With respect to failing insulators:
The moisture ingress into the porcelan post insulators is probably a
factor, if they are unglazed on the ends. There are two other reasons
to consider, that could also cause rapid breakdown of the insulators:
If they have metallized ends, sometimes the edge of the metallization
is sharp enough to cause some field enhancement around the points.
This leads to partial discharge, eventually corona, eventually bank.
We had a number of brown porcelan posts breakdown with very high
voltage, after 20 years of normal operation. Turns out that in a
bright light, you could see the streamer marks on the glazed surface,
which was indicative of breakdown along the surface. Going to some
machined polymer posts made from Polysuflone seems to have fixed it
two years ago.
There is another phenomina which kills post insulators. If there is a
trapped air space at the end of the metal screw, and there are metal
screws at each side, even if the ceramic is solid in the center,
there is high chance of partial discharge occuring inside this space.
It will be only time before it blows up, as the air becomes ionized
and eats away at the thing. You can tell if you remove the screws
later and see surface corrosion on the tip. For instance, a brass
screw might have greenish deposits. A polymer post WILL fail from the
ozone over time, ceramic holds up better. Whenever I use small
steatite ceramic post insulators, for hv up to say, 5 kV, I try and
use nylon screws to prevent E fields from enhancing at the air space
inside. When we make polymer insulators, or G4, G7, G10 glass/epoxy
insulators, we drill tiny air holes at the inner end of the screw
threads. Then apply some silicone RTV sealant (clear, red, white,
etc.) to the hole, screw in the fastener, and the excess goop will
squirt out of the weep hole. This ensures that you have filled the
cavity. Doesn't hurt for keeping screws tight too. Having done this
for years, I rarely have seen failures anymore. This works with RF,
DC, AC.
John
K5PRO
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