Arc Flash Analysis involving UPS

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TheIntern

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Fairbanks, AK
Hello,

I am an EE intern for the University of Alaska Fairbanks, which I am also currently attending for my EE degree. I also have some hands on electrical background and training from a vocational school.

I am currently tasked with drawing up riser diagrams for all our buildings in SKM-Power Tools for Windows and entering all the data for an arc flash analysis. (My boss reviews each one line). Each one of our buildings has a UPS, typically just for emergency lighting. For the UPS's that have just lighting loads, and no panels after, do they have to be included in the one line or can they be left out? If they have to be included, what data should I be looking to collect?

Thanks for any contribution.
 
JoeStillman said:
Assuming you want an A, don't leave them out. Enter a generic UPS in SKM and see what parameters it wants, then go read the nameplates of the units.
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Agreed, I'd try and be as detailed as possible. If you have the nameplate of the UPS that should be fine. The fact that they serve lighting loads shouldn't really matter since the furthest point of fault current, and therefore arc flash/PPE level, would be at the UPS themselves, and not the individual lights.
 
Thank you for the input.

Update on my progress:

I was able to get a hold of an electrical engineer that works for a company around my town who has done a significant amount of arc flash studies using SKM. He mentioned that because only lighting exists downstream of the UPS, I can model it as a simple bus and calculate the arc flash rating as if it was an MLO panelboard.

I also found this article done by Square D which has a section explaining what happens when a UPS has a potential fault down stream. The section for UPS is on page 13. It is a PDF download.

http://www.graybar.com/documents/short-circuit-coord-arcflash-studies.pdf
 
Do the UPS's (lighting inverter) have a bypass? If so, when a fault downstream occurs, does it go to bypass, or does it just go into a current limit mode?

If it goes to bypass, then modeling it as a bus, is correct. If it goes into a current limit mode and still provides current into the fault, then model it as a UPS (using the UPS component in SKM).
 
Since the UPS only has lighting downstream, and no other bus, do I even need to worry about the bypass/current limiting mode? My understanding is the 20A breakers in the UPS for the lighting would take care of any issue on those circuits.
 
I think you will find modeling anything below 480V is going to be a waste of time. There won't be enough energy at 240V or 208V to require more than the minimal level of PPE.

And lighting loads are not going to provide any level of fault current contribution.
 
kingpb said:
I think you will find modeling anything below 480V is going to be a waste of time. There won't be enough energy at 240V or 208V to require more than the minimal level of PPE.

And lighting loads are not going to provide any level of fault current contribution.
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I wouldn't be so sure. I've had 120/208V panels that were close to the main service that had higher levels of available fault current and required PPE. Additionally, even if your 120/208V panels have a low level of available fault current, they may have a high PPE required. This is due to the fact that the lower fault current will exist for a longer period of time than a higher fault current depending on how the breakers are set.
 
lielec11 said:
I wouldn't be so sure. I've had 120/208V panels that were close to the main service that had higher levels of available fault current and required PPE. Additionally, even if your 120/208V panels have a low level of available fault current, they may have a high PPE required. This is due to the fact that the lower fault current will exist for a longer period of time than a higher fault current depending on how the breakers are set.
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Please note I said minimal level of PPE, not no PPE.

I do not disagree that the time to trip may be extended due to the lower current, however it is very unlikely, especially for 208V system with a transformer smaller than 112.5KVA that it will be able to sustain an arc anyway.

What is stated is that the amount of energy present is not going to be high enough to require anything greater than the minimum PPE, and that minimum PPE (IMO) is what should be worn anyway (assuming working hot).
 
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