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 Post subject: System Grounding
PostPosted: Wed Jan 13, 2010 12:05 pm 
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Location: St. Louis, MO
The 1584 equations for arc flash studies result in higher energy levels for grounded systems, due to the K2 factor in the incident energy equation. This results in the calculated energy levels being ~30% higher for ungrounded or impedance grounded system. This runs contrary to recommended practices and most intuition, however, which recommends ungrounded systems to reduce the likelihood of arcing faults.

I have forgotten, or never heard, a good reasoning behind this, however.
I know that this is the direct result of the testing data form the 1584, but does anyone know WHY ungrounded systems would have higher incident energies?


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PostPosted: Wed Jan 13, 2010 1:55 pm 
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Location: Lawrenceburg KY
Quote:
30% higher for ungrounded
Quote:
WHY ungrounded systems would have higher incident energies?


Sorry, WDN
I am so dumb I just do not understand the question.

hmm, I was under the impression an ungrounded system had less arcing fault current.

Were do you see in 1584 that K2 for ungrounded is greater then 0?

But I do understand that ungrounded voltages do crazy things and instead of sagging may actually increase during certain faults maybe this is why the energy increases. I have read that transients are bad on ungrounded systems and can increase voltages up to 8 times the ratings.

I have little experance with ungrounded systems and hope to never have to figure one out. I know several places use HRG but I don't know much about that other then what I have read.


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PostPosted: Wed Jan 13, 2010 2:37 pm 
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THE CABLE GUY wrote:
Sorry, WDN
I am so dumb I just do not understand the question.


Considering the number of times you've agreed with me lately, I must respectfully disagree with this assessment. :D :D

THE CABLE GUY wrote:
hmm, I was under the impression an ungrounded system had less arcing fault current.

Were do you see in 1584 that K2 for ungrounded is greater then 0?

...
I have little experance with ungrounded systems and hope to never have to figure one out. I know several places use HRG but I don't know much about that other then what I have read.


CG -
We are currently moving to resistance grounded systems on most of our 480V systems. The other engineers don't understand when I tell them this is actually making our arc flash hazard worse! As you say, the impression (and probably the fact) is that ungrounded or HRG systems are safer.

The problem in the 1584 calculations come on page 11 of the IEEE 1584a-2004. Equation 4 is the equation to find Incident Energy from Arcing Current. The K2 factor is 0 for ungrounded, but -0.133 for grounded systems. This negative results in lower energy levels for grounded systems.

I'm at a loss as to the WHY an ungrounded or HRG system would have higher energy levels. I may have had it explained to me at one time, but slept since that discussion. :o


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PostPosted: Wed Jan 13, 2010 2:43 pm 
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Location: Spartanburg, South Carolina
See http://www.arcflashforum.com/showthread.php?t=344&highlight=high+impedance+ground


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PostPosted: Thu Jan 14, 2010 6:21 am 
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Thanks, jghrist, I knew I had seen it somewhere. It's fitting that that somewhere was here.

The explanations given seem to depend on the arcing current being higher for ungrounded systems, however. The 1584 formulas do not take grounding into account in the arcing current, but in the incident energy equations.

Taking the other thread as a baseline, however, I guess it would make some sense that, because the arcing current does not have a return path through the grounded enclosure, it may arc outward, away from the source, giving different energies to the person in front of the open enclosure.


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PostPosted: Thu Jan 14, 2010 6:56 am 
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I was thinking you seen a positive number for ungrounded but I suppose 0 is positive based on a negative number. I was thinking you seen something greater then 0. My bad.

After reading Jim's response on the link, I see how what you say makes sense for three phase faults increased IE calculations.

I have read that RG offers some protection on grounding faults but never had to calculate one. Just played around with it in the ESA software and I remember having discussing it in class but like my French I never use it.

WDN you should set up a poll and see how many are using or recommending RG or no ground? I suppose it’s on the increase these days.

Hopefully, I will remember this. Alzheimer’s does run in my family. :eek:

Thanks WDN that why your a PE and I'm a P-on


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PostPosted: Thu Jan 14, 2010 8:20 am 
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Location: St. Louis, MO
THE CABLE GUY wrote:
WDN you should set up a poll and see how many are using or recommending RG or no ground? I suppose it’s on the increase these days.


If I knew how to set up a poll, I might...
Unfortunately, also, participation in the polls here seems to have fallen. :(

THE CABLE GUY wrote:
Thanks WDN that why your a PE and I'm a P-on


Careful!
First, being an electrician first and an engineer second, I still consider myself an electrician. That being said, I know better than to think that my new PE makes me any better or smarter than many of those out in the field every day.
Second, having been an enlisted member of the Greatest Navy on Earth, I know exactly what "yes, SIR!" can mean!


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