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 Post subject: NEC 70 Article 700
PostPosted: Wed Mar 26, 2014 5:38 am 
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Hello,
I was curious to see if anyone has run into this issue before with a AHJ. The situation is surrounding a installation of a generator and ATS's in an existing health care facility and selective coordination. The facility has mccb's. The comments made by the AHJ concerning selective coordination are:

"Evaluation of selective coordination of overcurrent protection devices down to 0.01 seconds (one cycle) is insufficient per 2008 NEC 700.27 and 701.18. There is no time limitation exceptions included in the code. The full range of overcurrents irrespective of time is required to be evaluated."

Due to the inherent nature of mccbs in the instantaneous region this seems to be an impossible and the AHJ appears to be saying that coordination even to 0.01 s is insufficient.

Thank you in advance for any comments.

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PostPosted: Wed Mar 26, 2014 5:52 am 

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Have a look at

http://ewh.ieee.org/r3/atlanta/ias/NEC%20Mandated%20Selective%20Coordination%20-%20Challenges%20and%20Solutions.pdf

Some jurisdictions allow 0.1 as the cutoff.

The charts don't go lower than 0.01.


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PostPosted: Wed Mar 26, 2014 6:10 am 
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Here is an older[url='http://ecmweb.com/content/making-amendments']Article from 2007[/url] just before the 2008 edition of the NEC came out. I was asked about why this article was developed and was probably a bit too honest. :cool:

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PostPosted: Wed Mar 26, 2014 6:20 am 
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Thanks all. I think this would be clearer when this jurisdiction adopts later code versions as NFPA 99-2012 does actually state a 0.1 second time for selective coordination on essential systems.

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PostPosted: Wed Mar 26, 2014 6:54 am 
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wbd wrote:
Due to the inherent nature of mccbs in the instantaneous region this seems to be an impossible and the AHJ appears to be saying that coordination even to 0.01 s is insufficient.


Most manufacturers of molded case breaker have combinations of breakers which have been witnessed tested as being selectively coordinated regardless what the published TCC curves show. This concept is similar to fuse-ratio tables.

This is what our state requires.

This is a link to Schneider Electric's website. It contains a link to their Selective Coordination Analysis Tool.
http://www.schneider-electric.us/sites/us/en/customers/consulting-engineer/selective-coordination.page


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PostPosted: Wed Mar 26, 2014 8:46 am 
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Seems it defeats the intended purpose to allow any miscoordination at all on these critical systems. And choosing .01 or .1 seems pretty arbitrary, just a nice line on the graph. And all this discussion regarding the NEC intent? There is a process for determining intent.


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PostPosted: Wed Mar 26, 2014 1:39 pm 
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Even small MCCB's work faster than 1 cycle (0.016 s). Then we get to the instantaneous trip unit itself. Magnetic types typically require 1 cycle (0.016 seconds). Electronic trip units are designed with either peak sensing or energy sensing but regardless they don't work faster than 1/4 cycle, or 0.004 seconds, and there is still the pesky time delay for the breaker itself to trip.

Even if we bump up to larger breakers such as MV vacuum breakers, most of these today are tripping in 3 cycles (0.05 seconds), typically again with a 1 cycle delay for the trip unit. So it is pretty common to have instantaneous trips at least up to medium voltage of around 4 cycles or 0.067 s.

So from a very practical point of view in the old electromechanical days, I'd agree about "dynamic resistance" and selective coordination among breakers. But with electronic trip units, single cycle tripping is both here to stay and very real, and breaker mechanisms are frequently faster than 5 cycles and often operating in 3 cycles. The days of 12-15 cycle trip times are over. Thus 0.1 second coordination is a snail's pace these days.


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