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  • J Tiers
    replied
    Originally posted by darryl View Post
    I would certainly rather be wrong about the case I described- I got that info second hand. I was thinking also that the voltage would have been in the 480 to 600 volt range- maybe not high enough to launch an arc any appreciable distance. Still, I would not be the guy to want to pull the switch-

    A question arises regarding the duration of a lightning stroke- I wonder what it is for a typical one? Persistence of vision makes me think it's at least 1/4 of a second-
    Arc flash is essentially being close to a big welding arc. VERY close. Welders vary from 20 to 80 volts on the welding circuit, commonly. Just imagine the energy available from an arc driven by 480VAC, which is perhaps 12 times higher than many welding arcs.

    So, the local voltage from lightning, which has distributed some millions of volts over say, a mile or two......perhaps 500 to 1000 volts in 5 feet, dividing up a couple million over a mile long lightning channel. Could be more, I'm not claiming it as solid data.

    So, assume 2000A, which is a number I have seen for the current. The direct power during the stroke is 2000 x 1000, or perhaps 2 megawatts, 2 megajoules per second. But, it is actually 60 microseconds long, so the energy is 0.00006 x 2 megajoules, or 120 joules per stroke, equal to 1 second of a 120W light bulb.

    If there were 30 short flashes, that would add up to 0.00006 x 30 = 0,0018 sec, or almost 2 milliseconds. That would be 30 x 120, or 3600 joules total expended in a 5 foot section of stroke near you.. Youe would get the proportion of that according to how much angle you subtend vs 360 degrees around the stroke. Not necessarily a lot.

    Even the entire 3.6 kJ is not a lot, kill you, burn you, sure. Vaporize you? Likely not.

    This is highly dependent on conditions and assumptions. Choose your own according to lightning data and see how it comes out.........

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  • Jim Stewart
    replied
    Originally posted by darryl View Post
    I would certainly rather be wrong about the case I described- I got that info second hand. I was thinking also that the voltage would have been in the 480 to 600 volt range- maybe not high enough to launch an arc any appreciable distance. Still, I would not be the guy to want to pull the switch-

    A question arises regarding the duration of a lightning stroke- I wonder what it is for a typical one? Persistence of vision makes me think it's at least 1/4 of a second-
    Yeah, your persistence of vision is 1/4 second or longer.

    A quick consultation of our overlord Google led me to the Wikipedia article on lightning. It says, in part,

    "the average duration is 0.2 seconds made up from a number of much shorter flashes (strokes) of around 60 to 70 microseconds."

    Later in that same article it says the risetime is typically 1 to 10 microseconds.

    -js

    Leave a comment:


  • darryl
    replied
    I would certainly rather be wrong about the case I described- I got that info second hand. I was thinking also that the voltage would have been in the 480 to 600 volt range- maybe not high enough to launch an arc any appreciable distance. Still, I would not be the guy to want to pull the switch-

    A question arises regarding the duration of a lightning stroke- I wonder what it is for a typical one? Persistence of vision makes me think it's at least 1/4 of a second-

    Leave a comment:


  • RB211
    replied
    Video I watched was onboard a ship I believe, dealing with thousands of volts. The room filled with steam and only the rubber boots were recognizable after the camera iris caught up.

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  • PStechPaul
    replied
    A much more graphic demonstration of arc flash PPE and testing. Warning - some dummies were harmed in these tests.

    Leave a comment:


  • PStechPaul
    replied
    I am very skeptical about accounts of people being vaporized (even partially) by an arc flash, especially if it involves something like a mains switch or circuit breaker on low voltage (<600V) equipment, as would be the case for the facility power in a power station. The largest metal-clad switchgear I am familiar with are in the order of 4000-6000 amperes, three phase, and usually 480 VAC line-to-line. It is possible that there could be a mains switch in a cubicle for medium voltage (600 to 5000 volts), but I really doubt it. Also, standard PPE for operating switchgear is usually sufficient to prevent loss of life under the most severe conditions, and certainly an arc flash sufficient to vaporize a human body would also destroy a Nomex suit and face mask.

    Snopes has debunked a similar supposed human vaporization incident, stating it would take several million volts and something like 1/2 seconds to do so.
    https://www.snopes.com/fact-check/into-thin-air/

    Testing of Nomex arc-flash PPE:


    Last edited by PStechPaul; 05-23-2022, 10:02 PM.

    Leave a comment:


  • J Tiers
    replied
    Originally posted by darryl View Post
    ............................... The damage can be done before a big breaker somewhere opens.
    Which can take several half cycles, depending on the type breaker. Much longer than a stroke of lightning, although there can be multiple strokes, which your eye can see as one long one.

    Yes, it can happen, but it takes a major feeder line to have the sheer power capability. As I said, you can calculate the power needed, and if it is there, it can happen. Arc flash limits consider available power, and arc flash can do anything from fry the skin off you to vaporizing you, depending.

    Unlikely in the home shop, you will just be dead, with a couple small burn marks.

    Don't DO that.

    Leave a comment:


  • darryl
    replied
    I have an acquaintance that worked in a power station as it was being built. This isn't related to the power being generated, but the power coming in to run the equipment, lights, etc. During the building, it became time to throw the main switch, and an apprentice offered to do it. Just as he was being told to stand to the side while throwing the switch, he pulled it- should not have been a big deal, but there was a short somewhere- end result being that the top half of his body was vaporized by the arc-flash that came out of the panel. I don't know if it's possible to quantize the amount of power that can be concentrated in a small area, but a lightning strike certainly can do that, as can a short on a high voltage grid system. The damage can be done before a big breaker somewhere opens.

    Leave a comment:


  • J Tiers
    replied
    Originally posted by RB211 View Post
    .............................
    So when some one gets hit by lightning, why aren't they vaporized from the heat? I know when an electrician is dealing with high voltage systems and an accidental arc flash happens, often their rubber boots are the only thing left.
    People are made of water, which has a high heat capacity. The lightning stroke has a lot of energy, but is very fast. It has high frequency content, so it tends to travel on/near the surface rather than deeper in.

    Electric systems are low frequency, and have the capability to deliver constant power. Once the arc is established, it can persist, rather than being over in milliseconds. That gives time to deliver power to whatever "conductor" is across it.

    But, I know that even squirrel bodies are often recognizable after an incident, possibly because they were thrown clear. I do not know if I have ever heard of someone being vaporized, unless maybe with a very high voltage/power line capable of delivering megawatts.

    A 200 lb person contains maybe 70 kg of water, all of which would have to be vaporized. You can figure out the energy needed just to do that.

    Leave a comment:


  • RB211
    replied
    Originally posted by deltap View Post
    When I lived in FL there were rare documented cases of joggers being killed in bright sunshine by lightening strikes, the storm passing miles away. During summer months the weather pattern stayed the same with clouds building during the day and afternoon thunder storms you could almost set your watch by. By late summer this heat engine activity contributed to an occasional hurricane.
    Those are air mass thunder cells, very localized. The hurricanes we get are formed from Africa. Having flown in Africa a little bit, it was incredible seeing how quickly cells could form and grow above the altitude I was flying.

    So when some one gets hit by lightning, why aren't they vaporized from the heat? I know when an electrician is dealing with high voltage systems and an accidental arc flash happens, often their rubber boots are the only thing left.

    Leave a comment:


  • JohnMartin
    replied
    Originally posted by PStechPaul View Post
    Probably more than 25 years ago I was on a flight destined for Sea-Tac, and there were heavy storms predicted for the area. At one point there was a bright flash and loud BANG, and the lights in the plane flickered. The college age girl sitting next to me grabbed me ( I was OK with that ), and was very rattled. She said she was a pilot for her father's small plane, but had never experienced anything like that. The pilot came on the speakers after a minute or so and announced that it was only a "static discharge" and nothing to worry about, but I think that was just a euphemism for lightning strike. I think the landing was normal.
    That reminds me of a flight to Hawaii that we were on about 40 years ago.

    We were delayed at the gate in leaving SF as one passenger required extra attention in boarding - an older woman who spoke only some Eastern European language, who was seated in a window seat about 5 rows ahead of us. Her daughter was allowed to board with her, and it took the combined efforts of the daughter and a flight attendant to get her calmed down enough to fly. The daughter left the plane before departure.

    About halfway across, we had to fly through a large storm. The plane was pitching quite a bit, and there was much unintelligible loud talk - probably praying - from that seat. Well, then it happened. A huge flash right outside a window, the plane jolted slightly, and it sounded like a shotgun had gone off. And you can guess whose window that was.

    The screaming lasted for what seemed like many minutes. You didn’t need to understand Polish or Hungarian or whatever it was to get the gist of her words. We could hardly hear the pilot come on with his “static discharge, not uncommon” story. And yes, like your flight, he greased it in in Honolulu - he wouldn’t have dared a bumpy landing.

    Leave a comment:


  • deltap
    replied
    When I lived in FL there were rare documented cases of joggers being killed in bright sunshine by lightening strikes, the storm passing miles away. During summer months the weather pattern stayed the same with clouds building during the day and afternoon thunder storms you could almost set your watch by. By late summer this heat engine activity contributed to an occasional hurricane.

    Leave a comment:


  • lynnl
    replied
    Originally posted by darryl View Post
    We don't often get lightning storms here anymore, ........
    "anymore..."? Interesting that you say that. Here too (Alabama) I have definitely seen a substantial decline in the number of rain shower events that produce thunder and lightning. And those that do occur seem noticeably more docile, in terms of intensity, loudness, duration, pulse frequency, etc. I've wondered just why that is. Obviously the atmospheric conditions have changed. Yearly variations would not be surprising, but this seem to be a long term trend over the past 20-25 years. (Now I'm excluding those really severe, tornadic type systems we sometimes get.)

    Leave a comment:


  • PStechPaul
    replied
    I had lightning strike the line and phone line and probably also the huge sycamore next to my other house about 75 feet from where I was. I heard and saw a simultaneous flash-bang and it fried the land line phone and the modem.
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  • darryl
    replied
    We don't often get lightning storms here anymore, but last time we had one I stood outside while it flashed within about 1/4 mile on both sides of the house. What a show! I was waiting for that tingling feeling that would mean RUN FOR COVER, but it didn't happen.

    Reminds me of the time I was in Africa, living in the highlands district near Salisbury. We had a period of about two weeks where lightning would strike the pole in front of our house several times a day during the afternoon. You'd see the flash, then a fraction of a second later you'd hear the bang. Sometimes I'd stand in the yard and watch it. And of course, being a geek I'd wonder how to capture the energy. It's been said that one lightning strike has enough energy to run a household for a month or more- this would have ensured power all year long, assuming you could capture, store, and use the energy in some efficient manner.

    Our home insurance included a glass breakage clause, as it was common in our area for thunder to break windows.

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