assume you want fast blow.

If you still have a Radio Shack nearby, they have the correct fuses for $3.49 for a pack of 4.
https://www.radioshack.com/products/10a-250v-ceramic-fuse-for-microwave-ovens-4-pack?variant=5717232709

Home Depot charges more http://www.homedepot.com/p/Cooper-Bussmann-ABC-Series-10-Amp-Silver-Electronic-Fuses-2-Pack-BP-ABC-10/100207527

jack11

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Re: EQUIVALENCE OF AC AND DC FUSES
« Reply #6 on: August 01, 2016, 11:54:28 AM »
... as an update, ever since I've posted this I've been using the small (3/4" long) blade-type (but not mini) automotive fuses in one of my 120Vac circuits.
It works just fine.
I even blew a couple of them on purpose to see if an arc forms - it did not.

But, the problem is small physical size, and a small distance between the remaining wire ends once the fuse blows, and as Joe was saying the use in 240Vac circuits may be questionable.
I am sure I'll test them under 240Vac at some point in time, I'll post the result then.

kitestrings

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Re: EQUIVALENCE OF AC AND DC FUSES
« Reply #7 on: August 01, 2016, 04:04:55 PM »
I've never done this, but I have gone the other way - used AC glass fuses for low voltage DC circuits.

They also used to make replaceable link for AC cartridge types, where you unscrew the ends to release the link.  I still have these on my old (piston) water pump which now has a 48V motor.  I usually start with a link that won't quite hold the load, and then go up one size.  I probably wouldn't do this today - DC breakers are readily available - but it does work.

~ks

jack11

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Re: EQUIVALENCE OF AC AND DC FUSES
« Reply #8 on: August 13, 2016, 02:04:00 PM »
seems like a safer way to go would be to use DC fuses in AC circuits, not the other way around.

As oztules was saying, the AC arc extinguishes at 120 Hz, the DC arc does not, so the DC fuses should be more robust with respect to how they control arcing at their rated voltage.

jack11

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Re: EQUIVALENCE OF AC AND DC FUSES
« Reply #9 on: August 13, 2016, 02:22:51 PM »
... however, if this max rated voltage is say RV-volts for a DC fuse in a DC circuit, this fuse would be subject to 1.41 x RV (peak voltage, not RMS voltage) if it was used in a RV-volts RMS AC circuit, so you have to watch out there.