wilhendrix wrote:
Regarding the 125 KVA and below 240 volt "exception".
That exception was removed from in 2012 70E Standard. If the 2012 NFPA 70E Standard pointed that out, I missed it. I learned about the deletion because Mersen Fuse sent out a news letter detailing the changes in the 2012 Standard. They made a point about the exception being removed.
The exception was removed from 70E. It was not removed from IEEE 1584. But that's just one of the 8 arc flash models referenced in Annex D and the only one where the exception exists. The other models do not have an exception so it would be inappropriate to have it in 70E except with respect to the table-based method that 70E specifically supports. Thus it was entirely appropriate to remove the exception but it is not appropriate to conclude that this means that a significant risk exists for low voltage arc flash incidents.
I've summarized the exception as well as a whole lot more data here:
https://drive.google.com/open?id=0B6mGR ... Dl5VzlDSWMThe big thing to realize is that even though it is possible to die from an arc flash injury below 250 VAC, the likelihood of such a thing is extremely rare when you consider that OSHA has only documented one fatality in 20 years..that amounts to millions of worker-years and vastly below the generally accepted actionable threshold of "1 in a million" used by government agencies.
As a similar example, consider being struck by lightning. It should be pretty obvious that being struck by lightning has a pretty significant chance of being fatal, if not severely injured. The likelihood is about 1 in a million per year. This is close to the point where OSHA would require us to address it except with some simple common sense recommendations such as not playing golf in a lightning storm, or similar activities.
In the case of an arc flash injury, the odds of a fatality (or severe injury) from a system voltage that is under 250 VAC is much less than being struck by lightning (probably closer to being struck by lightning twice) so there's no compelling requirement to address it so long as workers are not taking undue and unnecessary risks such as attempting to disassemble a temporary construction panel while wearing flip flops and shorts instead of waiting for the lineman to arrive to open the cutouts (the incident that actually happened).
The rest of the review that I did attempts to quantify the hazard. The issue with using the IEEE 1584 empirical equation or really any existing calculation method for that matter is simply that all of them start with data from stable arcing faults. The model "works" because the incident energy is proportional to time once we can take into account the arcing voltage and arcing currents. Below 250 VAC and especially at 215 VAC and below, this assumption is violated. Arcs tend to spontaneously self-extinguish. None of the models have an upper bound on arcing time other than a rough rule ot thumb of 2 seconds. So they will grossly overestimate the incident energy for real world situations. The best way to data it is to upper bound it based on experimental data. Thus IEEE 1584 for instance upper bounds everything 208 VAC or below fed by a single 125 kVA transformer at 1.2 cal/cm2. IEEE C2 upper bounds everything below 250 VAC at 4 cal/cm2 or less.
But again...this is going down the road of ignoring the likelihood and focusing purely on the hazard side of things. If we look at it from a pure risk point of view, no significant action is required. This is also why for instance even though the IARC has concluded that red meat and processed meats "probably" cause cancer (along with some occupations like airline pilots and painters), we choose not to address it outside of some basic recommendations because the risk simply isn't there (injury is highly unlikely).