Arc Flash over 40 cal/cm2 1

[buddy91082]

I have some equipment that is listed as "dangerous" and over 40 cal/cm2. My unsderstanding is that NFPA 70E-2012 does not
explicitly prohibit work at locations with energy levels above 40 cal/cm2. Is that true that although dangerous per code it's not a problem so long as i have the proper PPE? Is there PPE that can be purchased to work on equipment over 40 cal/cm2? Is

Thanks.
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[davidbeach]

Nope, no PPE. At those levels it becomes essentially impossible to survive the concussion of the blast wave, the burns become a secondary cause of death.

 

[DistCoop]

There is clothing you can purchase over 40 cal/cm2... the highest I've seen is 100. This refers to the thermal protection however and, as davidbeach said, above 40 cal it is the blast wave we are concerned about.

 

[dpc]

Earlier versions of NFPA 70E had a PPE Category 5 up to 100 cal/cm2. Due to concerns regarding arc blast, this was removed, so the maximum incident energy level for which NFPA describes PPE requirements is 40 cal/cm2. The strong implication is that energized work should not be done above 40 cal/cm2, but as you note, it does not explicitly rule out working above 40 cal/cm2.

The main arc blast hazard is the pressure wave that occurs - basically it is an explosion. However, since the incident energy is directly related to the duration of the arcing fault, not all 40 cal/cm2 incident energy events are the same. 40 cal/cm2 released in 0.02 seconds is an explosive event. 40 cal/cm2 released over 2 seconds much less so. But at this point, in the interest of safety and simplicity, the NFPA 70E committee elected to use 40 cal/cm2 as a cut-off, since arc-blast injuries start to become a major concern above this level.

Bottom line, wearing arc-rated clothing over 40 cal/cm2, although it is available, is not a guarantee of protection against arc-blast events. Although NFPA 70E is a consensus standard, not a legal requirement, allowing work with incident energy over 40 cal/cm2 would also present a huge liability risk if anyone was injured.

Also, NFPA 70E-2015 is due out in a couple of months - there will be some significant changes.

 

[stevenal]

Any firm cutoff leads to a paradox. Exposure above X must be worked de-energized, but to be considered de-energized it must first be isolated tested and grounded thereby causing exposure above X.

 

[Zogzog]

DPC, as I recall HRC 5 never made it to a standard, just was a proposal for the 2000 cycle. Never been able to find it though so if you do let me know.

 

[dpc]

Yes, the voltage test requirement is definitely a Catch-22.

 

[buddy91082]

Thanks all for the helpful replies. Didn't know about the Catch 22.

 

[dpc]

Zog,

Yes, you are probably correct. It's definitely gone now, but you can still buy 100 cal/cm2 arc-rated PPE.

 

[ScottyUK]

The 100 Cal/cm² suits aren't significantly heavier than the 40 Cal/cm² types, so the additonal protection doesn't do any harm. Admittedly if you've just been blown through the switch house wall it ain't going to matter but even so...

 

[davidbeach]

From what I've read, you wouldn't hardly even need to be knocked over, the dp/dt of the front of the shock wave takes care of everything. After that it's all just about open casket vs. closed casket.

 

[ZeroSeq]

After talking with an arc flash PPE supplier, the question of working on live busses with 40+ calorie energy levels was brought up. I, like everyone here, was under the impression that the arc blast becomes lethal above 40 calories. I was then told that this, like the 2 second "rule", has been widely accepted as fact (somewhat incorrectly), but is not supported by any firm standard (keep in mind that I do not disagree with using a maximum 2 second clearing time, I'm just using it as an example). He cited that there have been zero confirmed cases where an arc blast has been the primary factor in any arc flash related deaths. He said that while arc blast most definitely leads to injuries, there is no evidence to support the claim that at 40 cal/cm^2, it now becomes lethal and thus MUST NOT be worked on live.
Can anyone refute this theory? I would be interested to see literature supporting either case.

 

[dpc]

ZeroSeq

I think this is basically correct. It is generally believed that arc-blast presents a safety hazard for very high-energy arcing faults. Pressure data was recorded during the testing used for developing the IEEE 1584 equations. High pressures were noted. I think based on this and whatever was known about blast pressure injuries they established 40 cal/cm2 as the upper limit, regardless of fault duration.

It may be overly conservative, but it would be somewhat difficult to come up with a different number that could be justified. Particularly from a liability standpoint.

The two second maximum fault duration is suggested in the Annex of IEEE 1584. It is simply a suggestion. It makes sense to set some maximum duration rather than just relying on the protective device TCC.