Re: Power Line Insulators

[John Denker <jsd@MONMOUTH.COM>]

At 02:21 PM 4/10/00 -0500, 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?

Shedding moisture and dirt is part of the story.

In fact good outdoor insulators might be better described as a stack of
cones (rather than disks).

                   /||\
                  / || \
                   /|x\
                  / || \
                   /||\
                  / || \


Up inside the cone (near point x) it stays relatively clean and dry.

Another factor is just plain length.  If you have a dirty surface, you want
to have a LONG path along the dirty surface.   Any sort of convolutions
will help with that.

==============

To answer a possibly-related question: the best _indoor_ insulators (such
as you find on a multi-megavolt van de Graaf) look like a stack of disks.

One reason has to do with nonlinear effects that occur at dielectric
breakdown in the material.

Another reason has to do with the fact that you want the central "stalk"
that holds the disks to be small (less conductance for any given
conductivity) but you want the disks to be large (less curvature ==> less
corona).

(Of course for outdoor neighborhood power lines, corona is not what they
are worried about.)

> An
> increased surface area would get rid of heat more efficiently, but these
> insulators shouldn't get that hot anyway, right?

If something goes wrong they can get hot, which is why they are ceramic not
polystyrene.  But you're right, heat dissipation isn't the main reason for
the shape.