3 phase vs. 1 phase arc flash calculations

What criteria is everyone using to decide when to use 3 phase i.e. IEEE 1584 vs. single phase (arc pro etc.) calculations. I know at lower voltages and in equipment like padmount compartments, switchgear etc. three phase IEEE models are used but say it is a 12.47 kv overhead line or a 115 kv line. Do you consider conductor spacing to be prohibitive for a single phase event to escalate to three phase at this level? Do you base a line-ground fault clearing on ground relay settings? We are just getting started with this.

single phase lines - single phase exposure. three phase lines...................you get the point. Arcpro handles multi phase faults - just use the multipliers provided with the software based on the exposure. Ground fault relays clear ground faults as do phase relays depending on the TCC's and fault current level. Your biggest unstated question is what fault current level are you going to model the arc flash at?

3 phase faults

Sorry but I cannot agree that 3 phase overhead lines should be considered as having the ability to progress to 3 phase faults. Greater phase spacings tend to not allow that to happen. It has been my experience that an escalation to a 3 pahse fault is the exception, not the norm. Expect that will not be a part of the standards any time soon.

Alan

So we don't agree - that's ok. I see way too many cases where jumpers, cutouts, arresters, switches, etc are mounted reducing the NESC minimum phase-to-phase spacings and it causes performance problems. Even on 8' crossarms, many lineworkers have moved phases to adjacent drilled positions to avoid trees, signs, buildings without regard for the consequences or an understanding of how they have reduced the BIL of the line. Three phase overhead lines have a > 0.0 probability of experiencing multi-phase faults, therefore from an arc-flash hazard analysis standpoint I treat them as such.

Just wondering if anyone feels confident using the IEEE 1584 equations for 3 phase distribution circuits in air (using SKM, Easy Power, EDSA, etc) for <15kV or if you would use Arc Pro and apply the multipliers?

Has anyone compared the two methods and results?

Single vs. Three Phase Arcs

Steve,

I don't believe Arcpro is valid for three phase arcs at all, and not sure where they get their 1.5 mult. for single phase arc in a box. We are not at all comfortable with Arcpro for the low voltage equipment and are examining the alternatives. Presently we are using the 1584 equations against the 70E stuff to see how they compare.

Alan

Ok. More specifically, if you were to model 12.47kV overhead distribution circuits, would you use Arc Pro and treat it as a single phase arc in air, or use the IEEE 1584 3 phase model?

For <1000V I am definitely going to use the IEEE 1584 model. It really bothers me that the <1000V, 4 cal/cm^2 wording got into the NESC.

Yes on the 12.47 kv question and single phase in air. I think we will end up with 1584 for the low voltage stuff. As I have said before, I do not consider the 4 cal exception in the NESC at all either.

Alan