Hi Kristiaan:
I have heard of this phenomenon and discussed various theories with other members of the PSES. Following are some thoughts: 1) If there is significant capacitance across one or both of the isolation barriers, the total surge voltage will divide in inverse proportion to this capacitance. For example, if Barrier 1 has a capacitance of 1000 pF and Barrier 2 has a capacitance of 9000 pF, 90% of the surge voltage will appear across Barrier 1. In theory, a cascade situation could arise where Barrier 1 breaks down first, followed by Barrier 2. 2) In the example you used of an AC mains barrier in series with an Ethernet barrier, the AC mains supply typically has far more capacitance across its barrier than the Ethernet barrier. Note that in the case of Ethernet, the Ethernet transceiver can be damaged even if the AC mains barrier does not suffer catastrophic breakdown. Even the small amount of current that couples through the capacitance of the AC mains barrier can be sufficient to damage some Ethernet port designs. I discussed the Ethernet problem in an IEEE paper that is posted on my web site, and was also published in the May 2015 issue of InCompliance Magazine. 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. 4) A particularly interesting situation was described by Rich Nute in the May 2014 issue of InCompliance Magazine. That was the case where one barrier was solid insulation and the other barrier was air. I believe that for that particular scenario, the failure would only become evident during a sustained hipot test, as opposed to a 1.2/50 uS lightning surge. In any event, Rich Nute is one of our resident experts on the failure of insulation, so hopefully he will chime in here. 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: Carpentier Kristiaan [mailto:[email protected]] Sent: Friday, June 12, 2015 11:06 AM To: [email protected] Subject: [PSES] Series Isolation Hi group, When 2 isolation components are put in series, my experience is that the total isolation breakdown voltage measured with an impulse voltage (ex. 1.2/50us) is always smaller than the sum of both. An example could be the isolation between 2 transformers, for example between the AC mains (transformer AC-SELV) and an Ethernet transformer (SELV-ETH). Another example could be between 2 open relay contact, with each contact 2.5kV but the sum "only" 3.5kV. Does anyone know of literature that explains this phenomenon? I'm also interested to understand the process of the breakdown of 2 isolations in series, like how can 1 insulation break down when the other did not yet. Thanks & best regards Kris Carpentier, - ---------------------------------------------------------------- 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]>
