arc flash boundary for synthetic fabric

[stevenal]

So the arc flash boundary is usually put at 1.2 cal/cm^2, although the implication of the new NESC rule puts it at 2 cal/cm^2. Either way, the boundary is meant to minimize rather than prevent injury. Since injury can occur, OSHA 1910.269(l)(6)(iii) requires only clothing that does not increase the extent of injury be worn at that boundary. So where must I stand while observing the work while wearing my best disco polyester? Is there a recognized incident energy level where synthetics will not melt?

 

[apowerengr]

They way I read it your disco suit is not appropriate for any exposure. NESC C2-2007 410 3. second paragraph provides language similar to other standards "... When exposed to an electric arc or flame, clothing made from the following materials shall not be worn:..." No caloric level or boundary is defined. Thus if any exposure is anticipated you can't wear it. Seems the janitors need to trade in their polyester suits too or be restricted outside some undefined boundary from work areas where exposed energized parts are present.

 

[dpc]

The 1.2 cal/cm2 is based on exposure of bare skin. Outside of the NESC world, most industrial facilities have required natural fiber clothing for some time. A lot of plants now have electricians and operators wear 8 cal/cm2 long-sleeved shirt and pants (or coverall) at all times, year-round.

NFPA 70E requires any clothing worn to be "Non-melting or untreated natual fiber"

 

[Grollor33]

I'm guessing Disco Polyester is somewhere between Dirty Skin and Untreated cotton...


NFPA 70E Typical Thermal Performance of Various Fabrics in Cal/cm^2

Material Total Weight (oz/yd^2) Rating (Cal/cm^2)

Bare Skin (Clean) - 0.5
Bare Skin (Dirty) - 1
Untreated Cotton 4 2
Single Layer FR cotton 7.5 6
Single Layer FR cotton 12.5 13.8
PBI Fiber Blend 4.5 6.1
Nomex III 4.5 9.1
Nomex III 6 13.7
Nomex III A 4.5 9.2
Nomex III A 6 13.1
Cotton (4oz) under FR cotton (8oz) 12 12.5
Nomex (2 layers) 12.2 22.6
Nomex (8oz over FR cotton (8oz) 16 31.1
Switching suit of FR coverall 24-30 40.0+

 

[stevenal]

It's that undefined boundary I'm looking for. What constitutes no exposure? Same planet where the arc occurs? Same hemisphere, acre, substation, office space on the other side of the factory area in the same building?

Jweav33,
Bare skin is preferable to the polyester. The polyester (and other synthetics) melts into the skin, trapping the heat inside for maximum injury. From your chart, I'm looking for a value somewhere between 0 and 0.5. Any ideas?

 

[dpc]

I don't really understand your question, but 1.2 cal/cm2 was selected because it generally represents the start of a second-degree burn. Less than that is not considered serious.

By NFPA 70E approach, beyond the flash hazard boundary defined by 1.2 cal/cm2, no special protective clothing is required.

The goal of NFPA 70E is not to eliminate injuries due to arc-flash, but rather to keep the injuries relatively minor and recoverable. The approach is reducing risk, not eliminating risk.

 

[stevenal]

Okay, practical example:

On a wet day in a substation, worker A is doing some switching with a hook stick. Arc flash analysis has been performed, and A is wearing FR clothing covered with FR rain gear that adds up to a value exceeding the calculated incident energy at his assumed distance from a possible arc.

Meanwhile worker B is digging a hole to replace a fence post some distance away. Since FR rain gear is expensive and his work is likely to destroy it anyway, he is wearing disposable PVC, non-FR rain gear. Arc flash analysis shows an incident energy level of 0.25 cal/cm^2 at this location if A's actions were to cause an arc. According to Jweavs chart, worker B would be fine if he was naked. But B is not to keen on this idea, and neither are the neighbors. Will B's PVC gear melt into his skin in the presence of the arc generated by A causing an injury that more than minor?

Meanwhile the general manager is taking the board on a substation tour. Hard hats and safety glasses are handed out by their qualified escort, and they stand under their nylon umbrellas observing the action from a 0.2 cal/cm^2 distance. Their suits are made of -- who knows???

Coward that I am, my disco polyester and I are outside the gate at 0.15 cal/cm^2.

Safe situation? OSHA or NESC violation? Hope this clears up my question.

 

[stevenal]

Seems like NFPA disagrees with my assertion that synthetic clothing is worse than none. 130.7(5) seems to suggest a boundary of 1.2 cal/cm^2 for meltable fabric and 2.0 for non-melting, non-FR. This makes the meltable synthetic actually protective compared with jweav's bare skin value above. I'm not ready to believe it though. My assertion comes from OSHA 1910.269(l)(6)(iii). And even NFPA forbids a meltable layer next to the skin once inside the boundary. My head hurts.

 

[dpc]

If the exposure is 0.15 or 0.2 cal/cm2, then I don't think it's an OSHA violation based on potential arc energy exposure. NESC violation - I have no idea.

Is it "safe"? I guess that depends on your definition of "safe". I wouldn't be concerned about it.

Tours inside a substation by "unqualified" personnel is a little different situation. There are a lot of reasons why I'd be concerned about that, with arc-flash being only one. At least one industrial client we have required PPE to be worn by anyone to walk into a substation.

There a plenty of gaps in NFPA 70E and probably the new NESC. But we know how to deal with a good majority of normal work activities. And we probably can't wait for a "perfect" set of guidelines before we do something.

 

[Zogzog]

OSHA has banned wearing clothing made of nylon, rayon, poleyster, or acetate either alone or in blends for 25 years now for any electrical work or to even be in a substation.

 

[stevenal]

Zogzog,

Got a reference #? I never saw the prohibition against being in the sub. 1910.269(l)(6)(iii) speaks only of exposure making no reference to a fence line. 1910.269(u) speaks of substations with no reference to clothing.

Although I used a substation is in my example, the scope of my original question extends beyond any such boundary.

 

[Zogzog]

It dosent need a reference to a fence line, it is when they are exposed to a arc hazard.

1910.269(l)(6)"Apparel."

(l)(6)(ii) The employer shall train each employee who is exposed to the hazards of flames or electric arcs in the hazards involved.

(l)(6)(iii) The employer shall ensure that each employee who is exposed to the hazards of flames or electric arcs does not wear clothing that, when exposed to flames or electric arcs, could increase the extent of injury that would be sustained by the employee.

Note: Clothing made from the following types of fabrics, either alone or in blends,is prohibited by this paragraph, unless the employer can demonstrate that the fabric has been treated to withstand the conditions that may be encountered or that the clothing is worn in such a manner as to eliminate the hazard involved: acetate, nylon, polyester, rayon.

 

[stevenal]

Agreed. My contention is that none of the people in my example above, except for A, are exposed. If you contend otherwise, where is the line? It can't, at least practically, be absolute zero.

 

[GordS]

Could you say that the line is defined by the limited approach boundary within which an unqualified person may not approach as per the 70E standard, item 2-1.3.5.2 and 2-2.4?

 

[stevenal]

I thought of that, but the 1.2 cal/cm^2 flash boundary extends beyond that in some cases. Also, the handbook note under the definition says; "..is related to mitigating exposure to shock only."

 

[apowerengr]

Stevenal, is is plausable that anyone inside the fence is exposed as outside the fence is general public territory and no facility owner can control that area (thus the fence)? I wonder, then, if we need to extend our substation fences beyond NESC requirements for shock hazards to arc-flash boundary protection?

Extending this reasoning, for an industrial application I'd proceed with anyone within the room containing exposed energized parts shall not be wearing clothing which can contribute to the severity of an injury (keep the janitor or mechanic out when energized equipment is exposed). Hard barriers such as walls and doors would reflect heat and thus contain the heat from arc-flash hazards to the space within that particular room.

This would be a good question for a code interpretation - OSHA perhaps? I would think this line of reasoning, if it were included in a facility's safety manual, would pass a review given my past experiences with OSHA investigators.

 

[Zogzog]

Sorry, I have been unavailable for a few days. I understand what you are asking now, and it is a great question.

You have a problem with your fences being within the FHB, however the proballity of an arc flash occuring is slim in a static state compared to when work is being done in the switchyard. I think you have found a loophole here and you might want to email your situation to the 70E commitee.

My personal recommendation would be to post the FHB when work is being done in the switchyard if the FHB would cross the fenceline. Sorry I cant give you something more concrete than my opinion, I dont think there is a concrete answer. My experence with OSHA inspectors is in a situation like this, they are looking for an effort to be made to protect persons exposed to electrical hazards.