Hi Doug: I don’t have any history. But I do have data for how air behaves as an insulator and protection methods for creepage distances. The data for air insulation as a function of distance (2000 meters altitude) and field shape was given in IEC 664, first edition, 1980. The data was not carried over to later editions. The upper line is for a homogeneous field, the lower line for an inhomogeneous field. I believe the changes in slope are due to anomalies in the measurement method (they should be straight lines). The lines converge at about 327 volts, the Paschen lower limit for air. (It makes no sense to require clearances for voltages below the Paschen limit!) Creepage distances were researched by Klaus Stimper, “The physical fundamentals of low-voltage insulation co-ordination,” VDE 57, 104 pages. I have this but it is too large a file to send by e-mail. If you are interested in studying this text, I can make this available in Dropbox. Best regards, Rich From: Douglas E Powell <[email protected]> Sent: Tuesday, January 25, 2022 10:27 AM To: [email protected] Subject: [PSES] UL Standards - Old School Spacings All, I'm interested in learning the history behind what I call old school spacings found in UL standards. In recent years there has been considerable progress toward harmonizing UL spacings to international standards, derived from IEC 60664-x. However, still today I encounter many UL standards with the old spacings table (interpolation is not allowed). Circuit Ratings V Minimum Spacings Through air Between parts of opposite polarity, live and non-current carrying parts and live and ground connections mm (in) Over surface Between parts of opposite polarity, live and non-current carrying parts and live and ground connections mm (in) 30 - 50 1.6 (1/16) 1.6 (1/16) 51 - 150 3.2 (1/8) 6.4 (1/4) 151 - 300 6.4 (1/4) 9.5 (3/8) 301 - 660 9.5 (3/8) 12.7 (1/2) 661 - 1000 19.1 (3/4) 19.1 (3/4) I've had more than a few "discussions" with design engineers about tables like this since the numbers seem very arbitrary, and I have to agree. The most frequent trouble I have is trying to explain why an increase of just 1 volt over 300V results in such a large jump in the over surface requirements. Usually I am forced to end with, "it is what it is, and we have to follow the rules". Any thoughts? -Doug Douglas E Powell Laporte, Colorado USA <mailto:[email protected]> [email protected] <https://www.linkedin.com/in/coloradocomplianceguy/> LinkedIn - ---------------------------------------------------------------- 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]>
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