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 Post subject: Current Limiting Breakers
PostPosted: Thu Jan 31, 2008 9:18 pm 
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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?


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PostPosted: Fri Feb 01, 2008 6:47 pm 
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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.


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PostPosted: Tue Feb 05, 2008 9:58 am 
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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.

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PostPosted: Tue Feb 05, 2008 12:59 pm 
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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.


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PostPosted: Tue Feb 05, 2008 1:57 pm 
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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.
I certainly appreciate your input!

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