This question came up and I'm unsure how to answer it. It seems irresponsible for NFPA to create such a table without knowing the incident energy levels available or arc fault currents available to everyone. Makes no sense to me; someone explain why this table is in there.

The tables are provided to be used in lieu of a formal arc flash analysis of the facility. This can be to the benefit of some organizations. The caveat of using the tables is that the parameters must be adhered to. These parameters are the maximum available fault current and the maximum clearing time. So someone has to do some analysis to insure that these parameters are met prior to using the tables.

The way to look at 70E (or NESC for that matter) is that these standards do NOT pass judgement on how to perform a risk analysis when it comes to arc flash. All that they do is to require it. In the case of NESC, they provide a table as one method and then suggest using an engineering analysis as an alternative without describing what is in that analysis. In the case of 70E, it also provides a conservative table and then in an Annex, they reference around 6-8 different models without any discussion of the accuracy of any particular model so again, a judgement as to whether one model is preferred over another is not being made.

This leaves it up to the user to determine what the best model to use is. In general the issue with the tabular methods is that they give recommendations for extreme values as far as determining the hazard (70E does a risk assessment). Using a calculation method using real numbers will produce lower values in most instances. The one caveat to using the tables which has been pointed out is that you have to adhere to the parameters which requires a particular maximum short circuit current and a maximum fault clearing time. Both of these would have to be determined with a short circuit study and a coordination study. However once you have gone this far, you've already done all the data gathering required for an arc flash hazard study using another method.

The tables were there at the same time as the calculations began. IEEE 1584 was not out yet and rough papers were in place on HOW to calculate. Ralph Lee's original paper was being used in a few cases knowing it overestimated the energy. The committee developed these tables based on experience. Many of the companies and experts on the committee had been using PPE successfully for years. The IEEE Yellow Book had some information from a paper from Bill Jordan with his experience as one of the first safety people to have a "flash suit" developed. They had not been tested until I did it in 1997. There were no ratings but EVERYONE who wore the "pickle suit" lived in arc flash events in his plant in Texas (they were made of Green Nomex and thus named "pickle suits").

The nice thing about an arc flash is that it is VERY focused. No more than 25% of the body will see the calculated energy (based on our study of about 40 incidents with full information and about 200 with partial information only in oil filled cable or oil filled transformer explosions do we see more than 25% body burn). So basically almost 95% of the people will live in arc flash events if we just wore arc rated gear. I'm into seeing better quality of life than 25% body burn and really think protecting the hands and face is critical to life quality so there is more to it but the tables were based on the collective experience of the group and backed up by research on the subject.

Calculations have made the chance of getting the worst burns much lower if you do all the engineering studies and do mitigation but it is still delusional (in my mind) to say I've engineered OUT the hazard and drop flame resistant daily wear for electricians.

The paper which supports the tables as a good approach is below. None of us recommend ONLY using the tables. We prefer to see energy reduced first and PPE used last but IF you cannot do calculations, the tables are quite successful and much better than what we had 18 years ago when I started doing arc flash research.

Doan, D.R.; Hoagland, E.; Neal, T., "Update of field analysis of arc flash incidents, PPE protective performance and related worker injuries," Electrical Safety Workshop (ESW), 2010 IEEE IAS , vol., no., pp.80,87, 1-5 Feb. 2010
doi: 10.1109/ESW.2010.6164450
keywords: {arcs (electric);electrical safety;occupational safety;protective clothing;ASTM test methods;PPE protective performance;PPE strategies;arc flash PPE;arc flash protective clothing;expanded field analysis;worker injuries;Arc discharges;Face;Hazards;Injuries;Protective clothing;Switchgear;arc flash;arc incident analysis;burn injury analysis;personal protective equipment},
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