The NFPA 70E handbook, on page 129, states "If the thermal energy exceeds 40cal/cm2, the accompanying pressure wave might injure any worker who is near." We all know that can't be the whole story because we have calculated IEs >40 that clear in 5 cycles but we have others that are calculated based on reaching 2 seconds if we have elected to use the 2 second rule. So surely the first would be a great pressure wave, and relatively speaking, the other would be a "slow cook" without the substantial pressure wave. I must give arc flash safety training to electricians in October. Are any of you close enough to the testing results to say if I can tell them that if the incident energy is calculated to be <40cal/cm2 the pressure wave will be survivable? If not, has it been determined to be survivable at some other IE without regard to clearing time? It would be easiest to get compliance with the PPE standard if everyone believed that it would protect them, rather than simply keep them from getting 2nd degree burns while being killed by a pressure wave.

From what I understand the 40 cal/cm2 isn't so much a cutoff for pressure waves, but all the 'secondary' effects of the arc blast combined exceeding what can be reasonably mitigated by PPE. Moreover, PPE is selected based on calculating only the radiated energy of the arc, at higher energy levels the IEEE 1584 model to calculate the hazard is no longer representative.

I actually once looked into this and found that the pressure wave probably isn't lethal of itself. The LD50 value for pressure waves I found was 18 atm, the pressure wave of a 100 kA arc is about 3 atm (I can dig up some links if you're interested). One difference is that the pressure of an arc flash pulses every cycle instead of a one-off blast, so at worst a worker is exposed to it for the duration of the arc.

On a different note, you're making it sound like a situation where the 2s rule is used is safer than a quickly extinguished arc for the same incident energy. Maybe I'm misunderstanding, but having such a situation and trying to solve it with PPE and relying on the worker to duck away in time isn't the right approach (imho).

Nothing is on the near term horizion to address this that I know of. We are all busy with other aspects of arc flash and this one is down on the list. You are correct, it is a matter of energy per time, not just total energy. 40 cal over 10 seconds would not have the same blast pressure as 40 cal over 5 cycles.

Ralph Lee published a paper about blast pressure calculations years ago. It is also posted on the forum.

I attempted to discuss the 40 cal/cm^2 issue in the article below but until we have a definite cut off of energy per time, it is going to be a judgement call - the kind of call no one wants to make (and should not make).

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