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 [email protected] LinkedIn <https://www.linkedin.com/in/coloradocomplianceguy/> - ---------------------------------------------------------------- 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]>
