Electriman
Senior Member
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- TX
I was wondering if you guys see anything wrong for to use a a 25 kV fuse on 15 kV line? it does not seems to be a mistake? Any idea?
25 kv Fuse on 15 kV line
- Thread starter Electriman
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rlundsrud
Senior Member
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- chicago, il, USA
While this is just a WAG, I don't think it's an issue. If it was the other way around that would be a problem. As long as it is rated for the correct current level it should be fine.
mivey
Senior Member
The 25 kV fuse might require a longer cutout. Depends.
However, using a higher kV fuse on a lower kV system will change the melting characteristics. Think of the fuse as a resistor. The link is melted by applying adequate energy during an overcurrent.
More energy is required to melt a higher voltage fuse so it should react slower than expected.
However, using a higher kV fuse on a lower kV system will change the melting characteristics. Think of the fuse as a resistor. The link is melted by applying adequate energy during an overcurrent.
More energy is required to melt a higher voltage fuse so it should react slower than expected.
petersonra
Senior Member
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- Northern illinois
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- Semi-retired engineer
Generally fuses don't work that way. They generate heat from the current flowing through them (I^2R). Voltage level does not make a lot of difference. It is like a resistor but the amount of heat is strictly a function of I^2R. note V is not in the equation.
It is possible that the TCC characteristics for 15KV fuses are different from 25KV fuses.
Electriman
Senior Member
- Location
- TX
Thank everyone. I will make sure that TCC curve matches with my application.
V is a factor
V = iR = drop across fuse or i = V/R
i^2 R = (V/R)^2 R = V^2 / R
so if R is constant i will be lower with a lower V, less energy = slower melt
how much? who knows
i^2 t is also a factor
if t is greater but i is small how does that affect let thru?
they make them in different kv classes for a reason
what are the specific fuses?
mfg/model/ratings
mivey
Senior Member
Look at a 10A 120 volt fuse and a 10A 69 kV fuse. A bit extreme but you get the idea. The characteristics will vary.
The difference between a 25 kV recloser curve and a 15 kV recloser curve is not much so there may not be a lot of difference in a 12 kV fuse and 25 kV fuse. I have substituted reclosers.
I have never switched fuses as I was always able to get the correct one whenever needed. Common stock item.
Add: also think about the arcing for an expulsion fuse. The arcing produces the gases needed to blow out the arc. The arc disintegrates the inside of the barrel to produce the gas needed. 25 kV will not arc the same as 12 kV.
mivey
Senior Member
It seems that using a higher voltage rated fuse may not be as big of a deal with expulsion fuses but it does seem to be a problem with current-limiting fuses because of the higher arc voltage and equipment BIL rating.
I'm going to have to read some more as my prior assumptions may be completely invalid.
kwired
Electron manager
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- NE Nebraska
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- EC
I was thinking along the same lines as you here, and I don't really know that voltage rating has too much to do with trip curves, they mostly should respond to current.
Sure more voltage on same circuit may possibly result in more current (and more power) being pumped into a fault - but how much current can flow is also dependent on impedance of the source and supply conductors just like it is for lower voltages isn't it?
How much volts drops across a fuse when operating volts is in the 15kV+ range? If you had 100 volt drop @ 10 amps that is 1000 watts of heat going to develop within that fuse regardless of if the supply is 500 volts or 25,000 volts.
But seems that would be an awful high resistance for a fuse, maybe you see such VD if there is bad fuse holder involved but not in the fuse itself. And at 15,000 operating volts that is "overload" trip levels, short circuit or ground faults is going to be much higher current and much more rapid heating of the fuse material.
I am not a MV guy, but Ohm's law doesn't change just because the voltage is significantly higher either.
meternerd
Senior Member
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- Athol, ID
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- retired water & electric utility electrician, meter/relay tech
Been a while, but as I recall, a recloser (and it's associated bushings) used on 7,200/12,470 is nameplate rated 15KV and a 14,400/24,940 is rated 25KV. Their ratings were based on line to line, not line to neutral, even if it was a single phase line to neutral recloser. If it was used on a 7,200 single phase circuit, it was rated 15KV. Not sure why. I think fuses were the same, but I'm not sure. Been retired too long.
Last edited:
kwired
Electron manager
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- NE Nebraska
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- EC
Been a while, but as I recall, a recloser (and it's associated bushings) rated 7,200/12,470 is nameplate rated 15KV and a 14,400/24,940 is rated 25KV. Their ratings were based on line to line, not line to neutral, even if it was a single phase line to neutral recloser. If it was used on a 7,200 recloser, it was rated 15KV. Not sure why. I think fuses were the same, but I'm not sure. Been retired too long.
But don't those voltage ratings have more to do with insulation rating and "open circuit" ratings more so then actual trip curves?
A 25kV switch shouldn't be a problem with any kind of flashing across open contact if only 15kV is what is being supplied, but the other way around could be a problem.
Yet a 10 amp fuse should have same trip curve on either voltage, it just needs to have proper voltage rating for safely interrupting same level of current, as well as interrupt ratings that are high enough for available fault current.
meternerd
Senior Member
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- Athol, ID
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- retired water & electric utility electrician, meter/relay tech
Sounds reasonable regarding a switch, but I'm no protection engineer. It could be that an expulsion fuse requires a certain amount of "power" to clear a fault properly. Since a fuse is basically a resistor, using a fuse at half of the voltage rating would require 4X the "current" to produce the same power available at the fuse during a fault. P=E sq/R Ohm's Law, not mine..... Could be that a lower than rated voltage would allow a longer duration arc, negatively affecting the trip time, especially on an expulsion fuse. I really have no idea, but I wanted to sound "Ejukated!):dunce:
kwired
Electron manager
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- NE Nebraska
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But at same time 4 the current is more of an overload condition and not a short circuit/ground fault condition, so you get a delay action on lower level of overcurrent then you do with the rapid rise in current that happens in short circuits/ground faults.
Now for the low voltage equipment I am more familiar with - a 600 volt rated breaker is fine for 240 volts, 480 volts, or 600 volts. Trip curve is likely the same at any one of those voltages. They will have different interrupt ratings for each of those voltages though.
mivey
Senior Member
That's true enough for some. They can fit a wide range of voltages. But if there is a catalog of voltages, they are specific for a reason. I just read that was important for current limiting fuses.
I'm sure there are variable depending on the fuse tech but I will have to read more.
Perhaps it matters due to the arc it has to create and how it extinguishes the arc. The length of the element, the element shape/size/configuration and difference for different voltages may play a role.
Maybe the arc length matters, maybe it is not that precise. I don't know.
Practically none before it is damaged. But when it starts to heat then melt it certainly rises.
But it could be the operating mechanism or protected system is sensitive somehow. Certainly seems to be the case for current limiting fuses anyway.
mbrooke
Batteries Included
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You are right- but you've never applied a 15kv fuse to a 4.16kv system or a 25kv fuse to a 12kv system because the voltage will latter be increased? Very common for POCO working on abandoning legacy systems.
mivey
Senior Member
Some fuses are multi-voltage, some are more limited.Been a while, but as I recall, a recloser (and it's associated bushings) used on 7,200/12,470 is nameplate rated 15KV and a 14,400/24,940 is rated 25KV. Their ratings were based on line to line, not line to neutral, even if it was a single phase line to neutral recloser. If it was used on a 7,200 single phase circuit, it was rated 15KV. Not sure why. I think fuses were the same, but I'm not sure. Been retired too long.
mivey
Senior Member
Our line fuses covered from 2.4 kV to 35 kV systems.
mbrooke
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Been a while, but as I recall, a recloser (and it's associated bushings) used on 7,200/12,470 is nameplate rated 15KV and a 14,400/24,940 is rated 25KV. Their ratings were based on line to line, not line to neutral, even if it was a single phase line to neutral recloser. If it was used on a 7,200 single phase circuit, it was rated 15KV. Not sure why. I think fuses were the same, but I'm not sure. Been retired too long.
Fuse rating change on how you apply them, ie you can often use some S&C 25kv rated fuse for a 34.5kv line to neutral connected pole pigs but must go to 34.5kv for a delta bank. You can click on the different types of cutouts and look at their links, ratings, ect.
https://www.sandc.com/en/products--services/?c=199
mbrooke
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But you've never had to place a 15kv rated cutout (and its link) on a 2.4kv system? Especially now that throttle fuses are no longer made (at least thats what I hear).
look at some fuse charts, typical, 50 A range
15 to 25 kv
the Z is 50% higher on the 25, 35 mOhm range
the breaking i is much lower: 15 kv:40 kA, 25 kv:25 kA
need info on the specific fuses and application
probably not an issue....until it is lol
call the mfg
15 to 25 kv
the Z is 50% higher on the 25, 35 mOhm range
the breaking i is much lower: 15 kv:40 kA, 25 kv:25 kA
need info on the specific fuses and application
probably not an issue....until it is lol
call the mfg
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