Hi Paolo and Jim!
I think I found the answer to your doubts in the IEEE Std. 142-1991, "IEEE
Recommended Practice for Grounding ...". Look at section "3.2.6.3 Humidity
Control" (that I largely quote in the following).
(If my understanding is right), the ambient relative humidity has a big
influence on the surface conductivity of many insulating materials (such as
fabrics, etc.). The higher the humidity, the greater the surface conductivity.
At normal humidity (30% or more) an invisible film of water provides an
electrical leakage path over most solid insulating bodies (and the clothes and
shoes of a worker), which drains away static charges as fast as they are
generated. When relative humidity is 30% or less the same materials dry out and
become good insulators and static manifestations become noticeable.
To summarize, my understanding of IEEE Std. 142-1991 is that low humidity makes
easier the "trapping" of charges on poor-conductive bodies entering into
contact. This makes ESD more likely and more "intense". The increased
dielectric strength of dry air should be (by far?) not enough to compensate for
the more intensive charging (and higher electric field).
This is my "drei Groschen" interpretation of IEEE Std. 142-1991.
Cheers!
Canio Dichirico
European Southern Observatory
Technical Division - Electronic Systems Department
Karl-Schwarzschild-Str. 2
D-85748 Garching bei München
Tel./Fax +49-89-3200 6500
Fax +49-89-320 23 62
email: cdich...@eso.org
website: www.eso.org
----- Original Message -----
From: Jim Hulbert <hulbe...@pb.com>
To: <emc-p...@ieee.org>
Sent: Wednesday, July 12, 2000 14:40
Subject: Re: R: R: Voltage Breakdown
>
>
> I must admit I am also confused. It is a fairly common practice and has
> proved
> quite effective to install humidifiers in enclosed environments to reduce the
> propensity for static electricity generation. Why does this work?
>
> Jim Hulbert
> Senior Engineer
> Pitney Bowes
>
>
>
>
>
> Paolo Roncone <paolo.ronc...@compuprint.it> on 07/12/2000 04:39:08 AM
>
> Please respond to Paolo Roncone <paolo.ronc...@compuprint.it>
>
> To: "'Rich Nute'" <ri...@sdd.hp.com>
> cc: "'emc-p...@ieee.org'" <emc-p...@ieee.org> (bcc: Jim Hulbert/MSD/US/PBI)
>
> Subject: R: R: Voltage Breakdown
>
>
>
>
> Hi Rich:
>
> thanks for your reply. Now I regret to say that I am a bit confused.
> Based on what you say I don't understand why in dry weather you have more
> chances of high voltage ESD than in humid weather. As I said this is a common
> experience that anyone can confirm.
> I thought the explanation is that dry air has higher dielectric strength so
> higher electrostatic fields need to build up before a discharge.
> Another point suggesting the dependence from humidity is that the ESD standard
> EN 61000-4-2 specifies relative humidity among ambient conditions to control
> during ESD tests.
>
> Regards,
>
> Paolo
>
> -----Messaggio originale-----
> Da: Rich Nute [SMTP:ri...@sdd.hp.com]
> Inviato: marted
--------------------------------------------------------------------------------
ì 11 luglio 2000 19.39
A: paolo.ronc...@compuprint.it
Cc: Product Safety Technical Committee
Oggetto: Re: R: Voltage Breakdown
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
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