According to NFPA 70E section 130.7 (C) (9) (a), layering of non-melting flammable garments is permitted to be worn under FR garments for added protection. However, the system arc rating of the innermost FR layer must be sufficient to prevent break open and ignition of the flammable under layer.
Only FR layers within the layered system are used to determine system arc rating. Arc ratings of individual layers cannot simply be added together. Any garment worn as the outer layer, including rainwear, must be arc-rated.
Melt-able fibers such as acetate, nylon, polyester, polypropylene, and spandex cannot be used in under layers next to the skin except that an incidental amount of elastic is permitted in socks and underwear.
Because of changes to the 2009 Edition of NFPA 70E, we can no longer advise wearers to simply add individual single layer arc ratings together to arrive at a system arc rating for layered garments. Annex M.3.2 of the 2012 Edition of NFPA 70E states that the total system arc rating cannot be determined by adding the arc ratings of the individual layers… The only way to determine the total system arc rating is to conduct a multilayer arc test on the combination of all the layers assembled as they would be worn.
Bulwark has conducted a series of layered arc tests to determine the total system arc rating of various combinations of base layer fabrics under shirts and coverall fabrics over shirts and pants. Click here to download our Layered Arc Rating Summary Chart.