After much speculation about the fate of the 125 kVA transformer “exception”, the 2018 Edition of IEEE 1584 – IEEE Guide for Performing Arc-Flash Hazard Calculations has finally been published and made it official. The 125 kVA transformer exception has been deleted!
In its place is the new language:
“Sustainable arcs are possible but are less likely in three-phase systems operating at 240 V nominal or less with an available short circuit current below 2000A”
The original language is from the 2002 Edition of IEEE 1584. It was based on a few tests that indicated sustaining an arc flash at lower levels of short circuit current and lower voltage would be unlikely due to a limited conducting plasma and limited voltage to support the arc. The original 2002 language stated:
“Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low-impedance transformer in its immediate power supply”
However, subsequent testing for the 2018 Edition of IEEE 1584, along with using new electrode configurations that were introduced indicate that under very specific conditions, it was possible to sustain an arc down to a much lower value of current which was used as the basis for the new 2000 Amp value. However even at that level, there are no guarantees that an arc would never sustain. This is why the words were added: “Sustainable arcs are possible but less likely.” Just like it states: It is unlikely but, never say never.
Another departure from the original language is to base the lower limit on short circuit current rather than using kVA as found in the 2002 edition. Why? Because the issue is not really dependent on transformer size. It is dependent on short circuit current and there is a relationship between transformer size and short circuit current. This time around, the focus is simply on the current – independent of what resulted in the lower current. That way if there is a case such as a long feeder has a low short circuit current but happens to be served by a larger transformer, the new language may be used regardless of the transformer size.
Some often refer to the 125 kVA exception as having its origins from NFPA 70E and think it was already deleted. That is not the case. It was added to the 2009 Edition of NFPA 70E but the language was derived from the 2002 Edition of IEEE 1584. However, it was subsequently deleted in the next Edition of NPFA 70E although the original IEEE 1584 source remained. Until now.