The IEEE 1584 125 kVA transformer less than 240V exception is presently under review. The existing language states:

4.2 Step 1: Collect the system and installation data and states:

[INDENT]
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. The idea is that an arc flash at a lower short circuit current and lower voltage will likely not result in a significant amount of incident energy. This or course depends on many other factors such as the enclosure size, bus bar spacing etc.[/INDENT]

Here is the question:

Have you ever experienced a “significant� arc flash on a circuit served by a transformer smaller than 125 kVA and at a voltage less than 240 Volts?

• Yes
• No

I assume you are excluding =240V (i.e. 120/240 1PH3W, 240/120 3PH4W, and 132Y/230 3PH4W) from this survey.

Correct. I am only looking at the exception as presently defined. We are still debating whether to include or exclude 240V. - Thanks for eveyone's input!

I have seen some of your posts about possible revisions to this exception. Been wondering myself if anyone has had a bad arc flash on the "exception" circuits. Is that why this exception was deleted from NFPA 70E?

It was deleted because it was a copy of what's in IEEE 1584, but with a typo: <= instead of <. So now they moved the wording to the source (with reference), so no more problem of incorrect wording.

Vincent is quite correct, the typo was a problem and is part of why it was removed. There was a Tentative Interim Amendment (TIA) during the 2009 Edition to remove it completely without reference to IEEE. The TIA was rejected however the 2012 edition addressed it instead.

There has been some research that shows you can have fairly significant incident energy at lower short circuit currents and lower voltages (208-240V) under certain conditions and that got people’s attention regarding this exception. I was involved with some of this testing last year.

Specifically, if you have a panel with no main and the arc flash begins as a 3 phase event (near the supply is better) and the bus bars terminate into barriers, the arc flash will sustain longer at the lower levels of fault current. Down to 4kA to 6 kA has been recorded depending on the voltage 208 to 240V. An arc flash at the low levels meeting all of the condtions can result in an incident energy greater than 1/2 cal/cm^2.

The problem is so many conditions have to be met for this event to occur, I believe the likelihood of a severe low current / low voltage arc flash is very low. (just my own opinion).

There is also a movement underway to better define the roles of NFPA 70E and IEEE 1584. NFPA 70E defines electrical safety requirements and practices while IEEE 1584 defines calculation methods. You are beginning to see calculation references disappear from the body of 70E and you will begin seeing PPE references disappearing from the next edition of 1584.