available fault current and 2011 NEC

[kwired]

The panels in question were almost all control panels. I believe the NEC requires the SCCR of an industrial control panel to meet or exceed the available fault current of the feeder or branch supplying it. Don't have the code handy to look up the articles but there are lots of papers and what not out there from the circuit protection manufacturers about compliance. Some sources I look at say this DOES affect arc flash calculations but I am not expert on arc flash at all.

If the SCCR of any component is exceeded by the ability of the supply, there is risk of a violent failure when a failure happens. NEC is in general not going to support exceeding ratings and any exceptions would be pretty rare. I would like to think in absence of any other requirements ratings are marked on a device it would at least be a requirement of 110.3(B).


I am no expert at details of arc flash calculations either, but more available fault current means there is more available energy to release in an arc flash event. Some kind of hang themselves thinking higher voltage means more energy, but voltage alone means nothing, it is the combination of voltage, current and circuit impedance that determines power and energy at the incident location.

 

[GoldDigger]

I am no expert at details of arc flash calculations either, but more available fault current means there is more available energy to release in an arc flash event.

Only if drawing that higher fault current does not result in a correspondingly faster opening of a protective device.
You are going in the right direction, but you left out the time element.
Energy in this case is voltage (often determined by the arc distance rather than the supply voltage) times current times time.
If the only working OCPD is a utility fuse or recloser on the primary, the time factor can be very long.

 

[kwired]

Only if drawing that higher fault current does not result in a correspondingly faster opening of a protective device.
You are going in the right direction, but you left out the time element.
Energy in this case is voltage (often determined by the arc distance rather than the supply voltage) times current times time.
If the only working OCPD is a utility fuse or recloser on the primary, the time factor can be very long.

You are right, when before I submitted that I was looking at it seeing the words current and energy used in same sentence and thinking something was not quite right because VA (power) and energy are not the same thing, I was missing the time component that is added for energy.

So back to Wallypiper: an incident with lower current but longer duration can possibly give you a higher arc flash hazard than an incident with higher current but shorter duration. But increase available fault current and leave everything else the same will increase the amount of energy released in an arc incident. Increasing available fault current can be done several different ways, larger source, increased size in supply conductors just to name the most two common ways.

 

[GoldDigger]

Whenever somebody mentions that the arc flash incident energy is often greater in a lower voltage situation, one explanation that comes up is that the available current is often greater at the lower voltage, while the arc voltage (a function of separation and ionization) could be just as high as the arc voltage fed by a higher voltage source.
Plus the time component in many cases being longer because of the OCPD situation.