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Air insulation
All,
Air is a pretty good and a pretty bad insulator. Air is free and
naturally present so to use it, we only need design our materials so
there is an adequate distance separating the two high voltage points in
question. All other insulators must be secured through purchase and cut
to fit an application.
Air is a gas or, more appropriately, a mixture of elemental gases. The
bulk of our air is nitrogen with a small amount of oxygen thrown it to
help us breathe. Argon is the only ohter gas worth noting and is presnet
to about 1%. This mixture is genrally assumed to have a stand- off
voltage of 50 volts per mil.(.001"). This is grossly affected by
moisture (we do have water vapor in the air also.) As most water vapor
is a distilled form and hard, it might be considered for the most part
non-ioninc and therefore non-conducting (pure water is a superb
insulator). The snag comes when the water accumulates on a surface and
becomes macroscopic (liquid) it is just itchin' to load up with ions!
Thi is when water becomes a detrement to HV insulation. Air transports
harmless water which condenses on solids to become an ionic medium.
Gases have a varying voltage breakdowns based on pressure and this is of
little concern to us for insulating purposes, for when we use air, it is
fixed at a normal, but slightly varying pressure of around 760mm of
mercury.
The really neat thing about air is it has a dielectric constant of about
1 (unity) or virually equivalent to the vacuum lof outerspace!! This is
nice and is airs one great saving grace as an insulator.
In the study of dielectrics, high voltage, and the like, it is quickly
realized that some of the best solid insulators have rather higher
dielectric constants than air. (k= 2-6). It is also quickly learned that
with a given field voltage, the field intensity grows to horrendous
levels at the interfaces of dielectrics of varying dielectric constant.
Have you ever seen sparks crawl along a sheet of mylar in air? I have!
the contact of two dielectrics of differing values is a collection point
for intense fields!
The lower the dielectic constant the less the field gradient becomes for
a given voltage. Thus, avoid close quartered high differential
dielectric constant materials and you won't see sparks seeming to prefer
to zip along this corridor, rather than leap out into air. Why should
they?! They are happy staying where the fields are intense. Therefore,
the best way to use air insulation is to allow for large radius of
curavature metallic charged surfaces to contact air directly without
other insulation.
Richard Hull, TCBOR