It is currently Sat Nov 18, 2017 10:09 pm



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: Question about using the Arc Flash Study
PostPosted: Mon Apr 25, 2011 11:06 am 
Offline

Joined: Thu Apr 21, 2011 1:58 pm
Posts: 3
Hi, I am glad I found this forum. I came across it while doing some research and it has been helpful.
I work in a tier 3 data center and we are having an Arc Flash study done. I have the job of checking out the results. The engineers that are doing the study have done a few more calculations than I have seen in other studies. The question that I have concerns our RDC's (remote distribution cab.). The RDC's have 4 panels, 2 in the front and 2 in the rear. The engineers did separate calculations for the front and the rear with a class 3 assigned to each side. They also did calculations for the 4 individual panels that were assigned a class 0. The RDC's are fed with a 225 amp breaker from our PDU's. The PDU's have a transformer (480 to 208 volts). The feeder terminates on the bottom of a contact block in the RDC. The contact block has two sets of feeders coming off the top to feed 225 amp main breakers at the 2 individual panels on each side. The panels have a small 3" X 8" opening at the top where the feeders terminate. The main breakers are just above this opening. With the feeders from each panel paralleled to the top of the contact block, would OSHA consider the tecnician to be exposed to the class 3 energy through the opening at the top? From a practical stand point, the opening is at the 5 foot level and if the tech was working on a lower section I don't believe the flash would reach him. I have attched pictures to show this application. I would appreciate any comments as I believe opinions might differ.
Thanks!


You do not have the required permissions to view the files attached to this post.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Apr 26, 2011 4:32 am 
Offline
Arc Level
User avatar

Joined: Wed May 07, 2008 5:00 pm
Posts: 679
Location: Rutland, VT
Typically, most studies will ignore the breaker inside the panel for providing any protection for an arc flash and rely on the upstream breaker. The reasoning is that the arc flash could engulf the panel main breaker rendering it useless.

But what I am interested in is why is the RDC a Category 3 hazard. From your description it sounds like the RDC is 208V fed from a transformer in the PDU (Power Distribution Unit??). I would suspect that this 480-208V transformer is probably less than 150 kVA (most likely 50 or 75 kVA). If this meets the Exception #1 in Article 130.3, then it could be considered Category #0. also are they using a 2 second cutoff limit on the arcing time?

_________________
Barry Donovan, P.E.
www.workplacesafetysolutions.com


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Apr 26, 2011 7:10 am 
Offline

Joined: Thu Apr 21, 2011 1:58 pm
Posts: 3
Thanks for the reply wbd,
Our PDU's are 225 KVA with 350 amp mains. The results table shows the RDC's trip time at 2 sec., a breaker opening time of 0 and incident energy of 20 with an (N3) listing. The engineers are using the SKM summary view option which lists the last breaker to clear the 80% fault threshold. They show the main breaker, at the PDU, feeding the RDC as the protective device. The individual panel calculations that show a class 0 have a .032 sec trip time, a breaker opening time of 0 and incident energy of .033 with an (N3) listing. As to the original question about worker exposure, the research I have done has been inconclusive. We have fashioned protective barriers for the area that I am concerned about but these are primarily for shock protection and to keep the tech from shorting anything out. As far as the RDC's are concerned we de-energize them at the PDU's to add or delete circuits but we can't do that to take amp readings. When working on a PDU, which has a similar opening, we turn off the main to the distribution panel but not the transformer main as this causes problems from the start up surge. Some of the information that I have researched considers that electrical equipment can at any time just self-destruct.
I originally had the job of putting the labels on the equipment but found so much information that did not make sense that I had to become very involved in the process. I am now reviewing the third revision. Allot of the problems came from incorrect input information. I was not involved in the original fact finding so I don't know where all the erroneous information came from. This facility came online in 1996 and my understanding is the information came from as builts and commissioning reports from the original construction.
As an example of what I was finding on the original study - on the UPS modules it showed some of them with the output breaker as the protective device and one of them showed a main breaker in a PDU as the protection.
I know that I can ask the engineers that are doing the study this question but I wanted more input because of the vagueness of the information that I have found.
Thanks again!


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Apr 26, 2011 11:33 am 
Offline

Joined: Mon Jan 26, 2009 5:00 pm
Posts: 46
Location: Midwest
RDC's / PDU's typically are installed with K Rated transformers. These end up having low impedances ~2.5-3.5%Z. This results in decent fault current levels on the 208V side, with everything usually requiring the 22K rating.

When you couple this with robust upstream distribution for fault coordination, the CAT 3 incident energies are common.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 4 posts ] 

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 1 guest


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
© 2017 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883