Paolo, I would suggest while in an air gap humidity does not contribute that much, on an insulating surface the surface resistivity would be lowered, making it dissipative if not conductive. This in effect, removes the ESD build up of potential. The differences between gaps and surfaces are similar to those that are seen in creepage and clearance issues when dealing with insulation of hazardous voltages. The breakdown mechanisms are not the same. In the ESD case we prevent breakdown by providing a discharge path and in the second case we prevent breakdown by improving the insulation.
-doug ================================= Douglas E. Powell Regulatory Compliance Engineer Advanced Energy Industries, Inc. 1625 Sharp Point Dr. Ft. Collins, Co 80525 mailto:doug.pow...@aei.com http:\\www.advanced-energy.com\ ================================= -----Original Message----- From: Paolo Roncone [mailto:paolo.ronc...@compuprint.it] Sent: Wednesday, July 12, 2000 2:39 AM To: 'Rich Nute' Cc: 'emc-p...@ieee.org' 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: martedì 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 ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
