Arc Gap and Working Distance for Higher Voltages

[DistCoop]

Has anyone given any thought to the appropriate arc gap or working distance for voltages over 15 kV for arc flash studies... say 34.5kV or 69kV? The suggestions by OSHA seem too small in my opinion. If you consider arc gap for 69 kV, they suggest an arc gap of 0.1* V_LG (approximate dielectric strength of air). That comes out to only 4". If you look at youtube arcing videos, arcs are far longer than 4". As energy is proportional to arc gap, that could be a significant underestimation.

15" seems a reasonable assumption of working distance for 34.5 kV (IEEE recommended for 12.47 kV), as people may glove 34.5. What would you consider a reasonable distance for 69 kV? Would you even worry about calculating energy at that voltage?

Thanks for your thoughts.

 

[wbd]

Not sure about the youtube videos but is it possible that what you are seeing on the video is after the arc has started and has elongated due to magnetic forces and ionization of the air. Until the dielectric threshold is breached there should be no arc. Of course, one could use a higher value of gap to be conservative but that will increase the incident energy. At what point have you gotten too conservative to the point where linemen have to wear cumbersome PPE?

OSHA states a 15 inch working distance for gloving, so that is the value that needs to be used. As far as 69KV, I assume this is hot stick work, so what is minimum stick length used? 3 feet? Of course, that 3 ft is the length of the stick which does not always equate to worker distance as the distance may be shorter based on how the stick is gripped and used.

I assume you are using ArcPro for this?

 

[bwen08]

For the arc gap, I used two equations, one for line-ground (VLG[kV] * mult * 0.1) and one for line-line arcs (VLL[kV] * mult * 0.1). The multiplier was pulled from the new OSHA standard for calculating the MAD distance based on the maximum transient overvoltage possible, mult=3 for 72.5kV and below. I felt including this transient overvoltage was proper, whether it's right or wrong, the transient multiplier results in a higher arc flash energy for ArcPro (worst case scenario is better in arc flash studies right?)

For the distance to the arc, I utilize two methods. For under 46kV where energized work is allowed, the working distance is 15". Although our safety standards do not allow us to perform energized work above 46kV, I utilized the MAD (from OSHA) as the working distance since the MAD must be observed at all times. This allows for the scenario where perhaps distribution under-build is being worked energized with both circuits set to hot line mode.

 

[DistCoop]

For the working distance, that's what I do. I use Table R-6 rather than calculating it, but that seems appropriate. Arc gap is more of a question to me. I'll often use 20", as that's the greatest gap that ARCPRO has been validated against. My thought is that it's far more conservative than OSHA's gap length, and the incident energy is often pretty small in my scenarios anyway. I just wish I had a little more technical justification than "seems about right."