Many authors with little knowledge of arc flash testing and arc flash phenomena have taken general statements from the early researchers and stated as fact the following myths:
Myth One: You can add up the ATPV’s of 2 or three materials and you will get a conservative arc rating of the system.The “air gap” between layers adds X cal/cm² protection.
FACT: In many cases this is true but not in all cases. Some common t-shirt under a shirt combinations have been shown to provide protection equal to the outer shirt due to superheating of the outer shell fabric in the arc. This superheating affects some materials more than others but we have many examples of materials which burns are predicted at just beyond the rating of the outer shell shirt. For example one FR Cotton/nylon shirt with an 8 cal/cm² rating and a specific AR 4 cal/cm² undershirt only received 8 cal/cm² when tested together.
Myth Two: The “air gap” between two layers adds insulation and X cal/cm² of protection.
FACT: Ablation (breaking open and breaking away to allow energy escape) is more predictive of the protection of a two layer system than adding the ATPV’s. The “air gap” in an electrical explosion (an arc flash) may provide no protection at all. Ablation, prevention of convective hot gas flow, thermal stability of the fiber blend at many different temperatures are better predictors but, to date, the only way to estimate the combinations of these effects is to arc test the system using ASTM F1959 or IEC 61482-1-1.
We have FREE Arc Flash Layering Data, and most garment manufacturers have data on their common systems. ArcWear can do arc flash testing to provide an arc rating of your system but we recommend finding free data.
NFPA 70E, Annex M makes this clear:
Don’t discount the power of layering. It is the most reasonable way to protect workers and still manage heat stress and costs. Avoid the short cuts of “adding up the layers” but there is good information out there and your arc rated clothing supplier should be able to help. Wearing arc rated underlayers, even if you do not know the added protection level eliminates the risk of ignition and has great value in work settings where high voltages are present and where arc flash levels are higher than 15 cal/cm²
Note: e-Hazard and ArcWear do not sell arc rated clothing. We provide unbiased information on the industry as a testing company and a training company.