> This is my recollection of where 240VA came from and > how it was used.
In a 1966 UL meeting with industry on the requirements in UL 478, the minutes report: "Where high current is available at potentials down to about 2 volts, enough energy is available to melt and splatter metal from neck chains, eyeglass frames, watchbands, bracelets, rings, and other personal metal objects unintentionally out across hot bus or between such bus and ground by operators or servicemen, thereby giving rise to a severe burn hazard. One of the industry representatives reported that his company reduces this hazard in such areas by limiting the apparent power available to 240 volt-amperes and the available energy to 10 Joules." Given that 1 volt-ampere-second is 1 Joule. If 10 Joules is the limit, 240 volt-amperes cannot be available for longer than 0.04 milliseconds! Because a Joule includes time, any source would eventually exceed 10 Joules! To be fair, the final requirement did not include the 10-Joule limit, although it was applied to the energy stored in a capacitor. In IEC TC108, in 2003, the Japanese delegation reported that testing showed that 5 volts, 2 amperes (10 volt-amperes) is generally enough for necklaces to become hot (more than 100 degrees C) if they are slightly tensioned. The 240 VA requirement did not do the job of protecting against a burn due to hot metal. The standard was never tested to determine if the requirement was effective. Rich - ---------------------------------------------------------------- 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 <emc-p...@ieee.org> 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 <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
