Fault Currents for ArcFlash Calc

[Modula2]

Has there been any concensus on which currents to use as bolted fault current on MV, Hi-R grd systems. I ask because arc flash standards caution against overly conservative fault calcs. However, fault calculation standards are just that. I have a max level from Ansi C37 momentary 0-1 cycle. For 1.5-4 cycles, there is the C37 interrrupting level, where motors have reduced contribution compared to mom. All motors OFF is a minimum level. L-L for all these is also a possible occurrence. That leaves max from C37 mom., and min with motors OFF, L-L fault. I did not see asymmetrical currents involved in arc flash calc standards (unless I missed it). Thanks

 

[dpc]

As far as grounding method, it has no impact - use three-phase bolted symmetrical fault current.

As far as motor contributions - there is no consensus. Some people use the ANSI C37 momentary network and some use the interrupting network. IEEE-1584 just says to use the "bolted fault current".

Per IEEE-1584, symmetrical current is used, not asymmetrical.

You do have to do more than just find the maximum fault current, since this won't always be the highest arc energy.

Also, keep in mind that the fault current at the arc will not always be the same as the fault current through the upstream protective device that will determine the arc time.

 

[wbd]

Another thing to keep in mind is that the 3 phase bolted short circuit may not be the most conservative, if you are close to the source, ie a generator.