On 4/16/2008, Robert Keller wrote:
How would you evaluate a TNV-3 circuit ,as defined in UL 60950-1, if it is "ground start"? I am running into problems with separation between TNV-3 and ground. Hi Rob: For ground start you have to use what I call the "permanent ground exemption" clauses in UL 60950. This typically requires a separate grounding lug on the equipment that is permanently connected to a reliable earth ground. You will find this exemption described fairly well in clauses 2.3.2 and 6.1.2.2, but less clearly in clause 6.2.1. All three clauses apply to a TNV-3 circuit. Also note that the integrity of the permanent ground must be evaluated per clause 2.6. The basic idea behind the permanent ground exemption is that if the SELV circuits are reliably grounded and the ground connection can handle the anticipated fault currents, the requirement for an isolation barrier can be waived. 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. 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
