One of the types of equipment in the Hazard / Risk Category Classifications [Table 130.7(C)(15)(a)] is "Other 600 V class equipment" and includes "Lighting or small power transformers (600 V, maximum). Is "small power transformer" defined anywhere? If not, what is the maximum KVA rating you would consider "small" for a power transformer? It's understood that regardless, to use this matrix the max short circuit current, fault clearing time, and working distance noted must be within the stated limits. Thanks.

Currently ieee 1584 would classify this as 125 kva or smaller with a maximum 208 volt secondary. It has been misstated in the past as 240 volt. This exception is going away though.

Paul, could you expand why the exception is going away?

The 2009 Edition of NFPA 70E mis quoted the IEEE language. IEEE says less than 240V and NFPA says 240V or less. In 2012, NFPA 70E deleted the language and referred to IEEE 1584. We have actually defined "informal" boundaries between IEEE 1584 and NFPA 70E. IEEE handles the calculations and NFPA takes care of safety practices, PPE etc. You can see the result of this as equations and any type of calculation has been moved to the Informative Annex section of 70E and out of the main standards.

As far as the exception, as part of the IEEE 1584 committee, I was heading up a task group to review this a while ago. The last place we left it was we do want to keep an exception but we all agree the value needs to be lower. Perhaps 30 or 45 kVA transformers. More recent testing shows you can sustain arcs to lower values of short circuit current than we first thought.

Also, we were thinking of a minimum default incident energy if these circuits are excluded. Perhaps 4 cal/cm2. This is because right now if someone uses the exception, the remaining question is what do you do about labeling and PPE. The default incident energy option would help with that. None of this is official and will likely still evolve with time before the next edition of IEEE 1584 is issued but for now, this is where we are for now. Stay tuned!

Jim,

Can the IEEE exception still be used today while it is being reviewed by the IEEE task group?

The best publicly available information is the comment (not truly a rule) in IEEE 1584. The best available rule is in IEEE C2-2012 which if I recall has a cutoff for all equipment below 250 volts but is also under review. Everything by the way in both documents is under continuous review. They are updated as our knowledge increases. C2 results came from PG&E experiments meant to simulate real world conditions. 1584 comment was a guess (back in 2002). More recent unpublished joint NFPA/IEEE lab tests show that arc flashes exceeding 1.2 cal are possible under special lab conditions. From around 2000 to 2011 there has been one OSHA reported case of injuries requiring hospitalization from a 240/120 temporary construction panel and one case of a tech holding a 120 volt cord in his/her hands when it exploded. The second case is an extreme working distance issue. I don't know what the transformer size was in the first case but since the geniuses in shorts that did it could not wait for Georgia Power to show up I would guess the only protection would be a transformer primary fuse. So it is even possible in the real world, just not very likely except in rather extreme cases.

follow up on the question...some of this is a judgement call. Engineers argue about theoretical things all the time but in terms of settled knowledge I would stick with C2 or 1584 but be aware that this is likely to change soon.