Re: Power Line Insulators

[William Beaty <billb@ESKIMO.COM>]

On Mon, 10 Apr 2000, Van E. Neie wrote:

> I've been watching new power poles go up around our campus and it
> occurred to me that the insulators that hold the wires always have the
> same basic shape:  what appears to be ceramic "disks" stacked together.
> I've tried to think through why this shape is important, but so far I've
> come up empty.  Does it perhaps have to do with moisture shedding?  An
> increased surface area would get rid of heat more efficiently, but these
> insulators shouldn't get that hot anyway, right?

I haven't thought about this issue in years, and now I see something I've
never noticed before:  as the tip of a "spark leader" grows, the speed of
the growth is higher when the e-field is large at the tip, but the SHAPE
of the spark is important.  We must note that the spark is made of
conductive plasma, and the tip grows fast because the spark acts like a
sharp needle. If the "spark leader" is following the surface of the
ceramic insulator as it grows, and if that surface curves so that the tip
of the spark leader is forced to turn back on itself, the conductive body
of the spark leader will provide an electrical shield, and the e-field at
the growing tip will fall drastically.  The spark will stop growing.  It
will "dead end" without ever reaching another conductor.

If the spark starts at the top in the crude diagram below, then as soon as
the tip curved around to face backwards, the growth of the spark would
halt in its tracks.  To continue, the spark would have to leap downwards,
leaping across the empty air between the insulator disks.  Sparks can grow
very long if they follow a surface, but they need a much stronger e-field
if they are to leave the surface and leap across an air gap.

___
   \
    \
     \      SPARK GROWS DOWNWARDS,
   |  \     THEN TURNS TO FOLLOW INSULATOR
   \__/     SURFACE, WHICH HALTS THE GROWTH



___
   \
    \
     \      SPARK CAN KEEP GROWING ONLY
   |  \     IF IT FORMS A NEW BRANCH AND
   \__/     LEAPS ACROSS THE AIR GAP TO
      |     THE NEXT INSULATING DISK
      |


A spark might make it across one air gap, therefore it makes sense to
stack a bunch of air gaps in series.  The gaps act like large value
resistors, and the potential between the tip of the spark and the
electrode is was "trying" to reach would become less and less as the
spark-leader lept across the successive gaps.



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William J. Beaty                                  SCIENCE HOBBYIST website
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