From: boris petkovic
Date: Fri, 3 May 2002 11:15:05 -0700 (PDT)
Subject: Re: [TSSP] Secondary voltage stress factor
Bert wrote: There are a number of studies that imply > surface fields of > 5-10kV/cm are sufficient to cause further leader > growth once a leader > has been initiated. But I believe Boris is also > correct - the point of > initiation will still require a local E-field in the > 26-30 kV/cm range to > trigger initial ionization. E-fields of this > magnitude might occur at > localized defects in the winding insulation, > imbedded particulate > matter, or defects in the surface coating. --- I mentioned 26kV/cm figure only in the context of ideal dielectric surface,where air strata in its vicinity starts to break down by itself (it will do that also in absence of insulator).However,perfect insulator ,or anything perfect in reality, does not exist.A real insultor transcendent surface-atmosphere envelope has some finite conductivity in quasy-uniform fields . I seem to remember reading somewhere the study where authors put very clean excelent insulators having smooth cylindrical walls, between two parallel big plate electrodes.Species were tested by 50 Hz and DC voltage.No matter how they tried hard no insulator shown more than 12kV/cm strenght before flashover occured.I can't be sure but I think they reported somewhat less strenght in DC voltage tests. Humidity and some other influences (appart from possible microscopic increase of local field magnitude due to never perfectly smooth insulator surface)may be reason for only 12Kv/cm limit? regards, Boris __________________________________________________ Do You Yahoo!? Yahoo! Health - your guide to health and wellness http://health.yahoo.com
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