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 Post subject: SLG factors in the arc flash study process ?
PostPosted: Thu Apr 28, 2011 1:26 pm 

Joined: Tue Apr 12, 2011 6:11 am
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Location: Oregon, USA
The questions of this thread were initiated by questions and comments found in another thread, entered under [url="http://www.arcflashforum.com/showthread.php?t=1421"]‘ground protection’[/url].


So, the consensus appears unanimous, that arc flash studies for low-voltage systems, as of yet, do NOT include results for other than 3 phase arcing faults. This is in view of the fact that the IEEE 1584 arc flash incident energy formula, at this time, STRICTLY assumes a 3-phase arcing fault. ( involving all 3 phases )

So that much is understood. However, this still leads to some contingent questions. Because the incident energy calculation is only the 3rd of 3 primary stages in the calculation and analysis process.

These 3 primary calculation and analysis stages are :

1. bolted short circuit fault current
2. arcing fault current
3. incident energy resulting from the arcing fault current

[ leaving the subsequent PPE level and protection boundary calcs still necessary, of course ]

Again, it is understood that as of the present, stage 3 -- the calculation of incident energy resulting from the arcing fault current, does not include direct SLG factors or questions at that stage.

Nevertheless, I am still not clear as to how necessary SLG factors may be in each of the 2 preceding stages.

Putting the question redundantly, but more specifically, at which stages must SLG factors be included, and then where do they become no longer of direct significance ?

OR, conversely, does the prevailing convention up to the present, of assuming an all-3-phase engulfing arc as inevitable, render separate SLG considerations unnecessary at all 3 stages of analysis ?

I don’t mean to be tedious, but I think these are genuine questions for the uninitiated seeking to gain a true understanding of the concepts involved. ‘Thank-you’ in advance to anyone so willing to further elucidate upon these fundamental arc flash analysis concepts.


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PostPosted: Fri Apr 29, 2011 2:56 am 
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I believe the others have provided very good information in the referenced thread. Here is a bit of extra detail. IEEE 1584 was written with only a few hundred tests and a minimal budget. Single phase was not studied and a general assumption was made that SLG escalates to 3P.

Is this always the case? No. It depends on many factors such as box size, conducting plasma cloud etc. However in the absence of new test data and equations, there are no SLG options with IEEE 1584. This means SLG considerations can not be made using IEEE 1584 methods.

Where SLG calculations are necessary such as overhead transmission and distribution, programs that model SLG such as ArcPro can be used. Hopefully we will have SLG methods included in future editions of IEEE 1584 but this is far off in the future.

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PostPosted: Sat Apr 30, 2011 7:37 pm 
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I agree that SLG faults are not covered by IEEE-1584 and that generally only 3P faults are considered for industrial systems. I question whether assuming 3P faults is conservative, however.

In the case of a SLG fault on the secondary of a delta-wye transformer, with fault clearing depending on primary protection, the primary current is only 58% of the secondary current (after adjusting for voltage ratio). In these cases, secondary SLG fault clearing may be significantly slower than 3P fault clearing.

Does anyone consider this? If so, how?


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PostPosted: Mon May 02, 2011 12:31 pm 
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jghrist wrote:
I agree that SLG faults are not covered by IEEE-1584 and that generally only 3P faults are considered for industrial systems. I question whether assuming 3P faults is conservative, however.

In the case of a SLG fault on the secondary of a delta-wye transformer, with fault clearing depending on primary protection, the primary current is only 58% of the secondary current (after adjusting for voltage ratio). In these cases, secondary SLG fault clearing may be significantly slower than 3P fault clearing.

Does anyone consider this? If so, how?


If you model the traditional 3 phase event, this should not matter however if it is a single phase model i.e. transmission/distribution systems using something like ArcPro, then the multiplier should be considered.

I talk about this issue pretty regularly in training and attached a slide that I use. It shows a 7500 kVA transformer with a secondary short circuit reflected through to the primary with 2 different fuse selections. Either way, it is a sluggish clearing time.


You do not have the required permissions to view the files attached to this post.

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