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 Post subject: Fault Current
PostPosted: Mon Jun 25, 2018 1:19 am 

Joined: Mon Jun 25, 2018 1:16 am
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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: Mon Jun 25, 2018 4:11 am 
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There isn't one. Part of the trouble is understanding failure mechanics. The effect of an arcing fault within a cable is vastly different from the effect of thermal degradation or magnetic forces. This is also why it is so difficult to "contain" an arcing fault within electrical enclosures.

Bolted fault doesn't apply. This estimate is intended for use for one and only one purpose. It is used to estimate magnetic forces which are proportional to the square of the current to estimate whether or not the cables, bus bars, standoffs, etc., can withstand the magnetic force generated during a dead short condition. Bolted fault current is used as a starting point for arcing faults. The scenario you are describing doesn't match the failure mechanism of a bolted fault.

With an arcing fault pressure is proportional to time and arc power. Even minor power arcs generate sufficient pressure to blow the doors off of relatively well sealed enclosures with 1 to 2 cycles when it reaches around 5-10 PSI of pressure. The pressure rise is essentially bounded only by the enclosure mechanical strength. Type MC cable is certainly much smaller in volume so I'd expect a much faster pressure rise. Some enclosures are designed to vent hot gasses safely during faults such as arc resistant switchgear or "explosion proof" (flame gap type) hazardous location enclosures, both of which use some kind of vent to release pressure in a controlled and predictable way. Type MC isn't designed for this so as with electrical enclosures during an arc flash, the metal peels apart at the weak point (the spiral). Experimental testing is required to predict the pressure at which this occurs but all the pressure data in the literature for equipment not intentionally designed for high pressure ratings gives way at around 5-10 PSI.

There are calculations available for the amount of metal consumed per second from an arc and this is used for a "burn through" scenario with say a wall in switchgear but well before that happens, the doors are either severely bent or blown off by air pressure. Since the scenario presented is "blow through", melt through really doesn't apply. This is more akin to the failure mechanism in the scenario presented but is highly unlikely as the actual failure mode. By the time a "blow through" the sheath occurs, the insulation will have melted or burned off the conductors and phase-to-phase arcing would have opened up the sheath from gas pressure alone. Burning through the sheath might be a better predictor of when the cable finally burns completely in two assuming that the overcurrent protection did nothing.

There are thermal damage curves for insulation. The Neher-McGrath model is generally used to predict maximum safe operating zones within this thermal limit (before any damage occurs) which applies to the conductors within the cable and gives rise to the concept of "ampacity". This applies more on the low end though when damage is more long term (minutes to hours). Given enough time or incidents if the overcurrent protection is insufficient the cable insulation will fail. At that point we get an arc and we can go back to discussing overpressure failures.

Further there is a basic problem in your scenario. Faults in cables do NOT stay in place. You get arc tracking. An arc on some kind of bus or cable travels rapidly (hundreds of feet per second) away from the power source via magnetic propulsion. It eventually stops when it encounters some kind of "barrier" (not entirely well defined yet) where the "arc flash" as we conventionally know it occurs. This arc tracking phenomena has confounded analysis of overhead power lines in terms of arc flash safety because the lineman may not be at the location where the arc started, or the arc that the lineman initiates might move rapidly away from the work site leaving them relatively unscathed.

So looking purely at hypotheticals you need to I guess be looking at whether or not an arcing fault can occur in the first place. It is possible to lower bound the solution, BARELY. Take a look at the articles section where I penned something collecting the data available on low voltage arcing faults. You can also consider Ayrton's equation or any number of others all of which predict the current in a power arc given the available fault current, voltage, and arc length (assuming it's equal to the arc gap). This would give you a lower bound of sorts but it won't be what you are looking for.

So far both theoretical and experimental analysis has yet to predict the lower bound on arc formation or extinguishment. It would be very useful if such a formula existed because we could categorically exclude a huge amount of low voltage equipment from consideration when it comes to arc flash, and we could even extend this up towards higher voltages by increasing gaps to the point where arcs are not stable. So far the closest thing I've seen to this is Schneider's patented "arc block" designs which basically place arc chutes at the entrances/exits of enclosures.


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