If you wear a 12cal/cm2 coverall over a 5cal/cm2 coverall are you now protected to 17cal/cm2? Or if you are wearing a pair of 8cal coveralls over denim jeans and a long sleeve denim shirt does this protect you to a level greater than 8cal? Is there a formula for figuring this out?

Read NFPA70E-2009 Annex M, Section M.1.1

Layering is allowed.

But needs to be tested.

Read NFPA70E Article 130(12)(a) it specifically mentions "layering system"....

Annex M is not part of the NFPA requirements. If you read the entire M.3.1 paragragh, it said in a "few cases" (no specific numbers or documented facts) the FR was decreased by layering. M.3.1 is a disclaimer statement. I would be very interested in reading study data where layering was actually tested. Anyone?

And don't forget HRC.
Two plus two equals 3, brilliant.

M.1.1 has some interesting arithmetic in the third sentence. Seems they added the 8 from the pants to the 8 from the shirt to the 25 from the suit to get 40 (rounded from 41 I guess). I would add the 8 only one time.

That is not what it says.
M.1.1 says "suit over (pants and shirt)". There is no mention of adding the individual component's values.

In fact, M.3.1 specifically says the combination must be tested.

Layering typically does increase the arc rating of the garment system, however the actual garment/fabric combination must be arc tested in order to determine the actual arc rating. Some fabric manufacturers have tested various combinations of their fabrics to establish a layered arc rating and many of these rating are available when requested of either the fabric or garment manufacturers.

So how did they get 40 given 25 and 8 and 8? No mention of testing in M.1.1. Looks like addition to me, with a bit of rounding.

M.1.1 simply gives an example of how layering can be effective, not how to determine the effect.

layering of FR clothing



Another FPN that is NOT part of the standard is:
"The arc rating for a particular clothing system can be obtained from the FR clothing manufacturer"

To layer or not to layer... that is the question.

NFPA70E Article 130(12)(a) specifically mentions "layering system". Annex M is not a NFPA requirement but doesn't say annex M can't be used as reference for layering.

Check out Article 130(12)....

"Personal protective equipment items will normally be used in conjunction with one another as a system to provide the appropriate level of protection."

My interpretation is layering is acceptable as long as the combined ATPV meets or exceed the minimum cal/cm^ requirements of the task.

Can this get any more confusing??

Don't you get a additional rating factor with the layering system because of the trapped air space between the layers?

For this you definitely need to test the combination. No accepted factor exists.

A little trip

to a web site or two will give you the combination ratings. Those numbers just go to prove you need the certification testing. This is NOT simple math.