The 2009 update to NFPA 70E no longer allows layering of FR garments to reach a composite ATPV rating.
For instance, you cannot layer an undershirt with an ATPV rating of 4.0 cal/cm2 and an overshirt with an ATPV rating of 8.4 cal/cm2 and then expect your total ATPV rating to be 12.4 cal/cm2.
When layering garments, the innermost layer must be heavy enough to not break open, nor ignite when exposed to an Arc.
You cannot layer a non-FR layer over an FR layer. If your outer layer were to ignite, or even melt depending on the fabric, the FR wouldn’t be able to protect you.
This is most commonly seen with rain wear. Many people don’t think twice about throwing on a raincoat to work in a downpour.
Unfortunately this can lead to catastrophic consequences. Rain wear is commonly made out of nylon, or PVC. Both of these materials will melt in the event of an electric arc. Your FR layer, while able to insulate you from the actual electric arc, cannot insulate you from the direct application of a melted material that covers your body.
Conversely, you do not want to layer your FR garments over any garment that has the potential to melt. The reverse of the scenario above would be true. Perhaps you decide to wear a polyester moisture wicking shirt under your FR shirt. An electric arc happens, and now your polyester shirt has melted under the intense heat of an arc.
The safest thing to do, is to layer FR over FR. And never forget that your Hi-Vis and rainwear needs to have an ATPV rating to be considered FR. Anything that just has the label FR and NO ATPV rating cannot be considered for electrical work under the NFPA 70E 2012 standards.