Re: [PSES] Series Isolation

[Douglas Powell]

All, When I mentioned e-fields and ‎non-linear responses I had in mind the example given of series contacts, these being air gaps between metallic electrodes.   While capacitance of these may be measurable it is quite small. In addition the construction is not identically perfect meaning one will have a slightly lower breakdown point than the other. First one set of contacts will break down or arc over and momentarily present a short in the that part of the series circuit. This in turn puts the full voltage across the second isolation boundary. The entire circuit is then likely to cascade into a fault with ionized gases making up the main conductive path.   This is also why two series spark gaps do not behave as one might initially expect.    As for the case of a capacitive voltage divider, with significant capacitance of more than a few picofarads, I think the problem is not the same. If the capacitor values are not well matched, one barrier may break down before the other because the bulk of the voltage will be impressed on the smaller value capacitor. First one capacitive isolation barrier breaks down resulting in permanent damage. Then the full voltage is seen on the other barrier, resulting in a full breakdown. It should be noted that the second barrier may breakdown on the next test impulse.   All the best,   Doug Douglas Powell http://www.linkedin.com/in/dougp01     Original Message   From: Carpentier Kristiaan Sent: Friday, June 12, 2015 3:34 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Reply To: Carpentier Kristiaan Subject: Re: [PSES] Series Isolation Hi Rich and all others,   Thanks for your explanation, make things more clear. I still don’t understand if the smaller capacitor with 2 points breaks down, then a current should flow (I think that’s the definition or result of “breaking down”) but if the larger capacitor with 2 planes still did not break down, then there can be no current flowing. Unless the explanation is like Joe said: 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   Do you have an idea about the question that the breakdown of 2 insulations does not equal the sum of both?   Best regards Kris Carpentier,    From: Richard Nute [mailto:ri...@ieee.org] Sent: vrijdag 12 juni 2015 21:48 To: Carpentier Kristiaan; EMC-PSTC@LISTSERV.IEEE.ORG Subject: RE: [PSES] Series Isolation       Hi Kris:     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.     Think of two capacitors in series.  Even though there is no conductor at the center point of the two capacitors, the voltage divides between the two capacitors inversely proportional to the capacitance.  That is, the smaller voltage is across the larger capacitor and the larger voltage is across the smaller capacitor.   If one of the capacitor’s dielectric is air, most of the voltage will appear across the air capacitor, while a small amount of voltage will appear across the solid dielectric.  This is because the dielectric constant of air is 1, while the dielectric constant of a solid dielectric is more than 1 (assuming the area of the capacitor plates is the same).    Thus, the air dielectric capacitor has more than half of the total voltage and can break down while the solid dielectric capacitor has less than half of the total voltage and does not break down.    This same phenomenon can occur when the capacitance is determined by the area of the plates rather than different dielectric constants.  Consider a worst-case of one capacitor comprised of two points and the other capacitor comprised of two planes.  In this case, the smaller capacitor is the one comprised of two points, and will break down while the other capacitor comprised of two planes will not break down.   Hope this explanation helps.     Best regards, Rich             From: Carpentier Kristiaan [mailto:kristiaan.carpent...@technicolor.com] Sent: Friday, June 12, 2015 8:06 AM To: EMC-PSTC@LISTSERV.IEEE.ORG 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. 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