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 Post subject: 2 sec rule
PostPosted: Mon Mar 18, 2019 1:59 pm 

Joined: Tue Jan 22, 2019 10:46 am
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On behalf of a colleague:

What if the Arc is initiated by a person falling into the equipment. Does the arc blast concussive energy remove the person from the AFB within the 2 sec? I guess at this point it is likely a lost cause anyway?


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 Post subject: Re: 2 sec rule
PostPosted: Tue Mar 19, 2019 12:58 pm 
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Apolcha wrote:
On behalf of a colleague:

What if the Arc is initiated by a person falling into the equipment. Does the arc blast concussive energy remove the person from the AFB within the 2 sec? I guess at this point it is likely a lost cause anyway?


I've heard the "concussive" argument before. It's utter myth. It is pretty obvious in the videos that the same trainers love to show and talk about it like it's a bomb going off. That's not what happens.

1. Arc blast is caused by sudden release of hot gases from within an enclosure that is sealed up enough to pressurize it. Testing has confirmed this and the fact that the pressure is not much to speak of. The "copper vapor" theory has been debunked via the same testing because the pressure changes do not correspond to the expected result if copper vapor played a significant role. There are several public inputs for NFPA 70E rolling back many of the completely unsupported claims that Gammon made in the annex. Another one is the "35,000 degrees" claim which is also completely unsupported by scientific evidence.

https://ieeexplore.ieee.org/document/7565738

Arc blast happens within 1 to 2 cycles. Aside from E-Hazard's tests above, see also CIGRE on modelling arc blast, confirming the same thing. Essentially unless you have some kind of "fat straw" such as an undersized arc resistance switchgear vent, the equipment ruptures at around 1-10 PSI and does not continue to increase. There is one reference where the authors claim to have measured a higher pressure but I can't find any corroborating evidence and a lot of their paper was all computer simulation anyways. The above article also shows why measuring arc blast pressure is so messy in the first place.

http://a3.cigre.org/content/download/39 ... e/A3-14(SC)01(B)-IWD_Draft-TB_A3-24__+rev+11-1_numbered.pdf

You might get your ear drums blown out, but you are not going to be blown off your socks by a 5-10 PSI pressure wave. In fact theoretical calculations show it can't get very far past the door. Think about it for a minute...we have a build up of pressure inside a cabinet that is well sealed (most are not), then "poof" it goes off. The air pressure is inversely proportional to the volume which is increasing with the cube of the distance so it very quickly falls off to almost nothing, far faster than the fall off rate of the arc flash (thermal radiation).

Human reaction times are more on the order of 0.3+ seconds. Watch an actual arc flash video and how long it takes until you see someone "thrown" from the equipment. Is it in 1 to 2 frames or several frames later? It's not arc blast...it's just fight or flight response. Slow the following down to as slow as possible (0.25 speed) right at the incident and watch the guy not "blown away" but quite literally running away. Of the videos out there they all show the same thing. "Blown" would mean that in frame X they are there, then in frame X+1 you see "flash" then frame X+2 everoone is now either repositioned or accelerating through the air to a new spot. That's not what happens...they are "repositioned" several frames later.

https://www.youtube.com/watch?v=2SopsQEfoc4

2. The 2 second rule is sort of just that. Basically we need to "cut off" the calculation somewhere. We could do a lot of hand waving and say that it's based on escape (see above...most escapes are within a second) or that it's based on conductors self-extinguishing or some other nice scientific explanation but the fact is that the 2 second rule is just a rule of thumb that was suggested in the 2002 edition of the IEEE 1584 standard with really no scientific merit at all. It has stayed with us since that time and it basically seems to work so despite a total lack of justification, we keep it because it works.

IEEE 1584, ASTM 1959, and NFPA 70E themselves are based on a lot of hand waving surrounding the "1.2 cal/cm2" value, burns, and an old chart about survivability. There is a huge amount of hand waving here. Do not attempt to get to scientifically curious about IEEE 1584 or it becomes obviously so flawed that there is no support for it...then what? Well, then we don't have anything else to go on. So now we have dozens of factors and calculations in IEEE 1584 that have refined the results to the extreme, based on some obviously contrived testing based on a contrived hazard model. The one and only saving grace here is that after almost 20 years of mounting evidence, the IEEE 1584 method, warts and all, seems to work in practice, every time. That probably means it's overly conservative. But we don't have a better model as of yet.

https://ieeexplore.ieee.org/document/6164450

If you can't escape (see point 1) and that's why the "2 second rule" works, then obviously 2 seconds doesn't hold up. The classic case is calculating arc flash in vaults or manholes where obviously escape is impossible. In that case most practitioners choose to either simply use the 2 second rule blindly (no justification...same as we've been doing) or calculate until a protective device triggers. It also doesn't consider in this case the fact that it's not an "open room" further increasing the incident energy, which is not a scenario that IEEE 1584 covers. Manholes are one of the situations where we really don't have good information. So if you don't follow IEEE 1584, then what? Yep...no answers.


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