JKlessig wrote:
"In general, lower voltages (120V 240V) cannot sustain an arc"
This sort of blanket statement has always bothered me.
Arc welders operate with open circuit voltages below this level,
and yet manage to sustain an arc for extended periods.
Clearly it is POSSIBLE, although unlikely
Need more clarification indeed. Lower voltages cannot sustain an arc over the arc gaps used for terminals with that type of equipment (1/4" or larger). Arc welders use much smaller arc gaps. Also and this matters, arc welders have some kind of insulating gas or liquid surrounding the melt zone (flux or gas). Without this protective envelope, the metals would oxidize rapidly and prevent further arcing. So the two are not directly comparable.
Also there is a difference there as far as "cannot be sustained". An arc CAN certainly be sustained, but may not be sustainable "indefinitely" where "indefinite" following current practice would mean for at least 2 seconds. So during those two seconds we need to know how much heat (incident energy) is released, and for how long (if it doesn't go for a full two seconds).
As to the comment that single phase arcs haven't been modelled, actually paradoxically this is not true. At least in open literature, 3 phase arcs are empirically modelled to a high degree of accuracy but not theoretically. In contrast single phase arcs have been modelled historically such as by the Duke Heat Flux software or ArcPro, and some empirical calculations are available from EPRI that do a pretty good job overall. These methods are used specifically for calculating incident energy above 10 kV, where the empirical model in IEEE 1584 at the current time does not work.
The inherent difficulty with single phase low voltage arcs (or even 3 phase arcs) is that they are not very stable. Predicting incident energy for these arcs will probably of necessity have to include a maximum arcing time value but so far the data is all over the board so it somewhat defies modelling attempts. So far most of the attempts try to "normalize" it to 2 seconds and then model it based on this but it is very obvious from looking at the data that this approach is not going to work.