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 Post subject: CB & Protection Coord Curve Question
PostPosted: Mon Mar 17, 2008 12:19 am 
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I am trying to use the IEEE 1584 Excel calculation sheet to calculate the incident energy and PPE level but my working knowledge of the CBs and protection curves is not that advanced. In the column, opening time, I assume that I have to key in the CB opening time and I'm trying to locate this information from the CB manufacturer data. My questions are:

1. is break time, open time and trip time for a CB the same?
2. what happens if the fault level is not covered by my protection coordination curve i.e. way too high that what is covered in the curves?

Thanks in advance for your help!


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PostPosted: Mon Mar 17, 2008 6:18 am 
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I'll just straight to #2. I can't say that what different manufacturer's specify is standard or the same. But if you have the curves in front of you, then the total time should be the upper part of the band for molded case, and will be the relay time + breaker operating time + any intermediate relay time for power circuit breakers.

If the fault level is greater then that covered by the curves, for the purposes of arc flash, I usually go with the instantaneous time (if any) or one cycle. It should be noted as well, of course, that this may mean that the breaker is not rated for the available fault current. In this case, a fault may be more likely to cause an arc flash event, due to the breaker breaking down. Other steps should be taken to ensure that the available fault level does not exceed the breaker rating.


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PostPosted: Tue Mar 18, 2008 4:38 am 
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There are a couple of other cautions. IEEE 1584a suggests also using 85% of the estimated arcing current to see if it becomes low enough to cause the breaker to time out in the overload region instead of the instantaneous. Long clearing times / trip time / opening time could also cause a problem with arc flash allowing a greater exposure.

Manufacturers tend not to draw breakers beyond their interrupting rating as WDN mentioned so you might want to check the short circuit current. i.e. if you calculated the bolted fault and you know the breaker size, type and manufacturer, look up the interrupting rating from the manufacturers website. The interrupting rating should be higher than the calculated bolted fault current. I also have a couple of free guides posted at my website that might help you sift though all this.

Good Luck!

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Jim Phillips, P.E.
Brainfiller.com


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PostPosted: Sat Apr 05, 2008 11:40 am 
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brainfiller wrote:
I also have a couple of free guides posted at my website that might help you sift though all this.



Nothing really to add to the other two replys, except a not to Mr Phillips. Your website is an amazing resource. I have just started to read through some of your articles but have already bookmarked the site and pointed a couple of my fellow engineers to your site.

Thanks for the resource


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