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Fault Current through air
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Author:  BTES [ Thu Nov 20, 2008 12:24 pm ]
Post subject:  Fault Current through air
Seems most programs have a standard arc distance (1", 4", 12" etc) but all seem to use the bolted fault current. Seems to me as the arc distance increases, the resistance through air will decrease the available fault current. How do you factor this into the calculations? What is the resistance of air?
Author:  McFlash [ Thu Nov 20, 2008 2:30 pm ]
Post subject: 
I am not sure what the resistance of air is, maybe someone else could chime in on that one, but the calculations do take into affect the gap distance between buses. Table 2 in the IEEE 1584 gives typical bus gaps based upon equipment types, its in mm not inches but I think this is what you are referring to. Any how these distances are factored into the Arcing Current equations in section 5.2 of the 1584. If you look at the equation for arcing current you will see that it takes factors such as bolted fault current, voltage, resistance based upon bus gap distance, and other factors.

You are correct that the distance does make a difference but the fact is that almost all commercial software packages take this into account as long as you describe your equipment properly. Actually there is probably a setting you could access, if you find that you have a piece of equipment that is not standard, where you could enter the actual gap in mm and it would calculate using that value.

Hope this helps!
Author:  Zog [ Thu Nov 20, 2008 5:44 pm ]
Post subject: 
BTES wrote:
Seems most programs have a standard arc distance (1", 4", 12" etc) but all seem to use the bolted fault current. Seems to me as the arc distance increases, the resistance through air will decrease the available fault current. How do you factor this into the calculations? What is the resistance of air?


What programs use bolted fault current for arcing faults? i have never seen one that does, the resistance of air is factored into determining the arcing currents.
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