My understanding that “clearance” is distance through air (the insulating medium for clearance), not through solid or liquid insulation. Clearance distance is a function of peak voltage and air pressure. The testing of solid insulation is generally done using an impulse withstand voltage test. Clearance is tested using steady-state d.c. or the rms equivalent of an a.c. test voltage.
The tables for clearance and creepage along with the requirements for solid insulation appear to have served the industry well over the past few decades and there seems no compelling reason to scrutinize their origin, unless it can be shown that those requirements are generally inadequate or draconian. I suspect there is a decent safety margin built-in to those numbers, maybe a 2:1 factor. Ralph From: Richard Nute <[email protected]> Sent: Tuesday, June 25, 2024 4:34 PM To: [email protected] Subject: Re: [PSES] dielectric strength question Hi John: My concern is an engineering-based prediction of clearances (air insulation) as a function of voltage. What is the basis for the clearance tables in the standards? I have never seen anything that allows me to independently verify the clearance dimensions as a function of voltage. My assumption was that the volts per unit distance through the insulating medium was an insulator constant. Not true. The volts per unit distance is a variable and depends on the distance. So, how do I generate a table of distance for each voltage? As near as I know, the tables are empirical. Your hypothesis is that the V/d curves are due to non-uniformity of the insulator is sort-of verified by the papers listed by Adam Dixon. However, the incident you describe seems to me to be due to partial discharge. Any V/d non-uniformity area of the insulating medium is a candidate for partial discharge. Air, because its V/d is very much less than a solid insulator V/d, is likely to have sufficient voltage across the void and can lead to a partial breakdown of the solid insulator. In the 1950s, I doubt that we knew much of the theory of partial discharge. Best regards, Rich From: John Woodgate <[email protected] <mailto:[email protected]> > Sent: Tuesday, June 25, 2024 1:26 PM To: [email protected] <mailto:[email protected]> ; [email protected] <mailto:[email protected]> Subject: Re: [PSES] dielectric strength question I feel that those curves support the hypothesis that the variation is due to non-uniformity in the material. I first suggested voids (because I recall a spectacular failure of a line output transformer design in the late 1950s whose HV winding was encapsulated in polythene. Air in the voids ionized and the ions gobbled up the polythene. Attempt to eliminate the voids in viscous molten polythene under vacuum were partly successful, but did not survive the moulding process. Voids are only one possibility; simple variations in density may be sufficient to concentrate the electric field just where it will do the most damage. Can some tests be done on a solid material that has been certified to be highly uniform? What happens with liquids, which should be orders of magnitude more uniform than the average solid? On 2024-06-25 20:35, Richard Nute wrote: Thanks to Adam for all the references. They address very thin solid insulations. But they confirm that dielectric strength is not a constant for very small distances, and they do not have an answer as to why. My concern is verifying clearances in safety standards. I’ve attached curves of three standards clearance requirements (logarithmic scale for volts per millimeter). The solid curves represent the clearances in standards and are close to power curves (dotted lines). The equations are for a best-fit power curve. The solid green curve is from an old standard and depicts actual withstand measurements. I suspect the electric strength curves are related to the reason for Paschen’s finding that gases do not break down at low voltages. My objective is to predict clearance dimensions without tables. Best regards, Rich _____ This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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