engrick wrote:
Would stand near this gap during a fault? or while racking a breaker in? The blast pressure could be huge.
Hugh Hoagland's group has had a major breakthrough when it comes to arc blast: they've actually measured it in a very reliable way. CIGRE has also done some monumental work in modeling it. The models are specifically intended to speed up the design/engineering/test aspect of arc resistant gear but nonetheless the information is quite useful if you sort of invert it and look at it from the point of view of non-arc resistant gear.
Hugh's group pretty much documented one crucial aspect of arc blast. There has been basically two prevailing theories as to what causes it. The first one is simply that the air inside the enclosure is heated and following basic understanding of gasses with no vent, it pressurizes. The second theory is that arc blast is caused by expansion of copper as it changes from solid to vapor form. Hugh's group in the most recent IAS-ESW documented that the copper vapor theory is either wrong or is so minor that it plays no role. This also matches the CIGRE work which uses the same basis for their modeling.
Where does this leave us? Ignoring arc resistant gear for a moment, we start with an enclosure. The enclosure may or may not be "sealed" sufficiently so that it builds up pressure. All of the documentation that I've been able to find on arc blasts measure a maximum of around 8-10 PSI within the enclosure and according to the CIGRE documentation this also happens very quickly...within one cycle. The reason that the pressure limits at 8-10 PSI is that smaller enclosures rupture at this pressure. Larger ones rupture at lower pressures. This stands to reason...10 PSI acting on a typical 1U bucket door which would be roughly 20"x20" has about 4,000 lbs. of pressure on it. That's guaranteed to blow off the hinges. I have been unsuccessful at finding anything credible that is higher than that.
Outside the enclosure though there are no claims about pressure at all. In fact except for some vague acoustical measurements and the data used by Lee to estimate arc blast which in itself isn't really representative because it involved an arc across two electrodes facing each other peaks out at around 2 PSI for high fault current cases.
So where does this leave us? 1-2 PSI AT MOST is the highest exposure we can expect. Maybe in some strange circumstance where someone is sealed in (and I mean SEALED) while an arc occurs we could get up to 8-10 PSI but that's it. At 1 PSI ear drums rupture and there are plenty of medical cases of this occurring. At around 20-35 PSI depending on which report you read is the threshold where a fatality is possible and somewhere in the neighborhood of 50-100 PSI fatalities are almost certain. That comes from military studies on the effects of a concussive blast which is what we are discussing here. So ear drums are definitely a possibility. Getting knocked around is also not out of the realm of reason. But fatalities or major injuries are flat out not going to happen. Effectively the claimed fatal consequences of an arc blast are a total myth. I can back it up another way, too. Doan, Hoagland, and Neal documented roughly some 50+ cases of arc flash and also listed secondary effects. Loss of hearing was one of them but nowhere in there are concussive type injuries listed. And I've had private exchanges with others that actually get to go out and do accident investigations on a regular basis who have also said that they have never seen any evidence of concussive "blast" type injuries outside of for instance blown ear drums.
As to people being "thrown back"...I think that's a myth, too. Look carefully at any of the videos on the internet of actual arc flash events and carefully single frame through them. Arc blast as previously noted happens in about 1 cycle so if that's the case we should see someone standing still in one frame and then suddenly thrown a significant distance in the very next frame and then stop moving by the third frame, assuming that the video frames are 60 per second (1 cycle) or slower. However what actually occurs is that about 0.1-0.4 seconds later, the person is suddenly moved away from the arc flash, way after the arc blast is already done. That's not arc blast...that's the "freeze or flight" reaction kicking in and the person is "propelled" away from an obvious danger under the own power. It might seem like they were "thrown" to people simply because the freeze/flight function in the brain is processed way down in the cerebellum and brain stem...it's more of an instinctive reaction than anything. By the time that the cerebrum figures out what is happening and begins to assess the situation, the cerebellum has already moved the person out of danger.
Anyways this is all opinion based on analyzing the available data six ways from Sunday. Based on this analysis I see no reason to make statements about not standing somewhere out of fear of an arc blast with the exception that for instance if you are going to be working off a scaffold or out of a bucket or somewhere else that you might get knocked out of (or jump out of), it's best to be in fall protection/prevention equipment, that you should stand to one side to avoid doors that might get blown off when an arcing fault may occur, and that ear plugs are probably a good idea.