Hi Paolo:
> what about relative humidity of air ?
> I believe breakdown voltage depends also on humidity content of air.
Contrary to "conventional wisdom," the water
content of air, humidity, actually increases
the dielectric strength of air by a slight
amount. (Sorry, but I cannot cite where I
read this.) As I recall, the increase is so
small as to be negligible compared to all of
the other factors affecting dielectric
strength of air.
Note that liquid water and gaseous water
(water vapor) have entirely different physical
properties. It is not unreasonable for water
vapor to have a greater electric strength
than air.
For gases, according to the Standard Handbook
for Electrical Engineers:
The relative dielectric strength, with few
exceptions, tends upward with increasing
molecular weight.
The noble atomic gases (helium, argon, neon,
etc.) are poorest... and have the lowest
dielectric strengths.
If we applied this generality (contrary to my
initial assertion), then the electric strength
of water vapor, H2O, should be less than that
of either oxygen, O2, or nitrogen, N2. However,
note that the actual number of molecules of
water vapor in air at any temperature is very
small compared to the number of molecules of
oxygen and nitrogen. So, the reduction of
dielectric strength by the presence of water
vapor, if any, will be very small.
Regards,
Rich
> From owner-emc-p...@ieee.org Tue Jul 11 01:33:30 PDT 2000
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> Message-ID: <01BFEB21.375DB740@roncone>
> From: Paolo Roncone <paolo.ronc...@compuprint.it>
> To: "'bogdan matoga'" <bogda...@pacbell.net>, Rich Nute <ri...@sdd.hp.com>
> Cc: "allen_tu...@pairgain.com" <allen_tu...@pairgain.com>,
> "emc-p...@ieee.org" <emc-p...@ieee.org>
> Subject: R: Voltage Breakdown
> Date: Tue, 11 Jul 2000 10:17:26 +0200
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>
> Hi,
> what about relative humidity of air ?
> I believe breakdown voltage depends also on humidity content of air.
> One thing anybody can experience is the much higher frequency of ESD jolts
> one gets in dry weather vs. humid weather. Anyone knows a quantitative
> relation or formula ?
>
> my penny's worth...
> Regards
>
> Paolo Roncone
>
> -----Messaggio originale-----
> Da: bogdan matoga [SMTP:bogda...@pacbell.net]
> Inviato: sabato 8 luglio 2000 22.57
> A: Rich Nute
> Cc: allen_tu...@pairgain.com; emc-p...@ieee.org
> Oggetto: Re: Voltage Breakdown
>
>
> Gentlemen:
> Let me add my penny's worth to Rich's information.
> a. Parallel plates provide a homogenous field only in their center, the
> field fringes at the edges. (One could configure the plates according to
> Rogowski form).
> b. The breakdown voltage is not a linear function, even for spherical
> electrodes. What applies is Paschen's law, which gives the voltage as a
> function of distance x atmospheric
> pressure.
> Have fun!
> Regards,
> Bogdan.
>
> Rich Nute wrote:
>
> > 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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>
>
>
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