I ran across a situation in an old building where one of the switches in the 208V Distribution Panel had a bucket with 600V Fuses in it. They are 60A as the circuit requires, but I was shocked to find 600V fuses. Upon inspection it appears that the fuse holders must have been swapped out for this to even work. As an engineering stand point this is absolutely incorrect just because of the rating. But my question comes after the statement of two co-workers who claim that you can safely use 600V fuses in place of 250V fuses but not vice versa. Obviously the 60A rating is the same but can 600V fuses sufficiently protect a 208V circuit?
600V Fuses in 208V Distribution Panel
- Thread starter ddderek
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petersonra
Senior Member
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- Northern illinois
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- Semi-retired engineer
600V fuses are fine in a 208V circuit.
kingpb
Senior Member
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- SE USA as far as you can go
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- Engineer, Registered
600V is rated voltage of equipment, the 208V is the system utilization voltage
hillbilly1
Senior Member
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- North Georgia mountains
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- Owner/electrical contractor
On some manufactures equipment, the switch is rated for 600 volt, but by moving the load side lugs and fuseholder to another pre-tapped position, alows the switch to use 250 volt or 600 volt fuses. The amperage that the fuse will blow at does not change, but the interupting capacity of the fuse is usually greater depending on the fuse type installed. As others have said, it will work fine at the lower voltage.
From a practical point of view and amp is an amp regardless of the voltage and as long s the fuse is rated equal of greater than the applied voltage, voltage is a non-issue.
But, when it comes to clearing time and its current limiting capabilities wouldn't heating be considered? A higher voltage with the overcurrent would equate to more heat than at a lower voltage which would clear the element quicker. As such a CL fuse probably will have different CL characteristics when applied at a lower voltage.
As such I believe that the actual application should be considered which may or may not be a factor. In a common application everyday application using a 600v fuse and applying it at 240v my be a non-issue as it may be like splitting hairs.
But, when it comes to clearing time and its current limiting capabilities wouldn't heating be considered? A higher voltage with the overcurrent would equate to more heat than at a lower voltage which would clear the element quicker. As such a CL fuse probably will have different CL characteristics when applied at a lower voltage.
As such I believe that the actual application should be considered which may or may not be a factor. In a common application everyday application using a 600v fuse and applying it at 240v my be a non-issue as it may be like splitting hairs.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
I see your point, but think of it this way. If that were the case, there would be a difference in clearing times between 600V and 480V even though the fuse is the same, and there aren't.
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- PE (Retired) - Power Systems
Voltage has nothing to do with melting time of fuses. - Period. Try to find a current limiting 250V Class L fuse or a Class CC, how many choices do you have?
Fuse voltage ratings have to do with re-strike and insulation.
According to Bussmann's SPD
Fuse voltage ratings have to do with re-strike and insulation.
According to Bussmann's SPD
Thanks Jim,
Then if I'm to understand you correctly a 600v applied at 240v would respond in the same way at both voltages. My reasoning was that the same current at both voltages would result in more heating watts of the fuse element which may change the fuse curve slightly. But it was only my theory and not a reality it appears.
I do understand the fuses ability to clear and the restrike and insulation issue which I delt with when representing medium voltage fuses. My thinking was that a fuse may clear slower at the lower voltage allowing for less CL ability which looks as if its not an issue.
I realize that that it is not reasonable to expect every combination of fuse ratings available for every combination as you basically stated.
I'm always interested in learning and understand.
600V Fuses in 208V Distribution Panel
Thanks for the replies... so it sounds like the voltage rating refers to the maximum voltage that fuse can be used at and anything less is ok? I incorrectly assumed that the combination of a higher voltage rating with the same amperage would cause different opening characteristics. That's why this forum can be so helpful, we can always learn from one another. And the moral of the story is I should have looked at the Buss website first!
Thanks for the replies... so it sounds like the voltage rating refers to the maximum voltage that fuse can be used at and anything less is ok? I incorrectly assumed that the combination of a higher voltage rating with the same amperage would cause different opening characteristics. That's why this forum can be so helpful, we can always learn from one another. And the moral of the story is I should have looked at the Buss website first!
kwired
Electron manager
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- NE Nebraska
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- EC
Amount of current that flows during a fault is going to depend on how much current the source can deliver, and impedance of the fault circuit. Amount of fault current during the incident will also make a difference in clearing time of the fuse regardless of what voltage rating may be.
It is perfectly legal to install a switchgear rated for 600 volts with 600 volt fuseholders on a 208 volt system. May cost a little more than 250 volt gear but is acceptable, and will give similar performance.
Remember that at lower voltages the available fault current typically will be higher than for same amperage installations at a higher voltage. If total energy available remains same but voltage goes down the current has to go up, so the available fault current on secondary of a 100kVA transformer is going to be higher if 208 volts vs if it were 480 volts and still 100 kVA transformer.
Good point. A certainly agree regarding you explanation of the available fault current if the availale fault current did in fact increase at the lower voltage. It makes sense. But when applying the 600a fuse of a 208 system that may have a lower fault current available then there will be less power to clear the fuse as quickly as if there where to be either a greater fault current available at the 208v and the same fault current available but a 600v. I know that with breakers it is the speed at which the contacts are opened that gives the breaker its CL ability. But breakers are mechanical and a fuse actually has an element that must be melted and/or broken so to speak in order to interrupt the current flow.
I guess my point is the speed at which the fuse element is cleared which may influence the fuses current limiting capability which may also limit down stream damage. This makes very good sense for fault currents but it is of my experience that the majority of faults are arcing faults which don't involve the fault current available which much less current flows. Wouldn't it then be a matter of E x I that would influence how fast the fuse element melts and clears as a 60a fuse is a 60 amp fuse whether it's applied a 208v of 600v.
Again, I agree that we are talking theory and it may be like shooting a dead horse but it make for an interesting discussion never the less. I may be even classified as "does it really matter?" or "who really cares?" or similar words to that affect.
kwired
Electron manager
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- NE Nebraska
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- EC
Some additional thoughts, and you have triggered some of them with things you mentioned.
If you have a long circuit you have less available fault current because of the resistance of conductors, so same 600 volt overcurrent device is going to have a slower response time if only thing different from one install to the next is the length of conductor from the supply.
Your typical fuses commonly seen for general purpose applications usually have higher interrupt ratings than most circuit breakers because the breaker is mechanical and not only must mechanically operate, but must also be able to reset when operated within its rating (seen this test done at Square D QO plant when I was in college, pretty impressive to a 18-20 year old anyway) A fuse only has to melt/ burn the element open without blowing up the case and does not have to be able to reset.
Arcing faults are more common on 277/480 than for 120/240 or 120/208. Higher voltage is able to jump a gap easier. The arcing fault does have higher impedance which limits the current so the application of a 600 volt fuse at a lower voltage is probably not much of a problem as the current in a fault will likely be higher as the chance of arcing fault is decreased. The 600 volt fuse is likely physically larger in an effort to make a longer gap so restrike of arcing or continued arcing is not likely. If you were to use a 250 volt fuse in a 480 volt application you may have higher risk of flashing across the fuse even though it did open the element.
I am not a fuse expert, these are just my observations and suspicions.
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