Having just spend the weekend with a few others developing Draft 5 of the next edition of IEEE 1584, a question about bus gaps came to mind.

I asked a similar question at the Arc Flash Forum many years ago but arc flash studies have continued to evolve since then.

Arc flash software will usually default to the bus gaps found in IEEE 1584 i.e. 25 mm, 32 mm, etc. This week’s question is about using gap distances that are different than found in IEEE 1584.

Here is the question:

Do you/consultants etc. use bus gap distances listed in IEEE 1584?
Yes
No - we use something else
It depends
We don't perform studies

If you use something else, please explain.

I change them if the equipment obviously doesn't match "typical" equipment (bus gaps are much wider) but typically I just leave it alone.

Of course the problem as bus gaps widen is eventually we reach a point where stability becomes a problem, which is entirely the principle behind arc chutes. And this becomes an inherent modelling issue that has to date not been solved.

We usually use the default IEEE 1584 gap distances. If we included time and expenses in our study proposals for measuring or looking up gap distances in all equipment, we would not do any studies.

Where we know that distances are different and easily determined, like on the secondary of padmount transformers, we have used different values.

It Depends….with that using the IEEE defaults 90% of the time, in reviewing some submitted SCCAF reports some things I have noticed, in the regional area I work / live in there are still some industrial / commercial clients that operate on a utility 4.8kV ungrounded delta utility source. With that the specified equipment is always 15kV rated gear sometimes if the study is done by someone not familiar with the system and how some of the commercially available software work, when you enter your system voltage as 4800 by default it will set you bus gap to 102mm [5kV], however when using the 15kV gear construction this has to be overridden to the 152mm as defined in IEEE to get accurate results when using a owners defined hazard level criteria that defines hazard levels [ex. 1, 2, 3, etc.] it can make a difference also knowing the system grounding I have seen reports that indicate the 4.8kV as a grounded system when it is installed as un-grounded and that compounded with the bus gap can make a difference.

With all that using the typical bus gaps unless otherwise defined by an equipment manufacturer or some custom piece of equipment. One item I always struggle with is when low voltage dry type transformer are to be labeled, not only are the bus gaps different there is also multiple voltages in a single enclosure [ex. 480-208Y/120V xfmr.’s] where you have the LV termination lugs intertwined with the HV lugs alternating H1-X1-H2-X2-H3-X3, etc. that can range in distance from a 30 - 300kVA dry type. I am not aware of any testing for this type of setup where you HV “bus gap� could easily be 70mm with a LV terminal in between….maybe I am wrong Jim??

There has not been any testing with the transformer configuration that you list - although that would be interesting!

The bus gap distance in Draft 5 of IEEE 1584 2nd Edition (just finished this a week ago) are:

208 - 600 Volts: 6.35 mm (.25 inches) to 76.2 mm (3 inches)
601 - 15,000 Volts: 19.05 mm (.75 inches) to 254 mm (10 inches)