Oly wrote:
The WPaper suggests the "conclusive" result that <= 0.24kV single phase systems are not able to sustain an arc and quickly self extinguish.
One bit of information stands out as a "hmmm" moment. The investigation was set up to simulate a typical lineman's exposure to an arc flash event. In short, the secondary leads of an un-energized, 7.2kV/240-120V XFMR were connected by a teaser wire across a gap ranged from 1/8" to 1". The secondary leads were energized by closing a 7200V primary riser fuse. So While the arc was in motion the transformer was in in-rush building a flux field.
What affect do you suppose this had on the arc? Would there be significantly different results if the circuit closure was on the secondary of an energized XFMR?
Its also a utility using their resources so no surprise there. Energizing the transformer would absorb energy so likely it would be hard to get past the first few cycles, but after that...since Bruce Power did something similar with 130 VDC and couldn't sustain more than 800 ms with 20 kA available current and optimized at around 1/4" gap, something that just doesn't happen in substation battery systems, it seems to suggest that at least at 120 V, arc flash is a non issue, AC or DC. So moving onto 240 VAC, the question is can anything be sustained. The Bruce Power data says yes for DC. PG&E did a similar study to the one in question which suggested 'no' but the wire gauge was the old IEEE standard which is now known to give false indications. If you use a short (1/4") gap, a barrier, and the right size box, exceeding 1.2 cal/cm2 should be possible but I'm interested in threshold conditions since the empirical formula does not provide valid results if self a extinguishing is going on and also because as voltage decreases, the minimum arc restriking voltage becomes a major factor. The current (circa 2000) IEEE 1584 test data set has a single point. All other tests failed to sustain an arc. Some subsequent data seems to suggest that there might be a hazard but again, how can we tell? Searching OSHA accident investigation data showed that there were a couple cases of what is clearly low voltage arc flash injuries requiring hospitalization but these were "close encounters" like an IT technician plugging a laptop into a faulty power strip which is effectively beyond what the IEEE standard addresses. The more interesting case is when two electricians in Georgia started disassembling a construction panel before the lineman got there. One died and the other was hospitalized. The voltage is not given but it seems unlikely it was anything other than 240 V. So it seems to suggest that in the exact scenario described (arcing fault on an energized distribution transformer), a fatality is at least possible. So the question isn't whether such a scenario is credible but at what is the threshold especially considering how common 240 V is.
