Please help me in understanding that why 70E disallow result of incident energy analysis to specify an arc flash category in Table Method?

Does it mean that calculations for calculating incident energy to get Available Fault Current and Clearing Time at each equipment cannot be used to choose PPE from Table Method? PPE from Table Method cannot be chosen until Available fault Current and Clearing Time is known. Thank you

Two different issues and questions here. As to the former when you do your own calculations, you are altering the assumptions baked into the tables. Doing your own engineering study is just that. You are thus free to adopt your own standards and tables. Annex H is the clothing "table" with just incident energy. Since you are not constrained by the somewhat artificial/outdated 4/8/25/40 selections you can pick values that match the PPE you have or better yet just use incident energy so that you can "future proof" your PPE as the requirements and technology improve over time. Oberon is already kind of walking away from 40 ATPV and offering 41, 43 ATPV in addition to much higher ratings, and there are tested combinations that might make a lot more sense. So there is no value in using the tables if you've done the calculations. Just use Annex H, a risk study, a hazard study, and move on.

As to being "required" to do opening time and available fault current, that is more of an assumption. If you're going to do the calculations, you will significantly improve on the tables and all you are getting is a clothing table so see Annex H. The purpose of the tables is for situations where the engineering calculations are unavailable. Take me yesterday. I went to a customer with a plastic injection molding machine that had a breaker tripping. They barely even got the office set up and had just moved the plant across the country. It was by my count a 4 man operation. Do you honestly think they're going to spend thousands on an engineering study when they didn't even have a place to put a lock on the machine? There was no practical way to do IEEE 1584. So as a service engineer i need some guidance for safety reasons. That's what the tables are for. It might mean I'm moderately underprotected or more likely severely over protected but it's some guidance.

The problem is some users years ago adopted the clothing table and IEEE 1584 instead of Annex H. There is also risk baked into the tables where IEEE 1584 + clothing does not incorporate risk so the rule was promulgated not to mix the two and read the PPE table "backwards". The PPE table is a consequence of the other tables. The ATPV numbers are for selecting PPE, not working backwards from PPE to a calculated value. This should be very obvious because no PPE is required regardless of incident energy under the 2015+ tables or is severely reduced under the 2000-2012 tables.

Here are two older threads where this was discussed at length. Hope it helps.

https://brainfiller.com/arcflashforum/viewtopic.php?f=8&t=655&hilit=PPE+fujita

https://brainfiller.com/arcflashforum/viewtopic.php?f=7&t=3545&hilit=fujita

Hello Sheel

Basically repaeating Paul's comments below. I experience similar circumstances up here with CSA Z462. Call it cross engineering, utilizing the tables represents a circumstance where obviously no detailed warning label exists so therefore no real guidance for PPE, therefore one has to begin the process of assumptions with regards to clearing times and short circuit currents as they are provided in the tables. With using the detailed labels or where the AFRA has been completed the short circuit current and overcurrent protection clearing time would have been known and part of the calculations. The tables I believe were intended for the contractor, who is perhaps performing work in various locations throughout the day- commercial, hospitals, shopping malls, all facilities that may not have done the AFRA due to budgets. Bottom line we cannot mix the 2 together. In the recent editions of NFPA 70E and CSA Z462 we moved a table from Annex H into the main body of the standard to hopefully help readers to dress according to the calculated incident energy level and moreover to help engineering firms insert on a detailed waring lablel the calculated incident energy in cal/cm2 in oppose to a PPE category- this taking the reader to the tables which of course as mentioned is cross engineering

Hope that this is of some help
Thanks

Z462 and 70E are word for word identical with two exceptions. First Z462 follows CSA formatting while 70E follows NFPA formatting so the sections are labelled differently. Z462 for instance is #.#.#.# format. The second difference is that while they share most of the annexes, Z462 has more annexes and a couple are different.

I disagree with moving Annex H to the body. The PPE table ALWAYS existed in the body of the text but whereas Annex H gives the PPE requirements in terms of incident energy ranges, the table in the body of 70E and Z462 is given in terms of somewhat arbitrary "H/RC categories" under the 2012 and earlier editions, and "PPE Levels" under the 2015 and later versions. There have been changes over time to both tables, particularly the table in the body which used to have a "2*" category. In prior editions there were two 8 ATPV PPE requirements, one of which did not require a balaclava and face shield, and another one that did. The 4 ATPV category also did not require a balaclava and face shield. More recent editions first did away with the extra "2*" category leaving only the more stringent category, and then pushed the balaclava and face shield requirement down to the "1" category which is what we still have today and is now identical to Annex H whereas in previous editions a lesser PPE requirement existed.

Regardless of the difference in terms of safety, there was a strong motivation to use the tables in the body of 70E/Z462 which did not have the face protection requirement in Annex H. As of the 2015 edition this "loophole" disappeared and in a related TIA which was later incorporated into the 2018 edition, finally Annex H and the table are harmonized as far as requiring the same PPE using the table method as it is in the engineering (Annex H) approach.

But there are further differences. As you alluded to the PPE table is tied to the equipment/task table. Prior to the 2015 edition these were mixed together into an "H/RC" equipment/task table in which lower PPE requirements were allowed when the likelihood of injury was reduced although the incident energy did not reduce. This is in keeping with a probabilistic point of view but fails to recognize that likelihood and incident energy are two entirely different things. That's the approach used in the "engineered" approach. The 2015 fixed this by breaking the table up into a risk (likelihood) table which determines whether or not PPE is necessary based on the equipment and task, and a separate equipment table that identifies the incident energy indirectly in the form of a PPE Level, followed by the PPE table.

Historically the tables allowed LOWER PPE for low "risk" tasks. For instance, if you were doing voltage testing in a MCC you could wear HRC 2 (8 cal clothing and a faceshield) but if you were racking a starter bucket in the SAME MCC you had to wear 40 cal. The reason for not using the categories then was one of two reasons:

1. If you mixed tables and calculations, some companies would reduce PPE for voltage testing even when they KNEW the incident energy could be much more.
2. If you had an 8.1 calculation, the PPE would be forced to 25 cal (if thats how you interpreted the tables).

Now it really doesn't matter if you mix the tables but typically IF you have calculations, they will be more or less than the tables, so why would you force to over protect or want to underprotect?

Hugh Hoagland
e-Hazard.com

You can still equate incident energy to PPE Levels as a site specific PPE. i.e. 8 cal/cm^2 can be considered level 2. 40 cal/cm^2 is level 4 etc.
But, it is better to just use the calculated incident energy or arc rating as permitted by NFPA 70E The reason is the original correlation between levels (formerly categories) and incident energy was based on arc ratings way back. Today there are many other ratings than when the original PPE tables were developed. 12 cal, 65 cal, 100 cal etc. so using Level 1, 2, 3, 4 would make using the other ratings more difficult.