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.

Medium voltage systems would not be considered as a 3P hazard on open poles. Only when buss bar spacing is closer than common powerlines should you consider 15kV systems a 3P hazard. It is quite rare to get three phases going on a powerline and the arc moves down the line at about 55 MPH at 16kA so it doesn't stay at the worker location more than about 2 cycles unless slowed by a crossarm or stationary in a cut out.

Hope this helps,

Hugh Hoagland
e-Hazard.com
502-716-7073

Nope. That's at atmospheric temperatures. The dielectric breakdown of air rapidly declines as temperature increases.

In a closed enclosure you can easily heat up all the air inside and get multiple arcs everywhere. That's what I typically see post-mortem when a tracking fault gets out of control.

In open air though that's about it. Reasonable phase spacing for open air would seem to hold to prevent arcs for going 3-phase. IEEE 1584 gives 3 phase open air arc gap data up to 153 mm (6"). After that point ehy couldn't maintain a stable arc. That seems reasonable for 5 kV spacing, and I've seen equipment down to 4" phase spacing at 5 kV but up to 15 kV, I tend to be more conservative than that.

There is a lot of data on the subject of how close you need to get before a flashover occurs in IEEE 516, which is what the shock tables in OSHA 1910.269 as well as NESC are based on. The data also includes multipliers for temperature and pressure which would apply.

I see what you mean that exposure to a three-phase system requires arc-rated faceshields at a lower exposure level (2-4 cal/cm2) then the single phase exposure level of (2-8 cal/cm2). I would think you can only justify it being single phase for the overhead distribution system and would you need to stick with three phase when working with pad-mounted switchgear.