Question about a railways third rail

Discussion in 'Physics' started by MrZeus, Sep 26, 2010.

  1. MrZeus

    Thread Starter New Member

    Mar 8, 2009
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    Hey guys, I'm not sure if this is the right forum to post this in, sorry.

    Anyway, I was watching an episode of Criminal Minds (Season premiere of Season 4, I think) and a guy stepped on the "third rail" of a subway in order to commit suicide - he was successful. One foot was grounded, the other was on the third rail. Should this have really killed him? It seems to me (3rd year in electrical engineering) that the current would go through one leg and directly to the other... Not passing his heart, possibly just frying his legs or something. But then again, as I was typing that, perhaps he fell and then that gave the current a better path to/through his heart as he lays there, right? Just curious here :) Thanks!
     
  2. sceadwian

    New Member

    Jun 1, 2009
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    MrZeus human beings are not solid conductors we're a complex networks of blood vessels and tissues. Think of lightning, ever seen a straight lightning bolt? =) While the bulk of the electricity would in theory (if a human were a solid conductor) travels as you say in the real world some of that energy is going to go through the rest of the body as well, it takes a pitifully small amount of current across the heart to put a person into arrest and besides that the simple Ohmic heating of the tissue when you have good contact to an high voltage source like that would be massivly destructive. All it has to do is vaporize a tiny portion of blood to send gas bubbles to the heart or brain killing you nearly instantly. At any rate as far as suicide goes that is one way I would NOT want to go, it's massively traumatic and there's no guarantee of death. I've been zapped a few times but the thought of actual lethal electrocution makes me queezy.
     
  3. MrZeus

    Thread Starter New Member

    Mar 8, 2009
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    Wow, that makes a lot of sense! I guess I was stuck on it having to directly affect the heart to kill. It does seem like a really horrible way to go either way. In the show, it was more like the only choice for the character.
     
  4. sceadwian

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    Jun 1, 2009
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    Yeah something like the third rail thing... that's a lot of nasty tissue heating there messy at best. Also the thought of large amounts of electricity traveling through my brain terrifies the crap out of me =) I have enough trouble with all the little electrons and chemicals up there moving around as it is! <chuckle>
     
  5. b.shahvir

    Active Member

    Jan 6, 2009
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    The human body is a network of series-parallel resistance dropping voltage across it as electric current passes thru it. If heavy current passes thru any part /section of the body, the resultant voltage drop (potential gradient) across the heart area might be enough to pass sufficient current thru the heart to initiate cardiac arrest!
     
  6. sceadwian

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    Jun 1, 2009
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    The capacitance from the vascular system is quiet high as well. I would love to see a detailed circuit equivalent of a specific human being but they can vary drastically from day to day let alone from person to person.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Shoes also make good insulators for the most part. Was this guy barefoot?
     
  8. DonQ

    Active Member

    May 6, 2009
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    Doesn't matter, this is TV! Anything is possible in that imaginary world.

    Ever see the guys climb up on a fence, and once they are fully above the ground, the owner turns on the juice to the 'electric fence'? Somehow, this can blow them right off the fence and halfway across the movie lot.

    Once, I saw some guys beamed up to a spaceship! It must be so, I saw it on TV. Does anyone have the circuit diagram for that? I want to make one in my basement.:D
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Perhaps, but the basic point remains, shoes made out of rubber will not do anything, bare feet where a power line has gone down is deadly. Come to think of it, I wouldn't want to be near a downed power line with shoes on.

    It can't hurt to give a reasonable question the best answer possible. Many times the answer will be questionable, but we try.
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
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    AWESOME!! Lets build one!

    ...
    ....we can call it........a TRANS-FER-ER !

    We will be RICH! ;)
     
  11. b.shahvir

    Active Member

    Jan 6, 2009
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    At 750 Volts DC, shoes might not be that effective an insulator...although the person might feel mild electrocution (due to moisture, sweat, etc.) but definately won't die
     
  12. sceadwian

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    Jun 1, 2009
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    Bill keep in mind that's only true in perfectly clean and dry conditions, I would personally not even on a dry day want to walk down a third rail in my work boots, if it were a wet day you wouldn't catch me anywhere near it.

    As far as the insulation resistance of boots go sorry I have to side with Bill b.shahvir. Asuming the boots are generally clean the insulation thickness of a pair of workboots would probably be good well over a couple thousand volts. They're generally polyurethane, plenty of isolation... under ideal conditions.
     
  13. retched

    AAC Fanatic!

    Dec 5, 2009
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    Unless you stepped on a nail or screw that protrudes 9/10ts of the way through the sole.

    Even while bending the foot forward to take a step, their is only the leather, then a STEEL toe, then your sweaty socks.

    If the sole of the boot was flat on the rail, and no other variables interacted, then yes. good inductor. Probably safe-ish.
     
  14. sceadwian

    New Member

    Jun 1, 2009
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    Safe-ish...
    retched, go work a medium industry job for 6 months to a year in a pair of boots.
    on then step in a stagnant puddle of water near a third rail and tell me you'll feel confident you'll be fine if you jumped right on it. Every step you take is one more chance of encountering the specific set of conditions that will result in your death, you don't take chances with that kind of thing.

    The electrical circumstances of 1 inch of rubber and dry leather encasing your foot in perfect protection only exist while the boots are still in their display case, my boots are about 8 months old right now you couldn't pay me enough to step on a third rail =)
     
  15. retched

    AAC Fanatic!

    Dec 5, 2009
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    If you cant recognize my wit by now... Geez. I was responding to your "ideal conditions" line.

    I have worked "medium" industry jobs.. And I stay THE HELL away from anything above 477v without proper mats, lockout/tagout, and other anti-grounding garments/procedures.

    We were not allowed to use steel "steel toed" boots. We had to use non-conductive eyelets, and toe-inserts.

    I wouldn't splash around in a puddle near a 3rd rail. I wouldn't even walk on the first 2 rails.. And neither should anyone else that is there for a non-maintenance reason.

    As the rails switch from segment to segment (long, not like maglev) there is a initial surge. This will blow frogs and snakes apart (like you cant find any pieces-parts left) during this period of a fraction of a second. This serves as the grass clearing service, as it will vaporize any vegetation that is close or touching the rail.

    Its nothing to play with... Nails, screws, steeltoe, whatever...
     
  16. sceadwian

    New Member

    Jun 1, 2009
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    reteched, I've been on this site for a month or so now that's it. Online emotional intonation is a fine art; sarcasm does not translate in plain text. You'll if you haven't already discovered by now when failing to see such fine detail in text I default to a literal view of things, because there is no where else to go! If I knew you personally it's be different, but it takes huge amounts of time to get to 'know' someone online.
     
  17. retched

    AAC Fanatic!

    Dec 5, 2009
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    BUT YOUR A "Senior Member" And in the rule book, that implicitly implies knowing and having the ability to convert retched's wit from text, regardless of emoticons used ;)
     
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