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Arc to ground wire

Discussion in 'Electrical Engineering' started by [email protected], Apr 24, 2008.

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  1. Guest

    Reportedly the line involved here was NOT energized and this is a case of
    induction from a nearby line that was.

    http://phil.ipal.org/usenet/aee/2008-04-24/funspark.mp4

    You can see the clear shape of the arc in the multiple reflections due to
    the many glass elements in the lens, below the point of arc.
     
  2. Those "ghost images" of the arc are unlikely to be from reflections inside the lens, since
    images due to reflections are invariably a different size or distorted in shape compared to
    the original (because they're reflected from curved surfaces).

    Instead, those images are probably generated in the CCD. Frame transfer CCDs accumulate
    electrons from light exposure for a period of 1/60 second, then rapidly shift the contents
    of all of the pixels vertically into a non-light-sensitive storage area. Then, over the
    next 1/60 second, the charges in the storage area are shifted out and digitized one pixel at
    a time while the light-sensitive portion of the chip is accumulating the next field.

    If there's no mechanical shutter, light that reaches the chip during the vertical shift
    period ends up in the "wrong place" in the image, vertically offset from where it ought to
    be. If the incoming light is continuous, it just causes a slight blurring because the
    transfer period is so short compared to the normal exposure period. But an arc is very
    bright and very fast, so it could record an image that's both bright enough to see and sharp
    despite the fact that the image is rapidly shifting across the CCD face.

    For further evidence, look at a still frame that has multiple "ghosts" visible. Each one is
    a slightly different shape - because each is a separate arc that took a slightly different
    path through the air.

    Dave
     
  3. Guest

    | writes:
    |>Reportedly the line involved here was NOT energized and this is a case of
    |>induction from a nearby line that was.
    |
    |>http://phil.ipal.org/usenet/aee/2008-04-24/funspark.mp4
    |
    |>You can see the clear shape of the arc in the multiple reflections due to
    |>the many glass elements in the lens, below the point of arc.
    |
    | Those "ghost images" of the arc are unlikely to be from reflections inside the lens, since
    | images due to reflections are invariably a different size or distorted in shape compared to
    | the original (because they're reflected from curved surfaces).
    |
    | Instead, those images are probably generated in the CCD. Frame transfer CCDs accumulate
    | electrons from light exposure for a period of 1/60 second, then rapidly shift the contents
    | of all of the pixels vertically into a non-light-sensitive storage area. Then, over the
    | next 1/60 second, the charges in the storage area are shifted out and digitized one pixel at
    | a time while the light-sensitive portion of the chip is accumulating the next field.

    I've seen a few nearly same size reflections from lenses. There are many
    layers often with no difference, but with an air gap, intended more for
    correction of color.

    Another possible source is reflection between front and back surfaces of
    a lens filter in front.

    | If there's no mechanical shutter, light that reaches the chip during the vertical shift
    | period ends up in the "wrong place" in the image, vertically offset from where it ought to
    | be. If the incoming light is continuous, it just causes a slight blurring because the
    | transfer period is so short compared to the normal exposure period. But an arc is very
    | bright and very fast, so it could record an image that's both bright enough to see and sharp
    | despite the fact that the image is rapidly shifting across the CCD face.

    That's believable.


    | For further evidence, look at a still frame that has multiple "ghosts" visible. Each one is
    | a slightly different shape - because each is a separate arc that took a slightly different
    | path through the air.

    I'll look again.
     
  4. Don Kelly

    Don Kelly Guest

    --------------------
    In this case it may well be simply capacitive coupling rather than inductive
    coupling. As the arc appears to be on breaking contact and not on making, it
    is likely inductive. In either case, it is not surprising. That's why the
    grounding contact is made before the worker touches the line.
    --

    Don Kelly
    remove the X to answer
    | Phil Howard KA9WGN (email for humans: first name in lower case at
    ipal.net) |
     
  5. Guest

    | In this case it may well be simply capacitive coupling rather than inductive
    | coupling. As the arc appears to be on breaking contact and not on making, it
    | is likely inductive. In either case, it is not surprising. That's why the
    | grounding contact is made before the worker touches the line.

    I would agree. Although there is not a huge amount of current there, it sure
    looks plenty lethal.

    I'd like to see a series arc on one of those transmission lines that is not
    connected at the far end, to see what the charging current looks like. My
    expectation is it would be similar to this.
     
  6. Don Kelly

    Don Kelly Guest

    ----------------------------
    No- it would be much more spectacular. An arc like the one shown could be
    drawn by just trying to connect an insulated lineman's bucket to the line-
    due to the distortion of the field.
     
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