SCCR - Packaged Air Compressor

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nhee2

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A customer has purchased a 75 HP packaged (skidded with enclosure) air compressor with integral control panel which includes 480 VAC dist. block, 75 HP VFD, control transformer (fused) and small ATL starter (fused). The panel has a sticker with SCCR of 10 kA. Available fault current exceeds that.

I am having trouble coordinating with compressor supplier to confirm what is the limiting component in the panel (I believe it is the distribution block as the drive appears be rated up to 100KA depending on fusing), and what can be done to eliminate it. I did not note any UL or other NRTL sticker on the enclosure.

I am considering recommending to the customer they replace the distribution block with one listed for the available current (bussmann pdb style) and installing applicable upstream fuses, assuming I will not get a solution from the manufacturer.

I could also tell them to increase their conductor lengths by around 150' to reduce the fault current, but that does not look like a practical solution.

Any other suggestions or cautions regarding changes to the vendor panel? As it is skidded equipment, I don't know if article 409 even applies, or if UL 508 would apply.
 
It might be as simple as feeding the thing with CL fuses. Most times the problem devices have a much higher SCCR when fed from CL fuses. BTW, UL508a does not require that the OCPD be located inside the control panel. There are some marking requirements that you would have to abide by if they are not inside the control panel though. In fact, the control panel manufacturer does not even have to supply OCPD inside the control panel as long as it is marked that the OCPD is located somewhere else. Unfortunately for you, that marking is normally done on the drawings and you can't update their drawings so you might be out of luck.

The only procedure I am aware of to determine the SCCR of a control panel is found in UL508a. You would need to follow the rules there to rebuild the control panel into something with a higher SCCR.

The problem is that you cannot just pick and choose what rules in UL508a you want to follow and than claim the end result is compliant with UL508a and thus compliant with the UL508a requirements for determining the SCCR. If you don't know what these rules are, you might as well just make up a new sticker that says 22kA SCCR and not change anything in the panel because what you end up with probably won't be compliant.

Incidentally, you can use UL508a to evaluate the SCCR of a control panel even if it is not listed. You just have to build it and follow all the rules found there, and skip putting the UL sticker on. So you end up with a control panel that is UL508a compliant but not listed.
 
Thanks for the feedback.

I am hoping the vendor will be able to verify that CL fuses can be used to raise the SCCR.
 
I ran power to same size, type, voltage compressor one time and the startup guy refused to energize the unit if there wasn't CL fuses in the supply, and made that clear in information they sent out with the unit, and we weren't even over the SCCR on that unit.
 
kwired said:
I ran power to same size, type, voltage compressor one time and the startup guy refused to energize the unit if there wasn't CL fuses in the supply, and made that clear in information they sent out with the unit, and we weren't even over the SCCR on that unit.
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In this case neither the label nor the installation instructions/installation drawings provide requirements other than to 'provide fused or circuit breaker disconnect per local codes.....'
 
nhee2 said:
Thanks for the feedback.

I am hoping the vendor will be able to verify that CL fuses can be used to raise the SCCR.
Click to expand...

maybe they would be willing to supply a nameplate that says something like this:

SCCR 65kA if fed from a circuit protected by Class J fuses not exceeding a rating of 200A.
 
petersonra said:
maybe they would be willing to supply a nameplate that says something like this:

SCCR 65kA if fed from a circuit protected by Class J fuses not exceeding a rating of 200A.
Click to expand...

That would be the best scenario and what I had originally hoped for. Just having difficulty getting to the proper person at the vendor.
 
nhee2 said:
A customer has purchased a 75 HP packaged (skidded with enclosure) air compressor with integral control panel which includes 480 VAC dist. block, 75 HP VFD, control transformer (fused) and small ATL starter (fused). The panel has a sticker with SCCR of 10 kA. Available fault current exceeds that.

I am having trouble coordinating with compressor supplier to confirm what is the limiting component in the panel (I believe it is the distribution block as the drive appears be rated up to 100KA depending on fusing), and what can be done to eliminate it. I did not note any UL or other NRTL sticker on the enclosure.

I am considering recommending to the customer they replace the distribution block with one listed for the available current (bussmann pdb style) and installing applicable upstream fuses, assuming I will not get a solution from the manufacturer.

I could also tell them to increase their conductor lengths by around 150' to reduce the fault current, but that does not look like a practical solution.

Any other suggestions or cautions regarding changes to the vendor panel? As it is skidded equipment, I don't know if article 409 even applies, or if UL 508 would apply.
Click to expand...

Seems odd the control panel didn't come with a disconnecting means of some sort.

JAP>
 
petersonra said:
what makes you think it didn't?

what makes you think it is required?
Click to expand...

It wasn't mentioned.
I didn't say it was required but generally a packaged unit like this has it's own disconnecting means which is usually of the breaker type.


JAP>
 
jap said:
It wasn't mentioned.
I didn't say it was required but generally a packaged unit like this has it's own disconnecting means which is usually of the breaker type.


JAP>
Click to expand...

no disconnecting means was provided.
 
jap said:
It wasn't mentioned.
I didn't say it was required but generally a packaged unit like this has it's own disconnecting means which is usually of the breaker type.


JAP>
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It would not surprise me at all to find that whoever bought it went for the cheapest options. The sale of this kind of stuff can hinge on a few dollars sometimes and manufacturers are always looking to find ways to reduce the price to make it more attractive.

In any case, you are pretty much stuck with what you have and have to make it work.
 
nhee2 said:
Thanks for the feedback.

I am hoping the vendor will be able to verify that CL fuses can be used to raise the SCCR.
Click to expand...

What's the available fault current at the compressor location?

JAP>
 
I have posted this comment on other recent posts, and I haven't the time right now to go into details. But I suggest that we do not place too much trust in the CL fuse as the solution to the SCCA concern. Just because the "C" stands for "current" and the "L" stands for "limiting," you can't automatically infer that this device will limit the current to a value below the SCCR of the component. The problem comes in when the component has an internal OCPD of its own, be it a fuse or a breaker. That second OCPD, in series with the CL fuse that you intend to install upstream, will alter the response characteristics of the CL fuse. The internal fuse or circuit breaker will possess a "dynamic impedance," meaning that when it begins to melt or open it will draw an arc that becomes a series element in its own right. The impedance of that arc, however briefly it exists, will cause the upstream CL fuse to see a reduced amount of fault current, and that will change how the CL fuse reacts to the fault.
 
charlie b said:
I have posted this comment on other recent posts, and I haven't the time right now to go into details. But I suggest that we do not place too much trust in the CL fuse as the solution to the SCCA concern. Just because the "C" stands for "current" and the "L" stands for "limiting," you can't automatically infer that this device will limit the current to a value below the SCCR of the component. The problem comes in when the component has an internal OCPD of its own, be it a fuse or a breaker. That second OCPD, in series with the CL fuse that you intend to install upstream, will alter the response characteristics of the CL fuse. The internal fuse or circuit breaker will possess a "dynamic impedance," meaning that when it begins to melt or open it will draw an arc that becomes a series element in its own right. The impedance of that arc, however briefly it exists, will cause the upstream CL fuse to see a reduced amount of fault current, and that will change how the CL fuse reacts to the fault.
Click to expand...

Thanks for input.
The discussion on the options to install CL fuses was assuming that the panel vendor could confirm that the component SCCR values would be higher when protected by a specific style fuse or OCPD. At least that was what I was assuming.
 
nhee2 said:
Thanks for input.
The discussion on the options to install CL fuses was assuming that the panel vendor could confirm that the component SCCR values would be higher when protected by a specific style fuse or OCPD. At least that was what I was assuming.
Click to expand...

The manufacturer could probably make that affirmation only if they had actually tested, not just done simple calculations. (Because of the dynamic characteristics of CL fuses.)
 
charlie b said:
That second OCPD, in series with the CL fuse that you intend to install upstream, will alter the response characteristics of the CL fuse. The internal fuse or circuit breaker will possess a "dynamic impedance," meaning that when it begins to melt or open it will draw an arc that becomes a series element in its own right. The impedance of that arc, however briefly it exists, will cause the upstream CL fuse to see a reduced amount of fault current, and that will change how the CL fuse reacts to the fault.
Click to expand...

But since the arc only holds the stage for fractions of a millisecond why is it considered a show stopper???? This is all about energy transfer so massive energy held up for a brief interval will still prevail and trigger the CL fuse. Without quantitative standards we are we not just painting bleak pictures of unlikely worst case scenarios???
 
GoldDigger said:
The manufacturer could probably make that affirmation only if they had actually tested, not just done simple calculations. (Because of the dynamic characteristics of CL fuses.)
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Presumably the manufacturer is following UL 508a. The procedure for determining the short circuit current rating found in UL 508a does not allow for testing. One could probably get UL to do the testing and change a specific procedure to allows otherwise untested components to be used together but since many manufacturers have already had their equipment tested with current limiting fuses it would probably be simplest just to go with some combination that's already been tested.
 
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