On Thu, 22 Aug 2013 12:04:41 +0100, The Other Mike wrote:

They are arcing horns and are fitted so any arcing from overvoltage that
occurs takes place across them and not across the porcelain insulators.

And you don't want the arc tracking along the insulator as it will
leave a mark/deposit that will encourage further arcing and/or
leakage along that path. Eventually the insulator will fail.

One of the ones at the top of "our" pole did that a few years back.
Every time they tried to restore power the insulator would breakdown,
arc across and trip the line. We just happened to be outside when
they tried a power restore and noticed, rang the DNO who passed the
message on. The guys that came to replace it where quite pleased as
they where having problems finding it though they had narrowed it
down to our spur or the segment of line that our spur leaves.

Got the impression they where using some form of ground
current/voltage test equipment that they moved between restore
attempts and noted the readings, higher the ground voltage/current
the closer they are to the fault, I think. Any body know?

--
Cheers
Dave.