We are performing a study using the IEEE 1584 - 2018 in SKM and when we run the AF calc a note is generated stating that the bus gap is outside of the IEEE 1584 range on the secondary side of a 35kV:480V transformer. Looking at the transformer secondary, the gap between phase conductors is about 6" (154mm) which is typical for this type of equipment so I am confused as to what the issue is. Any suggestions?

Take a look at the NESC.

Mike

I don't have that in front of me at the moment (hard copy at the office). Could you summarize?

I was conducing arc flash testing years ago on a padmount transformer / 480 volt side. I could not get it to sustain (don't use that statement as a formal statement) so I ended up bridging the gap with copper bus to get an IEEE 1 inch gap. Don't forget an arc flash could be from a dropped tool which will (initially) create a gap much smaller than 6 inches.

NESC

The 2017 NESC Table 410-2 for (arc rated) clothing systems uses the following gaps:
1 kV - 15 kV = 2 inches
15.1 kV - 25 kV uses 4 inches

For Pad mounted transformer up through 600 volts, NESC Table 410-1 references a technical paper by a couple of friends/colleagues Arc Flash Testing of Typical 480-V Utility Equipment Marcia Eblen and Tom Short. Their results indicate they could not sustain an arc flash on the secondary of the transformer and the NESC table indicates a maximum arc rating of clothing at 4 cal/cm^2 as a result of their finding.

Note: This is NOT an interpretation or recommendation - just referencing other work/standards.

Hope it helps.

btw, I can ask this as the "Question of the week" next week! - Watch this space.

Thanks for the reply Jim! I find it interesting that there seems to be this gap (pun intended) in 1584 and the results of the Elban/Short paper. In SKM, when we have the bus gap at 153mm, the AFIE is 20.8cal/cm^2 (Lee equation used); when the bus gap is set to 76mm, the AFIE is 71.2cal/cm^2; but the NESC says regardless of the bus gap - it will be no more than 4cal/cm^2. For the sake of our clients and future studies, how do people generally report the AFIE for the secondary side of a MV-LV pad mounted transformer? Do most just take the conservative approach and list the higher value on the labels?