John's analysis is accurate but may be insufficient in some cases: with ac or impulsive voltage stress the capacitance across each isolation needs to be considered as well as the resistance. Because of these capacities the voltage may divide between the two isolations in different ways at different frequencies.
This used to be of great importance in relation to the failure mechanism for paper-insulated high voltage cables. Richard Richard Marshall Laboratories, 30 Ox Lane, Harpenden, Herts., AL5 4HE, UK +44 (0)1582 460815 www.design-emc.co.uk Member of the EMC Industry Association -----Original Message----- From: John Woodgate [mailto:[email protected]] Sent: 12 June 2015 16:26 To: [email protected] Subject: Re: [PSES] Series Isolation In message <by2pr02mb16405b7796dd23601618b90f8a...@by2pr02mb1640.namprd02.prod.outlo ok.com>, dated Fri, 12 Jun 2015, Carpentier Kristiaan <[email protected]> writes: >Does anyone know of literature that explains this phenomenon? It's a very old issue, so modern textbooks may not address it. > >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. The resistance of an 'insulation' is very high but not infinite, so when you have two insulations in series, the voltage divides according to the resistances. The insulation with higher resistance gets the greater volts, and that may well exceed its breakdown voltage without the total voltage being equal to or greater than the sum of the two breakdown voltages. If possible, connect equal, very high value high-voltage resistors across the insulations so as to equalize the voltages. The resistance value clearly needs to be lower than the lower of the two insulation resistances, but with modern insulations, you may be able to use 50 Mohms or more. -- OOO - Own Opinions Only. With best wishes. See 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]> 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]> - ---------------------------------------------------------------- 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]>
