In a posting dated 98-02-18, Rich Nute writes: << I am assuming that the capacitors are connected between mains and secondary circuits. (If they are connected between a high-voltage secondary and a low-voltage secondary, then this discussion does not apply.) Such capacitors would be Y capacitors in the IEC scheme, or UL-Recognizied capacitors in the UL scheme. >>
Rich: I am interested in your parenthetical remark about the case where the capacitors are bridging the barrier between two secondary circuits. I have been having trouble getting a definitive answer regarding a special case of this situation. I would be interested to hear your thoughts on this. For telecom applications, EN 60950 requires basic insulation between a TNV-3 circuit (regular phone line) and a SELV circuit, per clause 6.2.1.2. A TNV-3 circuit is, by definition, a secondary circuit (clause 1.2.8.8). My question is, what requirements apply to capacitors which bridge the basic insulation barrier between TNV-3 and SELV? Two interpretations have been presented to me: 1) The capacitors simply have to meet the required dielectric strength test and provide the required creepage/clearance between their terminals. 2) The capacitors must be safety rated Y2 caps. Initially, my interpretation was #1 above. I believe that UL has a similar view. However, I am told that BABT has adopted interpretation #2. Reportedly, they justify this by pointing to EN 60950 clause 1.5.1, which says that "where safety is involved, components shall comply with the requirements for this standard or the safety aspects of the relevant IEC component standard." This leads them to require that the capacitors comply with IEC 384-14. However, my view is that IEC 384-14 is not relevant to this case, because IEC 384-14 is directed at capacitors which bridge the barrier from a *primary* circuit. Note 1 in clause 1.5.1 of EN 60950 states that "an IEC component standard is considered relevant only if the component in question clearly falls within its scope." I do not think that capacitors which bridge two secondary circuits fall within the scope of IEC 384-14. The difference between interpretations 1 and 2 has great significance for designers of phone line interfaces. In many of today's space-constrained products (such as PCMCIA card modems), it is often difficult or impossible to use Y2 caps to bridge the barrier. The use of interpretation 2 effectively eliminates certain design approaches that might otherwise be considered. Can you comment on this issue, and describe your interpretation of what requirements apply? I would also be interested to hear from any other emc- pstc members who may have comments on this subject. Thanks for any insight you can provide. Joe Randolph Randolph Telecom, Inc.
