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High Voltage Question

Discussion in 'Electronic Basics' started by Ben Weaver, Dec 12, 2003.

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  1. Ben Weaver

    Ben Weaver Guest

    Hi People.

    Disclaimer: All the below is theoretical - I'm not going to try it!

    I've been wondering. Let's say I'm standing in lab working on a high
    voltage thingy (computer monitor, whatever). Obviously I've got my left
    hand behind my back, I'm wearing decent shoes, no antistatic mats
    around, no grounded scope probes, etc...

    Now let's imagine that I accidentally touch a high voltage line. I'm not
    touching ground at the same time. How high a voltage would need to be
    present to do me significant damage?

    I've heard that mains voltage (240V in the UK) is okay in that scenario
    (not that I've tried it - and please no one try it) - but how high can
    you go? Assuming that you've got 30kV in a 15" CRT monitor would that
    still be safe?

    I guess that the weather might affect the situation somewhat?

    Any thoughts much appreciated.

    Ben
    ~~~
     
  2. grahamk

    grahamk Guest

    You and earth form two plates of a capacitor.
    Insulation such as shoes, carpets etc forms the dielectric.
    If the supply is DC and the voltage is high enough then the charging current could be high enough to shock you for an instant.
    If the supply is AC then the value of the capacitance, of which your body is a part, and the supply frequency will determine the AC current which will flow in the circuit which your body is part of.
    If the voltage is high enough then the dielectric could break down giving higher current flow.
    I often used to stick a screwdriver in the Live socket and hold the bare blade to demonstrate to students that a complete circuit was necessary to obtain current flow.
    Don't do it !!
     

  3. Your post reminds me of some similar questions I have. Specifically I
    wonder about birds and powerlines.

    Why don't birds burn their feet when they perch on powerlines? Or do they?
    Don't powerlines operating near capacity run at relatively high temperatures
    in the vicinity of say 70 deg. C? Or do birds selectively only land on
    powerlines that are reasonably cool? Do bird's feet have a sense of feel,
    and are they conductive? What about their legs leading down to the feet?
    It seems to me owls must have conductive feet/legs since they have
    controllable claws on their feet for picking up prey, but is that true of
    other more conventional birds you see on powerlines? Do powerlines ever
    injure/kill any birds?

    Do birds land on high voltage transmission lines (say around 200-1000kV
    lines)? If they do, do they experience a sensation of electric shock when
    they do? How much capacitive current flows through them at say 60Hz and
    750kV? How much corona current flows through them at 1 MV DC or 750kV AC?
    If the bird has some kind of flammable coating (or maybe just natural dry
    feathers with natural body oil) will the corona light them on fire? Would
    the corona and capacitive currents be enough for the bird to feel or be
    harmed?

    It is not that I especially care about the well being of birds, it is just
    these questions are all quite amusing to think about. Having the answers to
    these questions would help in my understanding of the nature of high voltage
    (which seems a bit different from low voltage).
     
  4. I stood in a plastic waste basket and put my hand on the globe of a
    vandegraaff static generator (30,000 to 50,000 volts) and my hair
    stood on end and crackled, but no ill effects.

    Like this:
    http://www.amasci.com/emotor/vdg.html
     
  5. David Harmon

    David Harmon Guest

    Have you ever seen the demo where a person standing an a well insulated
    platform puts their hand on top of a Van de Graff generator. The get
    charged up to thousands of volts, their hair stands straight up, etc.
    http://www.amasci.com/emotor/vdg.html

    This is DC, of course, NOT AC. AC and RF raise different issues.
     
  6. N. Thornton

    N. Thornton Guest

    it depends a lot, more useful is the q of risk vs voltage and current.
    People can die on 50v yet others can survive 25kV. Below 50v youre not
    as much risk, but that doesnt always hold true.

    50 people die each year from it, and I know from experence that
    standing on a dry carpet with shoes on is not good protection.

    - but how high can
    In a typical standing on carpet scenario you have dampness in the
    shoes, metal pins and dirt. In the carpet you have dirt and sometimes
    some damp. Concrete has been used as a good earth for half a century
    (see Ufer IIRC)

    So in short it isnt safe: you might be fine, you might be buried. Most
    times people survive no problem, but the risk is too large for many to
    mess about with. So dont. Some of us arent here because of it.


    Regards, NT
     
  7. Birds stay away from lines with that kind of capacity. The electromagnetic
    fields around those lines play havoc with a bird's orientation, I don't
    know if they could get close even if they wanted to.

    I think the lines that you'll usually see them sitting on are the ground
    wires that run across the top of a pole system, though not always.
    Sometimes a bird will connect to ground if its wingspan is wide enough,
    then poof, no more bird.

    --Russell
     
  8. Rich Grise

    Rich Grise Guest

    I've heard that (at least some of) the ultrahigh-voltage
    cross-country hi-lines (Intertie?) are DC. I just took that
    for gospel, especially when I saw the show on edjamakayshunal
    TV about maintenance on them. It was really quite cool. They
    fly up to the lines in a little helcopter, just a little
    bigger than two men. Well, I guess it's got a cargo thing,
    like a minivan with a side door. Anyway, the guy wears a
    full-coverage metal suit - I don't know if the chopper pilot
    is suited up like that or if they're depending on the skin
    of the aircraft - and on the inside of the suit, he's insulated
    from _it_. As they approach the chopper, the guy's got a
    shorting hook - about the size of Mary-had-a-little-lamb's
    crook - that he reached out toward the line with, and drew
    about a 3 or 4' arc. So, they neared the line, drawing a
    shorter and shorter arc, the guy hooked it, so the chopper
    was almost touching it, and clipped a big shorting strap
    to it from the chassis. Then he climbed out onto the line.
    There are actually two or 4 cables in parallel, so it's
    almost like being on a catwalk. He kind of crawled along
    the lines and replaced insulators or cleaned them or something.

    Anyway, I was wondering, would this even be possible
    with a line at a million volts AC? DC's easy - Q=CE.
    But what's the capacitance to ground of a small helicopter
    at a couple of hundred feet?

    Thanks,
    Rich
     
  9. You are correct. I used to work at Bonneville Power Administration as an IT
    person - I remember them talking all the time about their 500kv DC intertie
    - I think it was used to sell energy to california.

    Interesting operation, that. Except the IT people didn't get to see any of
    the really interesting facilities... :(

    --Russell
     
  10. Don Kelly

    Don Kelly Guest

    ------------
    Also the bird will distort the local field and will have corona discharges
    from feathers etc. and associated movement leading to what I would think
    would be rather uncomfortable "skin crawling" conditions. For higher
    voltages and larger birds, this could be painful and potential differences
    between parts of the bird's body could be appreciable.
     
  11. Don Kelly

    Don Kelly Guest

    --
    Don Kelly

    remove the urine to answer
    ----------
    It is done at 750KV - will work at 1MV - AC or DC. Yes, there is a
    capacitance between copter and ground- just as there is between the line and
    ground. The whole purpose of the hook is to bring the copter to line
    potential. The purpose of the mesh suit is to be a Faraday cage (not
    insulation which would have to be so bulky that the worker couldn't do
    anything but pretend he is the Michelin Man) to bring the whole of the
    surface of the worker's body to line potential as otherwise there would be
    varying high potential differences between parts of the body and the line.
    Also, any charging or corona current is through the mesh, not the worker.
    Much more comfortable.
     
  12. Bird sit on the line, but they are not connected to ground. So birds
    form a resitance to be connected in parellel with the (low) resistance
    of the line. Current always takes the way of lowest resistance
    (current divider principle), so current will always flow by 99,9999%
    through the line and not through the bird, so the bird will be safe.

    Other problems arise when the bird touches the other phase, then there
    will be a large voltage (400V in europe, 180V in US) drop at the bird,
    and there will be no more bird :->. This only happens for large birds
    though, as the lines normally have a large space between them.

    When you as a human will be hanging on a powerline, without touching
    ground, and with anough space to ground to be not causing a discharge,
    the same thing as above will happen. Current will not flow through
    your body as of its large resistance, so you will feel nothing.
     
  13. Roy McCammon

    Roy McCammon Guest

    lets say that a human body has about the same surface as a
    1 meter sphere. I don't remember what the self capacitance
    of such a sphere is, but lets say 100pF at 60Hz = 25 Meg reactive.

    So. hang on to a 1MV line, and you'll get 1/25 = 40 mA
    through your wrist. Definitely noticeable. Lets say
    that a helo is 10 to 100 times that. So, current from
    a 1MV line to helo could be 0.4 to 4A. You wouldn't
    want to get in series with that.
     
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