I model a transformer with multiple secondary windings.

Using the pre-2012 exclusion, results are like this:
480V, 350KVA, with NGR, CAT1, 2.8 cal/cm^2
240V, 10KVA, with NGR, CAT0, 1.2 cal/cm^2
208V, 40KVA, no NGR, CAT0, 1.2 cal/cm^2

Now, using the post-2012, report as calculated:
480V, 350KVA, with NGR, CAT1, 2.8 cal/cm^2
240V, 10KVA, with NGR, CAT2, 6.9 cal/cm^2
208V, 40KVA, no NGR, CAT3, 15.4 cal/cm^2

Perennial problem, the smaller secondary windings cannot easily affect the transformer primary side fusing.
Still, quite a jump from CAT0 to CAT3 at 208V, 40KVA.

As if the software is overstating the risk. My preference was the pre-2012 result.

What software were you using? What was the available short circuit current?

The 1st values come straight from the exception, so I'm curious if you've run the pre-2012 exception WITHOUT using both the exception and the 2 second rule just to see how those values compare. To me it seems as though the post-2012 result would almost have to come from a long duration event.

Perhaps I misunderstand what you are saying, but it seems to me that you shouldn't be using the exclusion in the first case.
The Transformer is clearly rated >350 KVA, and the exclusion is not on a "per winding " basis.


I also strongly dislike calling the "2 seconds" a "RULE". At best, as I see it, it is a suggestion, or a guideline, and comes out of thin air as far as I can see.

The IEEE P1584 Standard does include a "2 Second Rule" it is also noted in Annex D now in 70E and Z462.

The existing IEEE P1584 formulas currently calculate a very high incident energy, you will see this change in the new IEEE P1584 forumulas based on additional arc flashes at the lower voltages.

Currently the 125kVA rule still applies, no arc rated PPE required for less than or equal to 125kVA, single transformer, 240V.

Regards;
Terry Becker, P.Eng.
Owner, ESPS Electrical Safety Program Solutions INC.

I know I hate to saddle a dead horse on this issue but doesn't the IEEE 1584 say "The transformer supplying the circuit is rated less than 125 kVA"

Just checking my own sanity.