mivey
Senior Member
Riddle me these: Why can't I use 30 volt wire on 600 volt circuits? Why can't I run bus bars at 600 volts using the same circuit run spacing I have in my wrist watch?
A Fuse is a Fuse? No???
- Thread starter Dennis Alwon
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RichB
Senior Member
- Location
- Tacoma, Wa
- Occupation
- Electrician/Electrical Inspector
here's my favorite answer to questions like this--always seem to get it when no one has given a complete answer yet--"because we have always done it that way.":thumbsup::angel:
mgookin
Senior Member
- Location
- Fort Myers, FL
It's the insulation on the wire just like spacing on a circuit board. Volts are "jumpy" and the more the volts, the more the jump.
I reviewed the book on fuses that I referred to and found that voltage was used in some of the calculations but there was no references made to voltages in regard to how it may affect a fuse curve, only the maximum voltage rating of the fuse.
Even though it is of my opinion that a fuse curve is established at by its rated max voltage there the fuse curve must change when the voltage is reduced. For a fuse to work the must be a voltage drop developed across it as it response to an over current to cause heating to melt the fuse element. Should you consider a 600v fuse at 100a but use it to protect a 12v circuit one would think that the fuse curve would have to change based upon a lower voltage drop across the fuse when the element melts.
I think it should have to. The one thing that I regrets at this time is that I no longer have any access to the fuse engineers that I once had some 15 years ago. I would have had no trouble running this past them.
However, I have found nothing that links to applying a fuse at ratings lower than its maximum rating which seems to tell me that it's a mute point. I believe logically thinking that voltage would affect the fuse curve but is so minor that that the results must be insignificant at best. What I do know is that with TM circuit breakers voltage does is not factored in the trip curve when applied within its rating. I suspect fuses respond in a similar fashion.
Even though it is of my opinion that a fuse curve is established at by its rated max voltage there the fuse curve must change when the voltage is reduced. For a fuse to work the must be a voltage drop developed across it as it response to an over current to cause heating to melt the fuse element. Should you consider a 600v fuse at 100a but use it to protect a 12v circuit one would think that the fuse curve would have to change based upon a lower voltage drop across the fuse when the element melts.
I think it should have to. The one thing that I regrets at this time is that I no longer have any access to the fuse engineers that I once had some 15 years ago. I would have had no trouble running this past them.
However, I have found nothing that links to applying a fuse at ratings lower than its maximum rating which seems to tell me that it's a mute point. I believe logically thinking that voltage would affect the fuse curve but is so minor that that the results must be insignificant at best. What I do know is that with TM circuit breakers voltage does is not factored in the trip curve when applied within its rating. I suspect fuses respond in a similar fashion.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
There are fuses that are rated 250V as well as 125V . I know I can use a 250V fuse on a 120v circuit. I guess the question gets more complicated with wider ranges of voltages. IDK.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I know that this is the case but I don't know why other than voltage must somehow be related to heat. I have heard that high tension wires have no insulation because they would burn the insulation off but I assumed that was amperage not voltage but apparently voltage does come into play
K8MHZ
Senior Member
- Occupation
- Electrician
I don't think that's the reason high tension wires (cables, actually) have no insulation. If they got that hot, rain water would boil on them and we would see them steaming.
I think the reason is added weight. Just think of the thickness of insulation required for 315,000 volts.
It is true that open conductors are rated for more current, but I really don't think they get all that hot in the wild.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
The voltage between the two ends of the fuse when it is open definitely affects the fuse and holder design, but I see no way that it has any effect on the operation of the fuse while it is still intact.
For very high voltages, corona discharge from sharp points, etc. comes into play in the design of both the fuse and the holder.
I do not see any relationship between high operating voltages and presence or absence of insulation for the same current (except what ggunn mentions!). For high voltage circuits the voltage drop of the wiring becomes a non-issue and only the heat dissipation remains as a limiting factor. When you have to think about stretch resulting from increased conductor temperature, you are in the NESC world, not the NEC world.
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ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
I think the reasons are interdependent. If the wires were insulated for 315000V, it would indeed be very thick and therefore hold in a lot of heat as well, so ampacity would go down and voltage drop would go up. When you consider using insulator that would be practical from a weight and heat transfer perspective, it wouldn't be rated for anywhere near the voltage needed, so why bother insulating it at all?
I don't think is a burning insulation issue but more that insulation is not needed is you are referring to the high tension wires on poles and towers. Insulation when used may be more to protect the words from corrosion I believe I was told some years ago.
Remember that underground HV cable must be insulated.
JDBrown
Senior Member
- Location
- California
- Occupation
- Electrical Engineer
The issue here is that the voltage drop across a given fuse will be the same, regardless of system voltage (to a point), for the same current. For the sake of simplicity, let's say we have a 600 Volt, 100 Amp fuse with a resistance of 0.1 Ohm. If we try to pull 150 Amps through that fuse, the voltage drop across the fuse will be 150A x 0.1 Ohm = 15 Volts. Whether our system voltage is 24V or 600V, the voltage drop across the fuse link will be 15V and the I2R heating of the fuse link will be (150A)2 x 0.1 Ohm = 2.25kW.
But what happens when we put that same fuse into a 12 Volt system and try to pull 150 amps through it? Now we can't get a voltage drop of 15V across the fuse link, because we only have 12V available. You can't pull 150 amps through a 12V system with a 0.1 Ohm series resistance, because it's limited to 12V / 0.1 Ohm = 120 Amps. Still enough to blow the fuse, but we start to see a pattern emerging.
Now let's consider this same fuse in a 5V system (I know we're getting to the ridiculous point, but this is just a thought experiment). At 5V, we can only hope to pull 0.5V / 0.1 Ohm = 50 Amps even if the fuse is the only load in the circuit. It is, of course, possible to draw 100 or more Amps at 5 Volts, but not with this fuse in the circuit. We would need a different fuse with a lower resistance.
My apologies for thinking out loud (in a sense). I think what we can see here is that using a fuse over its voltage rating can create issues with arc quenching, and using a fuse far under its voltage rating may introduce an unacceptable (for the application) series resistance into the circuit. Of course, the examples above are a bit extreme -- the last time I measured a fuse with an ohmmeter, it was well under 0.1 Ohm -- more along the lines of 0.002 Ohm. I don't trust my el-cheap-o meter at that level of precision, but if we assume it's right then a hard short at even 5 volts would theoretically be able to pull 5V / 0.002 Ohm = 2500 Amps. (I'd like to see that 5V power supply
)I guess what I'm getting at is, while there is a theoretical low voltage limit for fuses, in real life it's probably pretty hard to reach. And as long as you don't have a voltage so low that putting the fuse in your circuit drastically alters the resistance in your circuit (and let's face it, if the circuit is that sensitive to a tiny resistance like a fuse, you're dealing with microelectronics), the system voltage has no effect on the I2R losses in the fuse, which is what will cause the fuse link to melt.
K8MHZ
Senior Member
- Occupation
- Electrician
I don't trust my el-cheap-o meter at that level of precision, but if we assume it's right then a hard short at even 5 volts would theoretically be able to pull 5V / 0.002 Ohm = 2500 Amps. (I'd like to see that 5V power supply
Spot welders have such power supplies.
5 volts and hundreds of kVA
This has certainly been a brain draining discussion. But I'm a big fan that you can't have I2r losses without having voltage Given that a fuse basically transforms itself into a load as it goes in its self destruct mode one would like to think that a lower voltage across the fuse would relate to less energy in order to clear the fuse. Is it slitting hairs? It looks as if it may be because if it were an issue I would like to think that there would be published about it.
JDBrown
Senior Member
- Location
- California
- Occupation
- Electrical Engineer
We probably are splitting hairs at this point. I agree that you need voltage in order to have current flow (I'd better agree with that statement
). My main point is just that the voltage drop across the fuse is not the same as the system voltage, unless there is a hard short just after the fuse.If we're talking about an overload of equal amperage on two different systems with different voltages, then the voltage across the fuse will be the same. Using my earlier example of a fuse with a resistance of 0.1 Ohm and a 150 Amp load current, Ohm's Law says that the voltage drop across the fuse has to be 15 Volts. It doesn't matter if the system voltage is 24V or 600V, the fuse only "sees" the 15V that we can measure across it.
If, on the other hand, we're talking about a short circuit, then the fuse will blow faster at 600V than at 24V, but that's because the current will be higher: 600V / 0.1 Ohm = 6000 Amps vs. 24V / 0.1 Ohm = 240 Amps. That doesn't mean we need different characteristic curves for different voltages. We're still on the same time-current curve, we're just at different points (6000A vs. 240A) along the curve.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Thank you all for the help. I guess it wasn't that simple a question. I guess what go me thinking was why they had fuses rated 250V and others 125V . That got expanded to what would happen if....?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I think that 250 and 125 are close enough that none of our devious speculations about problems using a fuse below its rated voltage apply.Thank you all for the help. I guess it wasn't that simple a question. I guess what go me thinking was why they had fuses rated 250V and others 125V . That got expanded to what would happen if....?
250 and 12, on the other hand.....
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Miniature 'glass body' fuses tend to be offered in many different voltage ratings where 'sand-filled' fuses tend to be offered in a more limited variety of voltage classes.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
If you look on the end caps of glass tube automotive fuses I believe you'll find that most of them are rated at 250V.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
That's 250 AC though! But point taken.
A fuse has very little voltage across it because there is an extremely low resistance, then the fuse is subject to an over current event that starts to destroy the fuse element, in the process of clearing the event would not the resistance of the fuse increase at some rate the voltage across the fuse increases to the applied voltage without considering other Dynamic contributing factors.
Obviously the fuse manufactures do not seem to be too concerned about voltages that are less than the fuse rating. They published one curve as I'm aware of. Would a curve for a 250v rated fuse be the same if the curve were to be recalculated at 120v or 24v etc? I don't think they would be the same as when the fuse element is in the process of clearing the voltage has to go up across the fuse.
Thus one would think that the increase in voltage would influence how fast the fuse clears the event. One would like to think that a fuse would start to react to the same over current even at all voltages up to its rating but it is what happens after that which may be determined by the actual applied voltage.
If a fuse has very little voltage across it because there is a very low resistance, then the fuse is subject to an over current event that starts to destroy the fuse element, in the process of clearing the event would not the resistance of the fuse increase at some rate the voltage across the fuse increases to the applied voltage without considering other Dynamic contributing factors.
Obviously the fuse manufactures do not seem to be concerned about voltages that are less than the fuse rating. They published one curve as I'm aware of. Would a curve for a 250v rated fuse be the same if the curve were to be recalculated at 120v or 24v etc? I don't think they would be the same as when the fuse element is in the process of clearing the voltage has to go up across the fuse.
Thus one would think that the increase in voltage would influence how fast the fuse clears the event.
It seems to make some sense but may be irrelevant in the real world. But you would have to wonder if you had to coordinate a 100a 600v rated fuse with another 100a 250v when applied a 120v. How would they respond when they are require to clear the same over current event.
What I do know is that a 480v 100at breaker will not have another trip curve should it be applied at 120v. If it doesn't have an electronic* trip it will have a bimetallic element* which responds to being heated as a result of over current. There are no trip curves based upon a given set of voltages. Even for DC other that kaic and voltage rating the trip curve doesn't differentiate between voltages.
Obviously the fuse manufactures do not seem to be too concerned about voltages that are less than the fuse rating. They published one curve as I'm aware of. Would a curve for a 250v rated fuse be the same if the curve were to be recalculated at 120v or 24v etc? I don't think they would be the same as when the fuse element is in the process of clearing the voltage has to go up across the fuse.
Thus one would think that the increase in voltage would influence how fast the fuse clears the event. One would like to think that a fuse would start to react to the same over current even at all voltages up to its rating but it is what happens after that which may be determined by the actual applied voltage.
If a fuse has very little voltage across it because there is a very low resistance, then the fuse is subject to an over current event that starts to destroy the fuse element, in the process of clearing the event would not the resistance of the fuse increase at some rate the voltage across the fuse increases to the applied voltage without considering other Dynamic contributing factors.
Obviously the fuse manufactures do not seem to be concerned about voltages that are less than the fuse rating. They published one curve as I'm aware of. Would a curve for a 250v rated fuse be the same if the curve were to be recalculated at 120v or 24v etc? I don't think they would be the same as when the fuse element is in the process of clearing the voltage has to go up across the fuse.
Thus one would think that the increase in voltage would influence how fast the fuse clears the event.
It seems to make some sense but may be irrelevant in the real world. But you would have to wonder if you had to coordinate a 100a 600v rated fuse with another 100a 250v when applied a 120v. How would they respond when they are require to clear the same over current event.
What I do know is that a 480v 100at breaker will not have another trip curve should it be applied at 120v. If it doesn't have an electronic* trip it will have a bimetallic element* which responds to being heated as a result of over current. There are no trip curves based upon a given set of voltages. Even for DC other that kaic and voltage rating the trip curve doesn't differentiate between voltages.
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