In a message dated 1/9/2004, Richard Hughes writes: > I hope that I have explained why the insulation is for a Primary Circuit > rather than for a Secondary Circuit (as could be expected because a TNV > Circuit is a Secondary Circuit).
Hi Richard: Thanks for your detailed description. As it turns out, I have heard variations of this rationale before. In fact, what we now think of as a "Nordic" requirement used to appear in the UK safety standard BS 6301 back in the 1980s. A fellow at BT offered a similar (but not identical) rationale when I inquired about it at the time. The same requirement was included in the initial "harmonized" telecom safety standard EN 41003, and at least one edition of that standard actually contained an explanatory note that summarized the rationale. However, I think the discussion has drifted from the original question, which had to do with interpreting what the latest requirements in EN 60950-1 call for. Under the latest version of Annex ZB there is no need to debate whether the supplementary insulation should be for a primary circuit or a secondary circuit, because there is no reference whatsoever to supplementary insulation. All that remains is a list of specific requirements on solid insulation. There is only one remaining place in the standard where it is necessary to decide whether the TNV insulation is for a primary circuit or a secondary circuit. That occurs in clause 2.3.2 where there is a reference to basic insulation. In this case the applicable requirements are those for a secondary circuit. As Ron Pickard noted in an earlier posting, the current edition still leaves room for people to have debates (as they have for years) about the assigned working voltage for the basic insulation referenced in clause 2.3.2. I avoided commenting on this earlier because I did not want to open another can of worms. For what it's worth, the conservative interpretation that I have been persuaded to use says the working voltage for a generic, unspecified TNV-3 PSTN line is 120 volts (max TNV) plus the SELV voltage on the other side of the barrier. This yields a range of 120 to 180 volts, depending on the characteristics of the SELV circuit. So, for compliance with the basic insulation requirement in clause 2.3.2, we have a working voltage (subject to debate) and we know that the applicable tables are those for a secondary circuit (not subject to debate, as far as I know). Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) [email protected] http://www.randolph-telecom.com This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
