OSHA 1926 Subpart V Differs With NESC 2017 On Arc Flash Calculations

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OSHA 1926 Subpart V Differs With NESC 2017 On Arc Flash Calculations


I am having trouble reconciling OHSA 1926 Arc Flash requirements with Table 410-1 in the new 2017 NESC.  In addition to the problems with notes 7 and 8 that are retained in the 2017 version of the NESC Table 410-1, there are at least two and perhaps three questionable testing references used for Table 410-1.

  1. Note 2 references B1 “208V Arc Flash Testing: Network Protectors and Meters” but the note is also shown in the table for Pedestals / pull boxes / hand holes and Open air (includes lines).  Upon review of the reference, there does not appear to be any testing involved beyond network protectors and self-contained meters.  Both IEEE 1584 and ArcPro, deemed acceptable by OSHA, infer sustained arcs are at least possible at 208V.  In addition, any arc testing at 208V must carefully consider source voltage support and the voltage drop across the distribution transformer and lines ahead of the arc.  If actual arc voltage falls below about 180V per Kinectrics(ArcPro) testing, the arc cannot be sustained.For example, systems under a 750 KVA 120/208V transformer are more likely to sustain an arc than those same systems under a 75 KVA 120/208V transformer.
  2. Note 9 references B29 “Arc Flash Testing of Typical 480V Utility Equipment”.   With respect to a pad transformer, the testing covered a number of reasonable  scenarios using a single 480V three-phase transformer of unknown KVA and impedance.  In all cases, the arc self-extinguished without operation of the protecting fuse.  While I consider the particular scenarios reasonable, I question if they are sufficiently representative since there are other published testing literature that shows higher Cal/cm2 energies with less probable but still feasible conductor bridges or assumptions.  The use of note 9 in both the 50-250V and 251-600V columns of table also infers that 208V arcs are represented even though fault currents would be substantially higher.
  3. Lastly, the subjective Exception 1 to 410.A.3 effectively undermines most of objective requirements of both OSHA 1926 and NESC 410.  While I believe some form of an exception is reasonable, I do not see where OSHA 1926 would allow the latitude of an exception that is so sweeping.

In OSHA’s 7/13/2015 interpretation regarding NESC 2012 Table 410-1, it appears that all data not based on Notes 7 and 8 is accepted.  In view of the above, I am somewhat at a loss that OSHA did not qualify the interpretation in that any referenced testing literature also be representative.  I had hoped that OSHA and the new NESC would be reconciled.

My personal opinion is that OSHA goes too far in an effort to remedy extremely unlikely events while NESC does not remedy events that are very possible.


First let me start out by saying that the point in the OSHA response is that if you have test data on specific equipment,  it is acceptable to use as a “reasonable estimate”.  For all the values in Table 410-1, it is the users responsibility to verify that their equipment matches the test configuration.

Second, since the time period for suggested revisions passed, NESC is creating a sub-committee to write a TIA and update table 410-1 to include all the test data and notes needed to correctly interpret use of the table.  Data without test backing or calculation bases, may be excluded.

Third, there is additional testing at 208V that shows gaps larger than 1/2″ can not self-sustain arcs at this low voltage level.  Testing was done with 40kA of available bolted fault current.  This data is also proven out by others who have tested at this voltage. The range of 208V/240V equipment that utilities deal with was examined and all was found to have arc gaps greater than 1/2″.  The self-contained meter bases tested presented the smallest found gap and arcs could not be sustained in that enclosure.  In addition, when testing at 480V shows that arcs can not self-sustain in specific equipment with certain configuration, it is irrefutable that give the exact same equipment and configuration, arcs can not sustain at even lower voltages.

Fourth, note 9 about 480V pad-mounted secondaries, extensive testing was done in extreme conditions.  While there was only one enclosure used, the arc gaps were artificially reduced by adding plates to the existing electrodes.  The arc gaps were reduced to under 3″, well within the known sustainable arc gap for this voltage, and arcs could not be sustained beyond a few cycles.

Fifth, by the very explicit nature of the hand protection requirements, OSHA acknowledges that there are higher level risks.  OSHA allow rubber gloves with leather keeper to be worn on the hands. This supports the exclusion of 410.A.3.  The risk of electrocution outweighs the need for arc rated protection on the hands.  While some may misinterpret this exclusion to cover other situations (like heat exhaustion), I don’t know that you can unequivocally say that 410.A.3 is invalid.  At current time, I do not believe this exception had any suggested edits; therefore, it was not changed.  Any clarification of this exception would need to be addressed in the next revision or via TIA in the interim.  You would need to provide significant reason for TIA. Sam Stonerock is the chairman of that section of the NESC.

Sixth, your choice to use Table 410-1 or not is totally up to you.  The OSHA letter merely states use of that table outside of notes 7 and 8 will be considered reasonable estimates.  NESC acknowledges the need to clarify table 410-1; hence, it has started a revision committee.  In the interim, my advice is to clearly review the test data quoted as the bases for Table 410-1 to determine if your specific equipment matches the tested configurations.

Lastly, Tom Short and I are writing an IEEE paper that will be presented at the 2017 ESW that will summarize all the sustainability test data in one place. Hopefully that will allow users to more correctly apply the use of Table 410-1.

Hope you find this helpful

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Marcia Eblen

Marcia is a California-licensed PE who has more than 30 years of experience in grounding analysis and design, high voltage and high power laboratory testing, encompassing nuclear power plant design and maintenance, arc flash program development, and more! Read more about Marcia.

This Post Has 3 Comments

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    Chuck Harvey

    Just remember. OSHA will probably cite for non-compliance with our standards before failing to follow NESC, NFPA or any other standards.

    1. Ken Sellars
      Ken Sellars

      Absolutely right. We teach this fact in our classes. The governing standard is OSHA, not necessarily the consensus document(s). Thanks, Mr. Harvey, for your reply.

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