http://www.ieee-pses.org/symposium http://www.emc2004.org/ -------------------------------------------------- At 09:14 PM 5/27/2004, Eric Meunier wrote:
Across the secondary winding of a DC/DC converter transformer powering an un-earthed SELV load, we have a 75V peak 350KHz square wave at 75VDC input. One side of the winding is connected to the un-earthed SELV ground and the other side is switched by transistors and goes through an inductor to produce an SELV power voltage. We also have a snubber circuit with a capacitor charged at about 90VDC and connected to the same SELV ground. If there is no risk the SELV output would exceed SELV limits in the event the insulation would fail across components, do we still need Basic insulation across the components and between the components and the SELV ground or can we apply Functional insulation ? Hi Eric: It sounds like you are asking whether it is acceptable for parts of a SELV circuit to exceed SELV limits under normal operating conditions. I have encountered this question before and I think there may be some ways to address it, but my first suggestion is that you should avoid this situation if at all possible. Even if you are successful, it will certainly complicate your approval, and it is possible that you will not be successful. Sometimes the circuit with the non-SELV voltages can be classified as a Limited Current Circuit. I think this method is sometimes used for the CCFT backlight supplies in LCD displays. I think that some test labs are willing to perform fault analysis to confirm that the non-SELV voltages never appear on accessible parts under fault conditions, but this is a tedious approach and is subject to dispute over interpretation. I suggest that you re-examine your design to see if there is any way to avoid this messy situation. If you feel that this problem can not be avoided, there are other members of this group who could probably provide more details on what your compliance options are. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) [email protected] http://www.randolph-telecom.com <http://www.randolph-telecom.com/> ------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. IEEE PSES Main Website: http://www.ieee-pses.org/ To post a message send your e-mail to [email protected] Instructions for use of the list server: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
