Treenails and fires

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Gary Johnson by way of Terry Fritz <twftesla-at-uswest-dot-net>" <gjohnson-at-ksu.edu>

About 30 years ago I was asked by an electric utility to help them with a
problem they had been having with fires.  On several occasions, the large
wooden poles supporting their 345 kV power lines had caught on fire at
conductor level (about 25 feet above the ground).  My conclusion was that
the fires started from the point of the nail holding a ground wire to the
pole. The point of the nail would go into corona and under just the right
conditions of pole age, chemistry, moisture, and wind, would catch the pole
on fire. The conductor might drop to the ground and interrupt service to
large numbers of customers. My suggested fix was to wrap the ground wire
around the pole (just a few turns) rather than run it straight down the
pole. The helix would act as a (poor) Faraday cage and would reduce the
electric field inside the pole just enough to prevent corona at the nail
points. They started installing grounds this way, and to my knowledge, never
had another fire.

The concern is that most of us operate Tesla coils at or above 345 kV fairly
close to walls and ceilings containing nails driven into wood. I would
expect some of these points to be at least close to corona. Apparently, we
get away with the practice because the wood is drier than a power pole and
because we do not operate 24/7. Still, it seems wise to have adequate fire
insurance on our test facilities. I know the Richard Hulls among us
regularly set our labs on fire with direct streamer strikes, but I wonder if
anyone has experienced a fire that occurred later, in a situation where a
direct strike would not appear to be a factor?

Gary Johnson