In message <003301c8a3d8$c2eec330$c600a8c0@PC323541548743>, dated Mon, 21 Apr 2008, [email protected] writes:
>Lack of access does not make the product safe. Rather, the mechanisms >that prevent the access must also comprise a scheme of double or >reinforced insulation. Agreed. > >I cannot imagine a membrane keyboard where the membrane meets the >requirements of double or reinforced insulation against mains voltages. >I suppose it is possible. When you consider that 1 mil of polyester film withstands around 7 kV, it's quite practicable to use 3 layers of it without making things too rigid. NOT that I'd encourage it! > >> Tens, probably hundreds, of millions of non-isolated radios >> and TV have >> been manufactured in the past, and the current safety >> standards do not >> prohibit the technique entirely, but in my opinion, anyone going down >> that route is at very high risk. > >Your comment brings back memories of non-isolated radios and TVs from >the '50's. One side of the line was connected to the metal chassis. >The knobs provided the isolation from the chassis to the hand! Well, later on we had plastic pot and switch spindles. And the knobs were large and did provide good insulation - once we eliminated grub-screw retention! >On the "better" units, the chassis was connected to the line through a >capacitor (before the days of Y capacitors). This prompted UL to study >and publish their classic reports on leakage current from radios. In Europe we had 'live chassis' TVs until SMPS technology was introduced. After half-wave rectification was abandoned (to get DC off the mains supply), the chassis was inevitably at half mains voltage to earth. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk Murphy's Law has now been officially re-named The Certainty Principle John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
