A Fuse is a Fuse? No???

[Dennis Alwon]

We all know that fuses are rated in amps but they are also rated in voltage. I don't understand why. If a 24V fuse is rated 10 amps does the voltage really matter. Why can't a 24v 10 amp fuse be used on a circuit of 120v if there is less than 10 amps? (ignore listing reasons) I know I am going to feel dumb when I hear the answer.

 

[templdl]

We all know that fuses are rated in amps but they are also rated in voltage. I don't understand why. If a 24V fuse is rated 10 amps does the voltage really matter. Why can't a 24v 10 amp fuse be used on a circuit of 120v if there is less than 10 amps? (ignore listing reasons) I know I am going to feel dumb when I hear the answer.

Think of it this way Dennis,
If a fuse were to simply melt and disconnect the power to the load that would be the simple way to look at is.
But is you were to think of the power at the fuses rating you wouuld have basically voltage x current that the fust would be seeing when it melts.
Now increas the voltage to let's say 120v per for example that poor punny fuse would have to deal with five times the power or 120v x 10a instead of 24v x 10a. That would equal a lot of heating and a the distinct posibility that you would tuen the fuse into a firecracker. Also, some thought should be give when the fuse does try to clear is there enough gap to overcome the 120v to clear the fault when before it was only 24v.
Dave

 

[K8MHZ]

We all know that fuses are rated in amps but they are also rated in voltage. I don't understand why. If a 24V fuse is rated 10 amps does the voltage really matter. Why can't a 24v 10 amp fuse be used on a circuit of 120v if there is less than 10 amps? (ignore listing reasons) I know I am going to feel dumb when I hear the answer.

The voltage determines the amount of arc over and the type of material the insulating part of the fuse is made of (Like glass, plastic or fiber).

When the fuse blows, the arc needs to quench quickly. If the voltage is too great, the arc will continue after the link has disintegrated, not providing adequate protection. Also, DC, not having a zero volt cross line, will arc longer.

Think of using a plastic 12 volt automotive mini fuse with 5kV. If the fuse blows, at 5kV it will turn into an arc lamp. That's a drastic amount of over voltage, but it makes it easier to understand why fuses have both voltage and current ratings.

 

[Dennis Alwon]

Thanks for the reply. I know that 600 amp fuses are much longer than 250V fuses so I assumed there was a reason but didn't wrap my head there.

So if a 250V fuse were used for 24 volts there wouldn't be that arc situation so would the fuse work effectively

 

[Dennis Alwon]

Think of using a plastic 12 volt automotive mini fuse with 5kV. If the fuse blows, at 5kV it will turn into an arc lamp. That's a drastic amount of over voltage, but it makes it easier to understand why fuses have both voltage and current ratings.

That part I sort of got but didn't think it would be as much a problem with the lower voltages.

Also why are some fuses rated 250V and others 125V-- seems odd unless it is because of other countries voltages.

 

[templdl]

Thanks for the reply. I know that 600 amp fuses are much longer than 250V fuses so I assumed there was a reason but didn't wrap my head there.

So if a 250V fuse were used for 24 volts there wouldn't be that arc situation so would the fuse work effectively

In theory one would think a 250v fuse would work knowing there is much less energy a 24v than 250v if you were able to get600a @ 24v. The fuse curve I'm sure would be severely altered a 24v. the melt time to clear the fuse would be extended wouldn'y you think? 600a x 250v Vs 600a x 24v.

 

[Besoeker]

We all know that fuses are rated in amps but they are also rated in voltage. I don't understand why. If a 24V fuse is rated 10 amps does the voltage really matter. Why can't a 24v 10 amp fuse be used on a circuit of 120v if there is less than 10 amps? (ignore listing reasons) I know I am going to feel dumb when I hear the answer.

Some 3.6kV fuses.



It's from a 350kW drive. From memory, those fuses on the left are, at most, 100A. For the PFC. And about a foot long.
As others have said, voltage matters.

By way of contrast:



It's a 45kW VSD panel. About half way down on the left hand side is a 100A ABB fuse switch. For scale, the trunking above and below is regular 40mm stuff - less than two inches.

 

[JDBrown]

In theory one would think a 250v fuse would work knowing there is much less energy a 24v than 250v if you were able to get600a @ 24v. The fuse curve I'm sure would be severely altered a 24v. the melt time to clear the fuse would be extended wouldn'y you think? 600a x 250v Vs 600a x 24v.

I don't think it works that way. The 250V or 24V is not the potential difference across the fuse (unless we're only talking about a hard short). Instead of talking about P = V x I, we ought to be talking about P = I^2 x R, since it applies regardless of whether the overcurrent is caused by a hard short or by connecting too much load. I see those little glass 250V fuses in UL listed 120V assemblies all the time (e.g. the fuses in Christmas light plugs, or in small DC power supplies). I've never seen/heard of a problem arising from using a fuse rated for a higher voltage on a lower voltage system (other than the fact that higher voltage fuses tend to cost more).

 

[K8MHZ]

I don't think it works that way. The 250V or 24V is not the potential difference across the fuse (unless we're only talking about a hard short). Instead of talking about P = V x I, we ought to be talking about P = I^2 x R, since it applies regardless of whether the overcurrent is caused by a hard short or by connecting too much load. I see those little glass 250V fuses in UL listed 120V assemblies all the time (e.g. the fuses in Christmas light plugs, or in small DC power supplies). I've never seen/heard of a problem arising from using a fuse rated for a higher voltage on a lower voltage system (other than the fact that higher voltage fuses tend to cost more).

I am with you.

The voltage rating is the maximum. As far as I know, there is no minimum.

 

[templdl]

I am with you.

The voltage rating is the maximum. As far as I know, there is no minimum.

I don't believe that I said that there as no minimum but one has to believe that the fuse curve itself would change at a lower voltage, that the fuse curve is base upon the fuses rated voltage and current. You can't change the fact that you a still dealing with power when a fuse clears at a higher voltage than at a lower voltage as well as the makeup of the fuse element.
I have a fuse bool "Electric fuses" by A. Wright & P.G. Newbery an Institution of Electrical Engineers power series 20. which has more formulas andcurves than you can shake a stick at and I'm not sure if I can make heads or tales out of it. When I'm on the john I have to take a look at to and see if I can figure it out.
The book is about 6"x9", 1/2" thick with about 200 pages. As such there must be enough about fuses a stretch 200 pages on the subject.
If not it at least it looks impressive.
Enough of this. I'll have to revisit this later as I have to go see the 10 year ol grand son play baseball this evening.

 

[JDBrown]

I don't believe that I said that there as no minimum but one has to believe that the fuse curve itself would change at a lower voltage, that the fuse curve is base upon the fuses rated voltage and current. You can't change the fact that you a still dealing with power when a fuse clears at a higher voltage than at a lower voltage as well as the makeup of the fuse element.
I have a fuse bool "Electric fuses" by A. Wright & P.G. Newbery an Institution of Electrical Engineers power series 20. which has more formulas and curves than you can shake a stick at and I'm not sure if I can make heads or tales out of it.

It will be interesting to see what you're able to find out. I'm used to seeing/hearing about time-current curves, and now I'm curious if there are any that include voltage as well. Let us know the results of your reading.

 

[K8MHZ]

I don't believe that I said that there as no minimum but one has to believe that the fuse curve itself would change at a lower voltage, that the fuse curve is base upon the fuses rated voltage and current. You can't change the fact that you a still dealing with power when a fuse clears at a higher voltage than at a lower voltage as well as the makeup of the fuse element.
I have a fuse bool "Electric fuses" by A. Wright & P.G. Newbery an Institution of Electrical Engineers power series 20. which has more formulas andcurves than you can shake a stick at and I'm not sure if I can make heads or tales out of it. When I'm on the john I have to take a look at to and see if I can figure it out.
The book is about 6"x9", 1/2" thick with about 200 pages. As such there must be enough about fuses a stretch 200 pages on the subject.
If not it at least it looks impressive.
Enough of this. I'll have to revisit this later as I have to go see the 10 year ol grand son play baseball this evening.

Enjoy the game! :thumbsup:

Glad you have your priorities straight!

Back to the topic, a fuse 'blows' because it's element is heated to it's melting point. Heat is a result of I (sq) R losses. I don't see voltage in the equation.

What I do notice is on dual rated fuses, the DC rating is always much lower than the AC rating, so in addition to the above, we must consider the zero voltage cross point. That said, a fuse designed for 120 volts AC may blow at a different current rating at 12 volts DC, not due to the lower voltage, but due to the lack of a zero voltage cross point.

So, if we are to compare a fuse with a 125 volt AC rating to one with a 12 volt rating, the 12 volts has to be AC as well. Which of course, won't happen, but we do have lot's of 24 VAC to work with.

 

[jim dungar]

What I do notice is on dual rated fuses, the DC rating is always much lower than the AC rating, so in addition to the above, we must consider the zero voltage cross point. That said, a fuse designed for 120 volts AC may blow at a different current rating at 12 volts DC, not due to the lower voltage, but due to the lack of a zero voltage cross point.

Along this line, be careful when comparing fuse curves. Most curves from manufacturers are shown as a single line, either a minimum melting curve or a maximum clearing curve. Software libraries, like those from SKM, often combine these curves into a single TCC which has width, similar to breaker curves.

 

[GoldDigger]

In theory one would think a 250v fuse would work knowing there is much less energy a 24v than 250v if you were able to get600a @ 24v. The fuse curve I'm sure would be severely altered a 24v. the melt time to clear the fuse would be extended wouldn'y you think? 600a x 250v Vs 600a x 24v.

The I/T curve of the fuse would not be changed at all. What would change is the I versus fault resistance curve, which could limit your ability to even reach the I/T curve in the first place, or force you toward a lower current, longer time, point on the curve.

BTW, a 12 volt automotive fuse, even if rated as high as 48 volts DC, will arc, melt, and/or burn, taking the fuse holder with it when used on a high voltage Photovoltaic input circuit, even without a high fault current availability. The rating of the fuse holder itself comes into play there too, as it can bypass the designed arc quenching path of the fuse.

 

[Dennis Alwon]

BTW, a 12 volt automotive fuse, even if rated as high as 48 volts DC, will arc, melt, and/or burn, taking the fuse holder with it when used on a high voltage Photovoltaic input circuit, even without a high fault current availability. The rating of the fuse holder itself comes into play there too, as it can bypass the designed arc quenching path of the fuse.

I think we all agree that the higher voltage is a problem for the low voltage fuse but will a 250V fuse work appropriately at 12V. I am not sure we have a clear answer on that

 

[GoldDigger]

I think we all agree that the higher voltage is a problem for the low voltage fuse but will a 250V fuse work appropriately at 12V. I am not sure we have a clear answer on that

The only problem I could foresee would be if the high voltage fuse was larger and required more energy to blow it for the same current rating. As a result the resistance of the fuse and the voltage drop, both absolutely and as a percentage of the system voltage, would be higher.
It would still protect just fine, but would be marginally more intrusive during normal use.

 

[Dennis Alwon]

The only problem I could foresee would be if the high voltage fuse was larger and required more energy to blow it for the same current rating. As a result the resistance of the fuse and the voltage drop, both absolutely and as a percentage of the system voltage, would be higher.

Are you saying that a 10 amp fuse at 24 volts has a finer fuse link then one at 250V?

 

[GoldDigger]

Are you saying that a 10 amp fuse at 24 volts has a finer fuse link then one at 250V?

No. I am saying that a 250 volt fuse might have a longer link than a 24 volt fuse, and also have more solid heat sinking of the ends (not by design though)

The fuse link wire small diameter portion of the distance between the fuse terminals will be longer on a 250 volt fuse, while a 24 volt or lower fuse could have a shorter "notch" with reduced cross-sectional area and high resistance. I doubt very much that you would be able to measure a difference in the overall resistance of the fuse without a very accurate low-ohm meter until you got up to very high voltages. I am willing to bet that the 3.6KV 100A fuses pictured will have a higher resistance than a 12 volt 100A fuse. But I have not measured them, so it is just speculation.

And, BTW, I would have expected the 250 to have a finer fuse link if anything. But I really think that the cross sectional area at the weakest point will be roughly comparable. Unless the material around the link in the HV fuse has a large effect on the heat dissipation from the element.

 

[jim dungar]

No. I am saying that a 250 volt fuse might have a longer link than a 24 volt fuse, and also have more solid heat sinking of the ends (not by design though).

The length of the fuse element does not affect its minimum melting curve. The I?r heating interacts with the cross sectional area of the element.

 

[mivey]

DC vs. AC is important as has been stated because of the AC zero crossing allowing for interrupting of higher currents.

For the most part, you can use any fuse up to it voltage rating. This general rule can fail when the fuse resistance causes a voltage drop that is a significant portion of the circuit voltage (like for circuits below 3 volts). This rule also fails when you jump to MV fuses as they operate differently. There are also some differences in interrupting ratings.

Here are some good reads for LV fuses that discuss these:
http://www.cooperindustries.com/con...ces/Data Sheets/Bus_Ele_DS_8005_Fuseology.pdf
http://www.littelfuse.com/data/en/Product_Selection_Guides/Fuseology.pdf

For some MV fuse information:
http://www.littelfuse.com/technical.../~/media/183A062E083248449510EE8E69E275B2.pdf
http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&dID=572634