FR Layering

Does anyone have any references or literature on the effects of layering with FR clothing? I have been told by some reputable FR people that by layering, you get at least 133% of what the two garments would be individually.

DuPont has a chart that shows the effect with some of their stuff, but I've been unable to find other references.

The effects of layering vary so much from different types of material that there is no way to really know what the protection level is without in depth testing. the insulating air gap between the 2 layers will provide extra protection but unless the garments are made as a multi layer garment and tested so they have an ATPV rating you cant make any assumptions. Bottom line is the ATPV's of the 2 layers are additive, you cannot take into account any layering benifits for determining what rating the PPE must be for protecting a worker from a calulated Ei.

I don't remember the ANSI spec, but I see on some labels that they rate the probability of reaching 1.2 cals. Something like if you have a Level 2, the garment is rated 8.0 cals with a 50% probability of skin reaching 1.2 cals. If you lower the exposure to 6 cals, then the probability of skin reaching 1.2 cals is only 5%. Layering would have a dramatic effect I would think of at least raising the probability of skin reaching 1.2 cals, when exposed to the outer garment rating (assume outer is highest rating) to a very low probability. So that doesn't answer your question exactly but the probability % is something to research.

I know of no regulation that authorizes a multiplier for the layering to arrive at a final overall rating. The NFPA table, at least the 2004 one, had layering permitted in some categories. I thought I remember seeing something like two level 2 coveralls to meet Level 3? Point is that layering done to the NFPA table would be authorized.

Dupont

Thanks for the replies. I'm looking for liturature and research so if the information is "there isn't any" then that is useful too.

Layering

Another source would be the clothing mfg. as well, but you need to get it in writing.

Arc Flash and layering

There are no rules of thumb which work in arc flash and layering.

We used to believe there could be a rule of thumb but this was disproven with testing in 2000 or before. Some systems are additive (that you can usually count on), or add the the arc ratings and you are pretty safe within a calorie/cm2 or two.

On layering some systems are double and others exhibit a double hump in which they are additive (or close to that) then get more protective as the outer shell breaks open. This is in the ASTM test method and the EN test method. The critical factor is the ablation effect of the outer layer. I recommend that you seek out manufacturer data but you can have the materials tested. The good news is that in real life most materials out perform their arc rating. The factors in the hazard analysis are many and the testing has a lot of safety hedge built in. There is a newer idea of testing clothing with a focused arc with parallel electrodes which we are using in ASTM as a blanket test. This method is the worst case to date and we are just exploring it. Faceshields and arc rated rainwear material do better under these conditions. The critical factor in arc protection is making sure the material is arc rated. Clothing that ignites or melts and drips is ALWAYS dangerous. Getting clothing that is arc rated by ASTM F1506 or rainwear with ASTM F1891 is critical! Differences in a calorie or two might make no difference at all in the real world. BUT clothing that doesn't ignite statistically makes your chance of survival go from about 50% to 95%.

That is the real power of arc rated clothing.

Well said Hugh, about time you showed up on this forum

Hugh,
Explain the testing statistics that I see on some Oberon clothes. They list a % of probability for reaching 1.2 cal versus on IE cal exposure. How do these statistics work.

You can get the ASTM 1959 test results for multi-layer oz weight fabric combinations for the same fabric from the manufacturer, I have them for Dupont, Oberon, Westex Indura Ultrasoft, Kermel and have requested them for Bulwark.

If the manufacturer is credible they will release this information to you on their letterhead, and you keep this on file with your Electrical Safety Program documentation in support of your layered ensembles.

Regards;
Terry Becker, P.Eng.
www.esps.ca