Group, Please see my comments below... Richard Hughes
From: [email protected] [mailto:[email protected]] Sent: Thursday, June 26, 2003 9:24 PM To: [email protected] Subject: Re: Dielectric withstand voltage for power supplies I read in !emc-pstc that Alexandru Guidea <[email protected]> wrote (in <[email protected]>) about 'Dielectric withstand voltage for power supplies' on Thu, 26 Jun 2003: > >I would raise the question about how a power supply with only basic >insulation between input and output is suitable for PCs with male D-subs, or >for PC headphones which are not isolated for 1500V rms. Well, now; do you suppose that this point isn't dealt with in the standard? Accessible circuits that have no connection to the protective earth conductor require double or reinforced insulation from the mains supply. But don't take my word for it, please read the standard and reassure yourself. [R_Hughes >] Technically incorrect: it is also possible to have a design where the secondary circuit is floating from earth and where you have a double-wound transformer with a screen connected to protective earth. It's called 'Method 2' and the requirements are in IEC/EN 60950-1 clause 2.2.3.2. In this case, breakdown from primary to secondary is rendered unlikely because breakdown from primary to protective earth is made to be more likely. It is also permissible to construct PCBs or other components this way. Admittedly in practice this is seldom done because by the time you have made the protective screen 'meaty' enough you may as well have provided insulation. [R_Hughes >] Procedurally quite correct: we are talking about safety here and there's no point in standing up in a court of law and saying "well, I never read the standard myself, but I did ask a bunch of folks on the internet and mostly they were of the opinion that..." The reason I thought it appropriate to mention the above was not to 'get one over' on John, but to emphasise the point that it really is necessary to check things out for oneself because relying on someone else to do it for you (especially when you expect them to do it for free) could easily lead one to false conclusions. [R_Hughes >] Dismounting high horse... -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc <http://www.ieeecommunities.org/emc-pstc> This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
