It is currently Fri Aug 14, 2020 3:30 am



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: SKM-UPS Modeling For Arc Flash
PostPosted: Wed Jul 12, 2017 9:35 am 

Joined: Wed Jul 12, 2017 6:06 am
Posts: 27
When modeling UPS's in SKM, what is the correct data to enter for three phase short circuit info? Is 100% the correct thing to do even though it reduces the bolted fault currents downstream? Or are you putting them in Bypass mode and making a "worse" case?


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: SKM-UPS Modeling For Arc Flash
PostPosted: Wed Jul 12, 2017 11:50 am 
Sparks Level
User avatar

Joined: Fri Jul 08, 2016 10:01 am
Posts: 227
Location: Indiana
If equipped with a bypass I just show the UPS as a bus.

_________________
SKM jockey for hire


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: SKM-UPS Modeling For Arc Flash
PostPosted: Wed Jul 12, 2017 11:57 am 

Joined: Wed Jul 12, 2017 6:06 am
Posts: 27
bbaumer wrote:
If equipped with a bypass I just show the UPS as a bus.


Thanks for the response. Yes it does have a bypass.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: SKM-UPS Modeling For Arc Flash
PostPosted: Fri Jul 14, 2017 4:13 am 
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
If you select bypass then it becomes just a bus. Thus the inductive loads will elevate the available fault current due to asymmetrical current which SKM accounts for. It will also generally reflect a much higher available fault current to downstream loads. This may be what you want for instance at a disconnect switch located on the load and line side of the UPS. However the effect on the overcurrent protection in most cases is to reflect a much lower incident energy than would exist with the UPS in service. It needs to be checked both ways...you can do this with additional scenarios but due to the coarse grained nature of that method often it is to test locally both ways. Due to the way that most breakers and fuses respond to reduced fault current (incident energy increases in the inverse time region and drastically increases if breakers go from instantaneous to inverse time), the assumption that lower fault current = lower incident energy is simply not true in many typical cases.

Mind you the idea that incident energy is approximately proportional to roughly I*t does not change (V plays a role but a minor one). The issue is that when these variables interact with each other rather than in isolation as they do with overcurrent protection devices where t varies as a function of I, the result is not the same as when they are analyzed in isolation.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: SKM-UPS Modeling For Arc Flash
PostPosted: Mon Jul 31, 2017 10:32 am 
Sparks Level
User avatar

Joined: Tue Oct 26, 2010 5:00 pm
Posts: 188
Location: Maple Valley, WA.
Be aware that you should run two scenarios for the equipment downstream from the UPS. One scenario should be when the UPS is in By-Pass. This will produce the maximum short circuit current but not necessary the highest arc flash energy. The second scenario is when the UPS itself is feeding the downstream equipment. This lower fault current may cause the output breaker to take longer to trip. The result sometimes is higher arc flash energy. Typical short circuit from a UPS is 2.5 to 5 X Ifl. However, some UPS units shut down very quickly for a fault external of the UPS. You may have to override the protective trip time and enter this UPS shut down time.

_________________
Robert Fuhr, P.E.; P.Eng.
PowerStudies


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 5 posts ] 

All times are UTC - 7 hours


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
cron
© 2019 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883