Layering to achieve the effective arc rating is allowed by NESC an NFPA 70E. However, the layering system needs to be tested to determine the total system arc rating per NFPA 70E.

It does not seem feasible since manufacturers don't test their FR clothing combos. If they do, there are a limited number of combos. This testing requirement only makes us to buy stuff that meet or exceed the minimum arc ratings per HRC.

I would like to know what policy your utilities have for layering. Layering without testing? No layering at all? Thank you.

Utility policies may not be a worthwhile comparison, if your workplace is a not-utility.

We're talking about clothing. Other than the hands, feet, and head areas, the general clothing (shirts and pants) rules are the same for both in terms of ATPV ratings.

The only source I know of is that Hugh Hoagland publishes quite a bit of this information on his web site. In general what I understand is that the value of layering is much larger than simply additive. The combinations that he has tested (and reported) show that the air gap that is created in between layers is worth far more than the sum of the parts.

Other than this general statement, the only way to figure out what works and what doesn't is by testing.