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 Post subject: Question on a size of Arc Flash
PostPosted: Thu Feb 19, 2009 7:52 am 
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Does anyone know what the results would be from a simple small distribution disconnect if it were short circuited and it had an arc flash. How large and destructive of an arc flash could this small of a device actually create?


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PostPosted: Thu Feb 19, 2009 8:03 am 
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Willis wrote:
Does anyone know what the results would be from a simple small distribution disconnect if it were short circuited and it had an arc flash. How large and destructive of an arc flash could this small of a device actually create?


Depends on the available fault current and the clearing time of the protective device. Could potentially be a devesating arc flash that can destroy the equipment and injure persons anywhere in the room, or could be very minor and not dangerous, all depends on the factors involved.


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PostPosted: Thu Feb 19, 2009 8:45 am 
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For small disconnect switch, sometime it is hard to find space on the switch to put a lable on. How do you guys deal with those devices? What if you have a row switches on the wall for pump/motor control?


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PostPosted: Thu Feb 19, 2009 8:52 am 
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Clearing time

What do you mean "clearing time"?


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PostPosted: Thu Feb 19, 2009 9:56 am 
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Willis wrote:
What do you mean "clearing time"?


How long it takes the fuse or breaker to clear the fault at different fault current levels.


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PostPosted: Thu Feb 19, 2009 10:45 am 
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Thanks!

I appreciate the information Zog.


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PostPosted: Thu Feb 19, 2009 12:02 pm 
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Welcome to the forum.


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PostPosted: Thu Feb 19, 2009 2:36 pm 
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How small is a small disconnect? If its fed by < 30A molded case breaker, it is likely to be < 4 cal/cm.
If it's fed by a 600A drawout breaker at a unit substation, it could be > 40 cal.
We actually have a case like this, where the breaker is used to feed several things, including a 70A 480V welding receptacle that is now >40 cal because they chose to feed it directly from the UnitSub.

Noah, if there is a wire trough above the disconnect rack, I would consider putting the label there, although I have also covered the factory markings at times (I do my best not to, though.)


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PostPosted: Thu Feb 19, 2009 2:54 pm 
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How small is a small disconnect? If its fed by < 30A molded case breaker, it is likely to be < 4 cal/cm.
If it's fed by a 600A drawout breaker at a unit substation, it could be > 40 cal.
We actually have a case like this, where the breaker is used to feed several things, including a 70A 480V welding receptacle that is now >40 cal because they chose to feed it directly from the UnitSub.


What is the cal/cm range that an arc flash becomes deadly or cause serious injury?


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PostPosted: Thu Feb 19, 2009 6:46 pm 
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Willis wrote:
What is the cal/cm range that an arc flash becomes deadly or cause serious injury?


1.2 cal/cm2 will cause second degree burns in 1/10th of a second. Put you in a burn unit, life changing event.


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