Hi Allen:
The voltage breakdown of air is presented in
IEC 664.
There is no fixed number (i.e., mm/kV) that
describes or predicts voltage breakdown of air.
The principle factor that affects voltage
breakdown is the shape of the electrodes
(degree of homogeneousness of the electric
field between the two electrodes). The more
homogeneous the field, the higher the breakdown
voltage, and vice versa.
For example, at 2 mm, the breakdown of an
inhomogeneous field is about 2.5 kV, while the
breakdown of an homogeneous field is almost 8
kv!
An homogeneous field is created by two parallel
planes. An inhomogeneous field is created by
a needle-point and a plane.
The second factor that affects voltage
breakdown of air is the voltage waveshape. The
more impulse-type waveshape, the higher the
breakdown voltage; the more sinusoidal-type
waveshape, the lower the breakdown voltage.
The third factor that affects voltage breakdown
of air is the air pressure. The higher the air
pressure, the higher the breakdown voltage, and
vice-versa (Paschen's Law).
There are still other factors such as temperature,
but these are minor effects compared to the first
three.
If you are interested, I can send a Powerpoint
4.0 file of the voltage-distance curves of the
homogeneousness effect. It covers the distance
range from 0 to 2 mm for both homogeneous and
inhomogeneous fields for sinsoidal waveforms.
The data is taken from IEC 664.
Best regards,
Rich
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