All, Paschen's Law is the result of two functions in the breakdown of gases. Both are related to the mean-free-path of ionized particles. As particles are ionized, they are then polarized and accelerate from one high voltage electrode to another. This can result the generation of secondary ionization by the collision of a highly accelerated ion into a neighboring particle. As pressure decreases the mean-free-path between any two adjacent air particles increases. Which results of a longer path for ions to accelerate and the energy of the resulting impact on a neighboring particle more easily removes an electron, creating another ion.
Atmospheric pressure is approximately 760 Torr at sea level. In IEC 60664 minimum clearances are normalized for 2000 meters altitude, or about 600 Torr. For altitude higher than 2000 m, using the table in IEC 60664 for altitude correction, you simply adjust the clearance number (interpolation of the table is not prohibited). As Rich states, creepage cannot be less than clearance, so you may need to adjust this as well. This can be applied to any safety standard based on IEC 60664. There are many standards that do not take this into account and it might be prudent to factor this into your creepage and clearance allowances. As an exercise, if you were to take an electrical product to the highest point in Austria (3798 meters), you would need to increase your clearances to 126% of the calculated value of the safety standard. This would surely impact your creepage distances. It is interesting to note, as pressure continues to decrease, the number of available particles in a volume of air becomes fewer and therefore ions will begin to "miss" hitting their neighbors and ionization will once again decrease. This the insulating characteristic of a vacuum. The point at which low pressure begins to raise the voltage breakdown point above the IEC 60664 minimum is somewhere between 0.1 to and 1.0 Torr. This air pressure is not normally found on the surface of the earth is of no concern to IEC 60664. But it is of great interest to manufacturers of orbiting satellite equipment. -doug Douglas E. Powell Advanced Energy Industries, Inc. From: [email protected] [mailto:[email protected]] On Behalf Of Richard Nute Sent: Friday, February 26, 2010 6:40 PM To: Petrie, Craig D Cc: [email protected] Subject: Re: Creepage/Clearance and Altitude Hi Craig: With regard to clearance, air is the insulator. As you go up in altitude, there is less air, so there is less insulation. And lower dielectric strength. So, to get the same amount of air (and the same dielectric strength) between the two conductors, you need more distance (clearance). Creepage is the interface between solid insulation and air insulation. Physically, creepage cannot be less than clearance. Degradation of the solid insulation surface (creepage) is a long-term effect largely due to deposition of foreign matter on the insulation surface. Best regards, Rich On 2/26/2010 04:12, Petrie, Craig D wrote: > Hello, > Can anyone tell me the link between creepage/clearance and altitude? Why > do these values need increased at greater altitudes? > Regards > Craig > *Craig D. Petrie > **Product Safety Engineer > * > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society > emc-pstc discussion list. To post a message to the list, send your > e-mail to <[email protected] <mailto:[email protected]>> > > All emc-pstc postings are archived and searchable on the web at > http://www.ieeecommunities.org/emc-pstc > Graphics (in well-used formats), large files, etc. can be posted to that > URL. > > Website: http://www.ieee-pses.org/ > Instructions: http://listserv.ieee.org/request/user-guide.html > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <[email protected] <mailto:[email protected]>> > Mike Cantwell <[email protected] <mailto:[email protected]>> > > For policy questions, send mail to: > Jim Bacher <[email protected] <mailto:[email protected]>> > David Heald <[email protected] <mailto:[email protected]>> > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> This message, including any attachments, may contain information that is confidential and proprietary information of Advanced Energy Industries, Inc. The dissemination, distribution, use or copying of this message or any of its attachments is strictly prohibited without the express written consent of Advanced Energy Industries, Inc. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
