Hi Doug:
I agree that there is a big difference in the breakdown of air for a homogeneous field versus an in-homogeneous field (think parallel plates versus two sharp points). The high electric fields generated around pointy objects induce ionization of air at a lower voltage for a given air gap. However, the way the question was posed, I think this effect should already have been accounted for. The question was why two barriers in series sometimes have a lower breakdown voltage than the sum of the two barriers tested separately. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) <mailto:[email protected]> [email protected] <http://www.randolph-telecom.com> http://www.randolph-telecom.com From: Doug Powell [mailto:[email protected]] Sent: Friday, June 12, 2015 1:12 PM To: [email protected] Subject: Re: [PSES] Series Isolation All, When dealing impulses of a few kV or higher and especially with many high frequency harmonics, it is no longer a simple matter of resistances or capacitance. You now must add into consideration electric fields and field enhancement effects of electrode geometries. Safety committees are aware of this as they sometimes mention the concept of homogeneous or inhomogeneous fields. A common way of doing this sort of testing is partial discharge (PD). Homogeneous field design does provide considerable reduction in clearance requirements but does not appreciably reduce creepage distances. In my experience, companies may avoid this sort of consideration as it does get a bit complicated and often routine PD testing is required on the manufacturing line. In the end, the breakdown mechanism is non-linear, and often the dominant factor is the radii of electrode geometries. Hence, the rounded corner design you often see in high voltage equipment. Thanks, Doug On Fri, Jun 12, 2015 at 10:15 AM, John Woodgate <[email protected] <mailto:[email protected]> > wrote: In message <[email protected] <http://randolph-telecom.com> >, dated Fri, 12 Jun 2015, Joe Randolph <[email protected] <mailto:[email protected]> > writes: 3) I?m not sure how to explain your example of two open relay contacts in series, since the isolation material (air) is presumably the same in both relays. However, when the two materials differ in composition, there can be unequal division of the surge voltage as John Woodgate has explained. The supports for the relay contacts can have different insulation resistances, and maybe capacitance but that would normally be very small. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk <http://www.jmwa.demon.co.uk> When I turn my back on the sun, it's to look for a rainbow John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - ---------------------------------------------------------------- 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.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) 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]> > -- Douglas E Powell [email protected] <mailto:[email protected]> Skype: doug.powell52 http://www.linkedin.com/in/dougp01 - ---------------------------------------------------------------- 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.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) <http://www.ieee-pses.org/list.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.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) 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]>
