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ekstra   ara
 Post subject: Fault Current
PostPosted: Fri Nov 09, 2018 7:14 pm 

Joined: Fri Nov 09, 2018 7:13 pm
Posts: 1
Hello everyone
I'm wondering if there's any data available that indicates the necessary available fault current, on a 277V system, to blow a hole through the sheathing of a MC cable (#12 awg, 3 conductor).


Assume worst-case - a bolted fault either between phase/ground or phase/sheathing.


If I know that necessary available fault current, I could then conduct a fault current analysis for my particular situation to determine if there's even sufficient current for that to hypothetically occur.
Thanks.


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 Post subject: Re: Fault Current
PostPosted: Sat Nov 10, 2018 10:39 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
If you are talking about an arcing fault then yes. Within about 1 cycle the pressure will blow your MC apart at essentially any sustainable arcing fault. Don't know of any test data but this is the case with electrical enclosures that generally rupture around 5-10 PSI. Simple air heating models actually work pretty well. See the CCITT testing on this. Also don't forget that line-to-ground faults aren't nearly so bad as line-to-line faults so there's really not much point in analyzing line-to-ground faults. When you have the amount of heat that you will have in a very small space within the cable from say a crush impact, the first fault is going to also burn through the insulation in the area and quickly escalate it into a full 3 phase arcing fault, no need to even consider the 1 or 2 cycles it will take for this to happen.

BUT this really isn't the correct approach to the problem. You screen this one off at the risk analysis level. Cable faults have roughly 1 million times more reliability than switchgear and MCC's. Formal risk analysis methods such as LOPA and the IEC methods ignore cable failures in risk analysis because the fault frequencies are so low. Even in cases where it's higher don't forget where the cable is at and what the exposures actually are. And when it ruptures, it will be an open air condition. Type MC cable is not even allowed where subject to heavy physical damage which is where only rigid metallic conduit is allowed so if the risk analysis shows there is a problem MC can't be used anyways. Of all the failures of MC that I've seen, every case that I can recall was a case of physical damage, not corrosion or some mystery spontaneous effect.


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