This week's question is pretty straight forward.

Do you/your clients ever layer PPE for a greater arc rating?
Yes
No
I/We don't need PPE

Don't layer because 70E won't let you assume a cumulative value without testing. Require that the highest individual layer be high enough for the hazard.

I realize this is very conservative, and a layered value is generally higher than the sum of the individual layers.

Yes, 70E is pretty clear about testing.

M.3 Total System Arc Rating.
M.3.1 The total system arc rating is the arc rating obtained when all clothing layers worn by a worker are tested as a multilayer test sample.

It will be interesting to see if people use a single layer or multiple layers. We'll see later when people begin to respond to the survey. As I write this only 3 have responded so far since I just posted this a while ago. You, Me and someone else.- Thanks for your comment!

Most AR PPE vendors supply "layering Data". I find using a light weight AR T shirt (moisture wicking) under a 8 cal/cm^2 shirt takes me easily into 12+ cal/cm^2 range and then don't worry so much about the 50% probability numbers for most general exposures (8 cal or less), and still have reasonable comfort even in "warmer" environments. Yes, testing of combinations is a requirement.

Is the T-shirt long-sleeved? Because a short sleeve T-shirt would leave your forearms at only 8 cal protection.

I do remember in the NFPA 70E training that the initial concern was to protect the worker's core and face and was not as concerned with the extremities when calculating distances and PPE arc ratings. As we know, our hands are often well withing the standards 'working distance'. That is when the instructor said the intent is to prevent burns to the torso and face. But as all this has been changing with the new editions I'm not sure that is still how they consider things.

The last poster is correct. The standard set up by 70E (matching the Stoll curve) with the standard working distances is intended to maintain a second degree or less at the face/chest area where survivability decreases significantly due to a burn. The hands/arms, which would in some cases be at "ground zero" (less than 1") from an arc flash are not covered at least as far as incident energy goes because the standard is based on survivability.

Currently the PPE requirements require UNIFORM head-to-toe protection with two exceptions. Below 1.2 cal/cm^2 there are no PPE requirements under the 2015 edition and again, uniformity applies. Although as certain posters will point out (Arcad) time might also be considered as a factor in the analysis and since the ATPV rating of the PPE is measured relative to the Stoll curve and thus captures the time element, the 70E standard and IEEE 1584 are looking at a uniform standard. Since the Stoll curve is based on BARE skin, if all of the math follows through, short/long sleeves, or any sleeves at all, is a non-issue at 1.2 cal/cm^2. Going up to 2 cal/cm^2, this is the cutoff under NESC and 1910.269 with no arc rated PPE. Going up to 4 cal/cm^2 (and up to 8 cal/cm^2 under single phase conditions under 1910.269), face protection is not required but full body protection (at or about the incident energy) is required. But again, the data is all based on avoiding anything worse than a second degree burn at the face/chest area. Extremities not included in the analysis but are included in the more-or-less uniform PPE requirement.

For the record, I only have long sleeve arc rated shirts. I typically wear a short sleeve cotton shirt under them except in summer, but I haven't had the cotton (unrated) shirt+AR shirt combination tested.