This week's question needs a bit of introduction.

Arc flash calculations based on IEEE 1584 include using the gap distance that an arc may jump across as one of the input values. This distance can obviously vary depending on where the arc flash occurs as well as the equipment design.

The existing 2002 edition of IEEE 1584 provides a table of "typical bus gaps" in millimeters based on equipment class and include:

Equipment Class / Gap Distance
15 kV switchgear 152mm
5 kV switchgear 104mm
Low-voltage switchgear 32mm
Low voltage MCCs and panelboards 25mm

These typical values are almost always used as the default since the actual gap distance is rarely known and obtaining it would be quite difficult. Software typically defaults to the typical values based on the equipment class selected.

The next edition of IEEE 1584 is progressing and a question has come up. Would you like to see the gap distance between bus bars listed on the equipment supplier drawings so a more accurate gap distance could be used as part of the calculations?

This is purely a hypothetical question since there is no such initiative to add this information to the drawings. Also this would only help for new equipment (but you have to start somewhere).

So here it is:

Would you like to see bus bar gap distances included on equipment drawings?
• Yes - More realistic data is better for the study
• No - Stick with using the typical values
• Other - please explain

Your comments, thoughts and opinions are VERY important so please feel free to provide your candid thoughts. Thanks!

This is a question I have been asking for some time now. I am pleased to see others asking the same.

Within my organization (college campus) high incident energy of the secondary side of most of our utility transformers has become an issue requiring evaluation. It seems our consultants have relied on the typical arc gap of 32mm when determining the hazards at the secondary terminals of 3ph padmount transformers. I believe this creates an overly conservative assessment of the hazards.

EPRI has conducted some testing for the utilities and results published in the National Electrical Safety Code (NESC-2012) Table 410-1 suggesting the maximum incident energy of pad-mount transformers up to 600V secondary is 4 cal/cm^2. Of course, I do not know their test criteria, but find it worthwhile to investigate. The link here, starting at slide 50 gives so information to Low Voltage Arc Flash and NESC ... http://www.rurdev.usda.gov/SupportDocum ... Update.pdf

Interesting question. I am somewhat mixed on this one. It would be good be more precise IF it has a significant effect on the results / PPE selection. However, it will also make the study more difficult - tracking down this information even if it is on the drawings. I guess this will all keep evolving.

1. Yes, this would be more realistic. However if we don't also model arc self-extinguishment, it it going to start producing more unrealistic results. We already know that the current methodology (IEEE 1584+ASTM 1959) works but is probably overly conservative based on no documented cases of >2nd degree burns so far, and that's based on the numerical 95% success rate in the IEEE 1584 standard.
2. For certain types of equipment despite a lack of standards for bus gap distance, at least at the 600 V class and the medium voltage classes I've found in practice that nearly all manufacturers use nearly identical bus gaps. Smaller bus gaps lead to more equipment failures when it is contaminated while larger bus gaps increase prices so there is a natural mechanism to keep everyone as uniform as possible.
3. This goes out the window on certain equipment where bus gap distance is not critical. At that point, distances are all over the place and generally decisions are made based on cosmetic appearance or mechanical considerations alone. This is especially true in transformer compartments for instance where space is nearly unlimited.
4. Increasing the bus gap increases the incident energy because the arc voltage increases and the arcing column gets longer, providing a larger radiant energy source. However, longer arcs are also more unstable. Once an arc ignites and temperature rise the room temperature flashover distance no longer matters though so something as simple as flashover distance is not a predicter of arc self-extinguishment in and of itself. At very long lengths EPRI has been doing research and has shown that the arc characteristic also changes quite a bit but for most industrial cases this would not apply.
5. Thus I'm a little ambivalent here. Changing the arc gap where it is appropriate can be more realistic, especially when decreasing the gap. But this is not likely to be the typical case and longer arc gaps are probably more likely the appropriate nrom. Thus we need to be careful in specifying the upper end if we don't want to continue to produce crazy, nonsensical results that end in destroying credibility of the standard.