OSHA 1910.269 Appendix E Table 3

[wbd]

OSHA 1910.269 Appendix E Table 3 contains footnote 2 for the columns which are 600V and Less and 601V to 15kV. This footnote states "At these voltages, the presumption is that the arc is three-phase unless the employer can demonstrate that only one phase is present or that the spacing of the phases is sufficient to prevent a multiphase arc from occurring."

So the question becomes, what would be acceptable demonstration of spacing sufficient to prevent a multiphase arc?

OSHA does mention in the standard that the dielectric breakdown of air is 10kV per inch. This would give some indication that if the phases are outside this range would that be sufficient?

Or another possibility is if the phases are far enough apart that the other phases would not be engulfed in a arcing incident but how does one know?

For example: In the 12.47kV system in my area, the phase spacing on a typical pole with a crossarm and pole top pin is approximately 3 feet. Would that be considered sufficient to prevent a multiphase arc from occurring?

One reason for all these questions is that it directly affects when a lineworker has to wear a AR face shield or a AR hood/faceshield with balaclava. The values where this is required is much lower for 3 phase exposure.

Thanks to all in advance for your thoughts/opinions.

 

[Duwe6]

I have difficulty using the open-air wire spacing - 3-feet at 12.47/13.8 KV, for a dimension for declaring that arc faults cannot be 3-phase. One little strand of metallic [or carbon fiber] thread between two phases will turn to a track of plasma and generate a huge arc that usually causes the third phase to 'blow' also. And for most man-made accidents, it was the worker that brought a conductive item in proximity of two phases, usually himself. So for personnel protection, you probably should continue with the 3-phase assumption.

 

[wbd]

The other point to mention is in my area it is the practice of the utility to rubber up the other phases not being worked on, which would include the neutral. This would line hose and blankets.

 

[Duwe6]

After 'rubbering up' I agree that the assumptions can be dropped to single-phase.