> Since you say that the noise is better when it rains there
are two
> possible explanations. First, the insulator surfaces are
cracked or
> have arc damage, in this case the rain is filling the gaps
and causing a
> steady current instead of arcing. These should be
replaced. The other
> is that the insulators are dirty.
I constructed and serviced power line noise locating
equipment in the 70's, and we did subcontract noise
investigation as an off shoot of that business.
Virtually all insulator problems that appear in dry weather
and vanish in wet weather are caused by the pins in ceramic
or glass bell-type insulators.
These insulators have considerable capacitance (dozens of
pF) between the pin holding the wire and the mounting pin.
The main pin is a loose fit pin that slides through two
holes in two metal bars that extend from the large "cold"
end of the bell. The big pin is held in position with a
small spring clip or pin that prevents the main pin from
falling out.
This big pin locks the bell to a metal eye, either another
insulator or a eye-bolt through the pole.
Since this is all a loose mechanical fit and since the pin
normally has high voltage via the leakage capacitance, the
insulator system depends on high mechanical force to prevent
tiny pin arcs. The most common defect is installers leave
slack in the spans. It is a bad installation habit caused by
poor crew training with regard to RFI. (The entire city of
Barnesville GA is wired that way, as is most of the crap
around Cleveland Ohio.)
When the pin and socket corrodes over time, it arcs. When it
gets wet, the arc stops.
Pin arcing is a sharp buzzing noise. It breaks up when the
pole is tapped or shaken, and is a dry weather problem.
Slack spans with bell insulators were by far the largest
single problem at HF.
Defective insulators are a very deep pitch with little
effect from moisture, or they may actually get worse when
wet. In general most of the defective parts were bad either
wet or dry.
I actually found considerably more loose clamps and wires
than bad insulators. I don't think I found more than one or
two bad insulators out of thousands of noise complaints,
although we would sometimes change out really old bells with
newer polymer insulators since the new insulators could
tolerate having slack in the spans. ( The polymer insulators
were a long fiberglass core with a flexible plastic ribbing
to increase leakage path.)
I'm not sure about dirt on normal distribution lines. I
can't recall ever running across that problem, except on
some high tension lines in an industrial substation. In that
case the noise was all up on TV channels, with very little
HF noise. In my experience, HF problems require actual
arcing between "big things" rather than corona.
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