It is currently Mon Feb 19, 2018 2:53 am



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: Cleared Fault Threshold
PostPosted: Mon Jan 28, 2013 7:15 pm 
Offline
Sparks Level

Joined: Sat Apr 04, 2009 9:50 pm
Posts: 89
Location: San Antonio, TX
Is there any white paper from IEEE or any other reputable organization that will evaluate when the integration process of an arc flash event should be stopped? SKM software has an option called Cleared Fault Threshold which default value is 80%. If you keep this default value, SKM will stop the energy integration when 80% or more of the BOLTED FAULT current (not the arcing fault current), is interrupted. This value seems reasonable to me, but I would like to have a reliable back up to justify its use.

Thanks.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jan 29, 2013 8:55 am 
Offline
Arc Level

Joined: Mon Jan 18, 2010 11:35 am
Posts: 445
Location: Wisconsin
The assumption is; after the threshold is reached the remaining arcing fault current is not sufficient to maintain the arc and should not be included as accumulated incident energy. The result of this study option is that incident energy will be accumulated until either the Fault Cleared Threshold or the Maximum Protection Trip Time is reached, whichever occurs first. There are no recommendations in NFPA 70E or IEEE 1584 for determining a fault cleared threshold.

My company uses a 99% threshold with a 2 sec cut off time.
Decisions like this are why some companies feel arc flash studies should not be DIY projects.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jan 29, 2013 9:31 am 
Offline
Sparks Level

Joined: Sat Apr 04, 2009 9:50 pm
Posts: 89
Location: San Antonio, TX
JBD:

Thanks for your answer. This is precisely why I am after some kind of backup for the assumption of the cleared fault threshold (no matter what value it is). Every engineer has a strong opinion regarding this number. The majority of them are PE with years of experience y knowledge. Our assumption is not better than other, though we justify it with passion. It is time to have a more precise guidance regarding the proper practices for performing are flash studies. I though that after years of performing arc flash hazard analysis, there was an IEEE paper that would give an institutional backup for a specific value. Have a great day!


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Sat Feb 02, 2013 8:49 pm 
Offline
Sparks Level

Joined: Thu Jul 07, 2011 6:03 am
Posts: 59
Location: Netherlands
SKM's default of 80% will cut off motor decay when the main device trips. The main difference of using a higher value of 99% like JBD said would be seeing the entire decay of the motor which is not interrupted. Even at 99% it will usually only show up for the bus the motor is connected to, not at upstream buses (so your results table is not littered with motor decays).

For reference: a 100 kW LV motor gives about 0.2 cal/cm┬▓ difference between the 80% and 99% setting. It's not huge but it can make the difference between HRC 0 and HRC 1 so the hidden assumption of the default setting - no arcing from just a motor contribution - could leave you underprotected.


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