John Wrote: > The carbon track resistance progressively reduces as more degradation occurs.
True, but at that moment the creepage insulation path is already broke down. A leakage current is flowing and "heating up" the surface. It's the end of the process, and soon after a fuse will blow. The time span in which the insulation degrades without a full continuous "burned spots" conductive path is IMHO linear with average or rms current. Another analogy, but don't shoot me if it's not entirely fitting: As a piece of paper is wetting in the rain, the time to become conductive from one edge to the other will depend on the total number of rain drops (RMS or AV */ time), not the rain rate at a given moment (transient). Stimper describes all mechanisms you stipulated, and declares the cited one to be predominantly harmful. Who am I to deny.....a 104 pages long paper... ? Or did I overlook something .... Gert -----Oorspronkelijk bericht----- Van: John Woodgate [mailto:[email protected]] Verzonden: dinsdag 13 mei 2014 17:22 Aan: [email protected] Onderwerp: Re: [PSES] Creepage and RMS In message <[email protected]>, dated Tue, 13 May 2014, "ce-test, qualified testing bv - Gert Gremmen" <[email protected]> writes: >The theory assumes that most damage is done by small local discharges >caused by a discontinuous conductive path (imagine condensation drops >or small wet particles) called scintillations. This is not really an assumption; it's one of at least two mechanisms that are known to occur. Another is the electrolytic decomposition of conducting salts (no necessarily NaCl) on the surface. This can liberate gases in an activated form that attacks the insulating material. >These are micro sparks (visible with the right equipment) that create >local temperatures well above 1000 degrees, Because they are sparks, the peak voltage is relevant, at least initially. >and contribute to a local burning disintegration of the (carbon >containing) base material in areas of about 0.2 mm. Once enough areas >have been affected a full continuous (carbon) leakage path is created >on the surface that may lead to further damage. This is a more or less >statistical process. > >The size and frequency of these scintillations and affected areas seem >dependent of the voltage and total damage is therefore a time >integration of voltage over the creepage path. Not really. The carbon track resistance progressively reduces as more degradation occurs. This is probably a mixture of thermal and electrochemical degradation, thermal dominating when/if the current becomes high enough. The time-integral of the current is relevant but because the resistance decreases with time, that is not proportional to the integrated voltage. > >In my opinion this leads to the conclusion that an average voltage >value should be choosen, Instead of rms, to determine creepage path, >but I may overlook certain aspects. Correct me where I am wrong. I think there is avoidable risk if one ignores peak voltage. > >Stimper states that leakage currents due to fully continuous conductive >paths can be neglected, That's a VERY bold statement. There is a third mechanism that can catch one out if unlucky. This is electromigration of metallic film across insulation, and it can occur at quite low voltages. Silver is particularly prone to electromigration, as I found long ago when trying to explain why some reverse-biased Ge 'power' diodes failed short-circuit. Attacking the glass insulator with metal polish restored diode operation! >and only in the last phase of drying, sufficient discontinuities exist >for scintillation. Alternating humidity conditions such as day/night >rhythms, in relatively cold (need for condensation droplets) designs >are most liable to creepage problems Those effects occur but they are not all of the story. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk Nondum ex silvis sumus John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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