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 Post subject: Last Protective Device Question
PostPosted: Fri Nov 14, 2008 7:39 am 

Joined: Fri Nov 14, 2008 7:33 am
Posts: 1
I've had this come up multiple times now and I'm hoping someone here can explain this to me.

Why does the system for calculating arc flash not take into account the next upstream protective device?

I get this every once in a while...I will get a higher level arc flash directly after a low level when all that is in between is one protective device. Even though the most recent protective device will not trip for a longer period of time (leading to higher arc flash), the previous protective device will trip ahead of it anyways....so why the higher arc flash level?

(Basically this goes for an uncoordinated scenario).


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PostPosted: Fri Nov 14, 2008 8:00 am 
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Joined: Mon Sep 17, 2007 5:00 pm
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Location: Scottsdale, Arizona
It is really not suppose to work that way. (i.e. the upstream faster device gets ignored at the downstream bus)

If the upstream device (device #1 at bus #1) is faster and gives a lower I.E. at bus #1 and the downstream device (device #2 at bus #2) is slower and gives a higher I.E. (obviously mis-coordinated as you point out) device #1 should be the limiting device for an arc flash at bus #2.

The only wrinkle I can think of is if there is either a transformer between 1 and 2 or significant motor contribution or other source between 1 and 2.

I know at least one of the commercial S/W programs searches upstream to find the quickest device which may not necessarily be the closest.

Hope it helps!

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Jim Phillips, P.E.
Brainfiller.com


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PostPosted: Mon Nov 17, 2008 7:13 am 
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Joined: Thu Jul 19, 2007 7:54 am
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Location: St. Louis, MO
If you are using software to perform the analysis, I suspect you have missed some options. You really need to go through and understand all of the options available for performing the analysis with electrical simulation software. Depending on the settings, you can get a very wide range of values.


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PostPosted: Mon Nov 17, 2008 8:10 pm 
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They're right most commercial software suites will take this into consideration. Some even have options for seeing the trip time upstream if a closest device should fail to trip.


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