In a message dated 7/1/99, George Sparacino writes: > I have a modem product that failed the hipot requirement defined in > cls 6 of 950. My understanding is that an isolation xfmr between Telco & > SELV is needed and clearance from Telco & gnd is to be 3mm minimum. Does > anyone have any other general design guidelines that they can share on this > topic ?
George: The isolation requirements that apply to your modem will differ based upon several variables, including the types of cables attached, how the SELV circuits are grounded, and where the modem is installed. However, in the vast majority of cases (such as a typical box modem) IEC 950 requires "basic" insulation between the TNV3 circuits and the SELV circuits, per clause 6.2.1.2. This translates to 1.0 mm clearance, 1.6 mm creepage, and 1000 VRMS hipot. In my experience, a circuit that provides the required creepage and clearance distances will easily pass the hipot test. You referenced the "950" spec, but did not mention whether you are using UL 1950 or EN 60950. If you are using the second edition of UL 1950, there are some provisions that allow you waive the creepage and clearance requirements and use a simple 1000 VRMS hipot test. The second edition will be withdrawn in April 2000 and replaced with the third edition, which does not allow this option. If your target market includes Europe, you should be aware that Norway and Sweden have taken exception to the requirement for basic insulation that appears in EN 60950, and have inserted national deviations that require supplementary insulation. This requires 2.0 mm clearance, 2.5 mm creepage, and 1500 VRMS hipot. For supplementary insulation, there is an additional requirement for at least 0.4 mm distance through any solid insulation that is used in the barrier. Basic insulation does not have this requirement. If you are looking for a manufacturer's app note on this subject, you might check Midcom at http://www.midcom-inc.com. I seem to recall that they have such an app note. Also, there was a lengthy discussion on this topic about a year ago on the treg listserver. You can search the archives of postings at http://www.rcic.com. Lastly, if you are still confused about this subject, give me a call. If you can describe the specifics of your application, I can quickly identify the applicable clauses in IEC 950 and walk you through the calculation of the requirements for your specific application. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (voice) 781-721-0582 (fax) --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
