Arc Flash Layering

[mgmsparks]

In NEFPA 70E Table 130.7 (C) (10) footnote, a cat 3 rating can be obtained by using 2 sets of fr coveralls with the inner rated at 4 and the outer rated at 5 over non melting or untreated natural fiber clothing. There is no explanation for allowing this in the article. In order to meet the cat 3 level table 130.7 (C) (11) requires a fr coverall with a rating of 25.

Since a fr 4 and fr 5 do not equal 25, there must be some additional protection by layering clothing, but the code article does not explaian what it is.

Can anyone explain the formula that the author of the tables are using in footnote 9?

If I would wear a fr 8 rated coverall over a fr 8 shirt and fr 8 pants, would I be able to meet the same protection stated in footnote 9?

 

[jim dungar]

In NEFPA 70E Table 130.7 (C) (10) footnote, a cat 3 rating can be obtained by using 2 sets of fr coveralls with the inner rated at 4 and the outer rated at 5 over non melting or untreated natural fiber clothing. There is no explanation for allowing this in the article. In order to meet the cat 3 level table 130.7 (C) (11) requires a fr coverall with a rating of 25.

Since a fr 4 and fr 5 do not equal 25, there must be some additional protection by layering clothing, but the code article does not explaian what it is.

Can anyone explain the formula that the author of the tables are using in footnote 9?

If I would wear a fr 8 rated coverall over a fr 8 shirt and fr 8 pants, would I be able to meet the same protection stated in footnote 9?

The foot note says the inner coveralls must be => 4 and the outer ones => 5, but when layered they must meet or exceed 25.
So, your 8+8=16 is not suffcient even thoug it meets the minimum per layer.

 

[zog]

There is no magic formula for layering. 8 + 8 =16, thats it. Now when a multi layered garment is tested, the values are typically higher that that, but it has to be tested.

There were several proposed changes to that footnote for the 2008 70E, dont expect it to be the same after 2008.

 

[WDeanN]

I would just like to note also that Table 130.7(C)(11) is an example of a typical clothing system, as stated in 130.7(C)(11).

The FPN there states "The arc rating for a particular clothing system can be obtained from the FR clothing manufacturer."

The layering is not a requirement. It may be what is necessary to reach the minimum ATPV for that category, but if you can reach it with one piece of FR clothing, then do it.

 

[jim dungar]

There were several proposed changes to that footnote for the 2008 70E, dont expect it to be the same after 2008.

The next edition of NFPA 70E will be in 2009. I believe it has just entered the ROP stage. Submit your comments.

 

[davidr43229]

The layering is not a requirement. It may be what is necessary to reach the minimum ATPV for that category, but if you can reach it with one piece of FR clothing, then do it.

If I would wear a fr 8 rated coverall over a fr 8 shirt and fr 8 pants, would I be able to meet the same protection stated in footnote 9?

Mgmsparks, no because Note 9 calls out for 2 sets of FR coveralls, what you didn't consider is the air gap created between the two sets of coveralls and the air gap between the non-melting or untreated clothing and the air gap between the cotton undergarments. Clearly the layering effect plays a significant role.
This is in addition to the flash suit hood that has to be worn at that level.
Just my $.02

 

[davidr43229]

mgmsparks,
I suspect the "math" is within the ASTM standard of F1959. I have the Dupont clothing standard that goes much more into detail, should you want a copy of it, I can send it to you.