Layering and Undergarments
According to NFPA 70E section 130.7 (C) (12) (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 breakopen and ignition of the flammable under layer.
If Table 130.7 (C) (9) is used to determine the HRC category, 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 FR.
Meltable 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 our customers to simply add individual single layer arc ratings together to arrive at a system arc rating for layered garments. Annex M.3.1 of the 2009 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 is currently conducting a series of layered arc tests to determine the total system arc rating of various combinations of shirts over T-shirts and base layer fabrics, coverall fabrics over shirts and pants, and outerwear over shirts, pants and coveralls. This information will be communicated to the market as soon as it is available.
How flame-resistant fabrics work
Flame-resistant (FR) fabrics and garments are intended to resist ignition, prevent the spread of flames away from the immediate area of high heat impingement, and to self-extinguish almost immediately upon removal of the ignition source.
Normal work apparel will ignite and continue to burn if exposed to an ignition source such as flame or electric arc. Everyday fabrics will continue to burn until they are extinguished or all flammable material is consumed.