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 Post subject: Ground protection
PostPosted: Mon Dec 06, 2010 12:04 pm 
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If there is a fault, is it possible that ground fault protection clear the fault first before the overcurrent protection do their due.


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PostPosted: Mon Dec 06, 2010 12:38 pm 
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Noah wrote:
If there is a fault, is it possible that ground fault protection clear the fault first before the overcurrent protection do their due.


Yes.
In a properly coordinated system we would want a single phase to ground fault to be cleared before it escalates into an arcing line to line fault.

But ground fault relays will usually not operate fast enough to be considered when performing arc flash calculations.


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PostPosted: Tue Dec 07, 2010 8:29 am 
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IEEE 1584 has not done AF testing for line to ground faults, only for three phase systems. The AF energy and boundary calculations were derived from three phase testing. Future revisions to the standard will include single phase line to ground faults.

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Robert Fuhr, P.E.; P.Eng.
PowerStudies


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PostPosted: Wed Apr 20, 2011 11:09 am 
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Begging the question . . .

Would it be valid for me to assume then, that this means SLG faults and effects are generally NOT included in most arc flash studies at this time ?


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PostPosted: Thu Apr 21, 2011 11:11 am 
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I don't have IEEE 1584 in front of me, but I believe they prohibit GF devices and trips to be used in the calculation.


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PostPosted: Thu Apr 21, 2011 12:58 pm 
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Joules Vernier wrote:
Would it be valid for me to assume then, that this means SLG faults and effects are generally NOT included in most arc flash studies at this time ?


Yes. The assumption is that SLG faults will progress into 3P faults and that 3P arcing faults have more energy than SLG arcing faults.

An exception would be electric utilities where the NESC arc flash hazard tables (above 1 kV) were developed assuming SLG faults. This is because of larger phase spacing on OH lines compared to typical industrial enclosed equipment.


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PostPosted: Thu Apr 28, 2011 1:32 pm 
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My sincere appreciation to each for answers to my SLG related questions. However, this still leads me to a few contingent questions. But since further discussion along this line would be tangential to the origin of this thread, I have posed these questions as the beginning of a new thread. Please see, [url="http://www.arcflashforum.com/showthread.php?p=8148#post8148"]“SLG factors in the arc flash study process ?”[/url]


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