It is currently Tue Nov 29, 2022 2:37 pm



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: Reasonable estimate for utility MVA
PostPosted: Sat Nov 12, 2022 8:36 pm 
Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 517
Location: New England
The utility refuses to provide fault values to the service transformer primary. They tell me to assume an infinite bus. That's fine for sizing the AIC of equipment, but with arc fault I find that sometimes lower fault current values can generate higher incident energy values as some breakers never reach their instantaneous trip threshold.

Does anyone have a typical estimate on the reduction in fault current when calculating a finite bus. I remember seeing a reference that a 10% reduction is common. This is a typical street feed 13.8KV overhead line supplying a 300KVA 208/120 transformer.

Before anyone starts, I realize this is not ideal and if I ran the utility I would make it available. But that fight is not winnable at this time. I'm thinking its in the 10 to 20 percent range, but would like your opinion.

thanks


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: Reasonable estimate for utility MVA
PostPosted: Sun Nov 13, 2022 1:20 am 
Sparks Level
User avatar

Joined: Fri Jul 08, 2016 10:01 am
Posts: 241
Location: Indiana
Rather than a single estimate you could run, say, 5 scenarios. I usually work in amps so say, 2,000, 5,000, 10,000, 20,000 amps and infinite or something like that based on what your experience is in that area.

Run the scenarios to automatically select worse case. Not ideal, I agree, but it would probably get you there.

Don't know what software you are using but in SKM that would be easy to do.

Good luck.

Oh, if you choose to try this, please report back with which scenario was typically the worst case. It would be good to know.

_________________
SKM jockey for hire


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: Reasonable estimate for utility MVA
PostPosted: Sun Nov 13, 2022 6:40 am 
Sparks Level
User avatar

Joined: Fri Jul 08, 2016 10:01 am
Posts: 241
Location: Indiana
I had nothing better to do this morning so I briefly played around with what I described above. Anyone have any better ideas or find fault with this approach?

Here's the vid:

https://youtu.be/LAktCt3RjA0

_________________
SKM jockey for hire


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: Reasonable estimate for utility MVA
PostPosted: Mon Nov 14, 2022 8:01 am 
Sparks Level

Joined: Sat Apr 04, 2009 9:50 pm
Posts: 121
Location: San Antonio, TX
There is an IEEE publication that could help solve this issue.

Impact of available fault current variations on arc-flash calculations

There, you will find an excellent method, with two scenarios, to estimate the arc flash incident energy at the line side of the service equipment.

I teach this method in the seminars I deliver for the SKM arc flash studies.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: Reasonable estimate for utility MVA
PostPosted: Mon Nov 14, 2022 8:32 am 
Sparks Level
User avatar

Joined: Fri Jul 08, 2016 10:01 am
Posts: 241
Location: Indiana
RECS wrote:
There is an IEEE publication that could help solve this issue.

Impact of available fault current variations on arc-flash calculations

There, you will find an excellent method, with two scenarios, to estimate the arc flash incident energy at the line side of the service equipment.

I teach this method in the seminars I deliver for the SKM arc flash studies.


This: http://www.eatonfuller.com/ecm/groups/p ... 372889.pdf

Conclusion: Run an infinite bus scenario and a 50% scenario. This is indeed better than infinite bus alone but probably not as conservative as running more scenarios than just infinite and 50%:

Attachment:
IEEE excerpt.jpg


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

_________________
SKM jockey for hire


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: Reasonable estimate for utility MVA
PostPosted: Mon Nov 14, 2022 8:44 am 
Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
Posts: 596
See https://brainfiller.com/arcflashforum/viewtopic.php?f=23&t=4665&p=21385&hilit=Impact+of+available+fault+current+variations+on+arc+flash+calculations#p21385 for a discussion of the process proposed by the IEEE paper RECs spoke of.


Top
 Profile Send private message  
Reply with quote  
 Post subject: Re: Reasonable estimate for utility MVA
PostPosted: Mon Nov 14, 2022 9:02 am 
Sparks Level
User avatar

Joined: Fri Jul 08, 2016 10:01 am
Posts: 241
Location: Indiana
stevenal wrote:


Thanks!

_________________
SKM jockey for hire


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 7 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:  
© 2022 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883