Informative Annex M is titled, “Layering Protective Clothing and Total System Arc Rating.” Its major point is that layering of arc-rated clothing is an effective approach [M.1.1].
But the layers must be tested together to determine the composite rating. You can’t just add the rating of each layer together [M.3.2]. And in some cases, adding a layer will actually decrease the total arc rating. Garments that are not arc-rated should not be included.
You could wear a 40 cal/cm2 arc flash suit over cotton shirt and pants to achieve a 40 cal rating [M.1.2]. But what if you don’t have a 40 cal suit? You could wear a 25 cal suit over arc rated pants and shirt (each having an arc rating of 8 cal) to achieve a total system rating of 40 cal. Notice that 25 plus 8 is only 33, not 40. But these work as a system, and thus the higher rating is achieved. This can provide the same protection with less discomfort.
We all know not to wear polyester around electrical equipment. But this does not mean natural fiber is always safe to wear. “Under some exposure conditions, natural fiber under layers can ignite even when they are worn under arc-rated clothing” [M.2.1]. Those conditions are then described in M.2.2. This information is worth reading carefully. It may be prudent to simply assume that all natural fiber is an ignition hazard and either replace it with arc-rated clothing or go without. Consider that ignition of the natural fiber clothing under the arc rated clothing means to remove the burning natural fiber clothing from your skin you must first remove the outer layer(s) of arc-rated clothing. Ditto for simply extinguishing the burning natural fibers.
You’ll find an additional explanation of the total system arc rating in M.3. Given all of this complexity, how can you determine what works and what does not? It goes back to the testing. Contact the manufacturer of your arc-rated clothing to get their recommendations based on test results — and don’t assume anything that isn’t backed by testing.
