Arc to ground wire

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

On Thu, 24 Apr 2008 19:40:40 +0000 (UTC) Dave Martindale wrote: | snipped-for-privacy@ipal.net writes: |>Reportedly the line involved here was NOT energized and this is a case of |>induction from a nearby line that was. | |>

| |>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.

-------------------- 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.

| 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.

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.