It is currently Sun May 28, 2023 3:39 pm

Author Message

 Post subject: Current Limiting BreakersPosted: Thu Jan 31, 2008 9:18 pm
 Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 519
Location: New England
How do you deal with Current Limiting Breakers?

Say you are feeding a remote panel from a Fused Switch Disconnect. You have 200A Class R Current Limiting Fuses upstream of the panel. You calculate bolted fault to the panel and you get 16KA. You calculate Arcing Current at 11KA. But the fuse has a maximum let through current of 8KA. It would seem to me that since you know Arcing Current is greater than Let Through that the fuse will clear in less than 1/2 cycle and that the max let through would be 8KA. So for Incident Energy I would use the 8KA@1/2cycle. Is this correct?

If the arcing current was less than the let through, I would then use the clearing time based upon the fuse curve for that arcing current value.

Point I am trying to make here is that current at arc can never exceed let through current of the fuse.

IS this logic correct?

Top

 Post subject: Posted: Fri Feb 01, 2008 6:47 pm
 Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 519
Location: New England
Sorry, I was thinking Fuses but typed breakers. Thread is in reference to current limiting fuses. Although I guess it would apply for breakers that incorporate current limiting fuses.

Top

 Post subject: Posted: Tue Feb 05, 2008 9:58 am
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1702
Location: Scottsdale, Arizona
Current Limitation

Haze,
You have the right idea but there are a few different formulas to use. The current limiting devices can reduce the short circuit current and arc flash energy. That is a major advantage the fuse manufacturers promote. There are some current limiting fuse formulas in IEEE 1584 which are also copied in the Annex of NFPA 70E that are used for RK1 and Class L fuses. You input the bolted fault current and the formulas predict the incident energy. You might also contact the fuse manufacturer for other classes of fuses.

_________________
Jim Phillips, P.E.
Brainfiller.com

Top

 Post subject: Posted: Tue Feb 05, 2008 12:59 pm
 Sparks Level

Joined: Thu Jul 19, 2007 7:54 am
Posts: 201
Location: St. Louis, MO
I must be pretty dense, because it wasn't until this point that I really realized the reason behind the other set of equations for current limiting devices. (The lower let through current.)
I think they should have clearified this in the 1584 report, as well as perhaps giving the equations the same or similar format to the regular equations.

Top

 Post subject: Posted: Tue Feb 05, 2008 1:57 pm
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1702
Location: Scottsdale, Arizona
WDN - Yep just when you thought it was safe.. more forumulas! Guess we are all trying to figure this stuff out together. Can't wait to see what the next forumulas look like after the latest round of tests. I'll be meeting with the 1584 committee in a few weeks and if any new info shows up, I'll post it here.

_________________
Jim Phillips, P.E.
Brainfiller.com

Top

 Display posts from previous: All posts1 day7 days2 weeks1 month3 months6 months1 year Sort by AuthorPost timeSubject AscendingDescending
 Page 1 of 1 [ 5 posts ]

 All times are UTC - 7 hours

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

 Jump to:  Select a forum ------------------ Forum Library / Articles The Lounge    Question of the Week - What Do You Think?    Arcflashforum.com Feedback and Announcements    Off Topic Discussions    News in Electrical Safety Arc Flash and Electrical Safety    General Discussion    Electrical Safety Practices    Equipment to Reduce Arc Flash Dangers    Personal Protective Equipment (PPE) Arc Flash Studies    General Discussion    Arc Flash Labels    Software for Arc Flash Studies    System Modeling and Calculations    NEW! Electrode Configuration Library â 2018 IEEE 1584 Codes and Standards    CSA Z462 Workplace Electrical Safety    EAWR Electricity at Work Regulations, HSE - Europe    OSHA CFR Title 29    IEEE 1584 - Arc-Flash Hazard Calculations    NFPA 70 - National Electrical Code - NEC (R)    NESC - ANSI C2 - National Electrical Safety Code    NFPA 70E - Electrical Safety in the Workplace    2015 NFPA 70E Share It Here    Arc Flash Photos    Your Stories    What's Wrong Here? by Joe Tedesco
© 2022 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883