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LETTERS TO THE EDITORCreepage and Clearance Clause To the Editor: In the article "Calculating Creepage and Clearance Early Avoids Design Problems Later" by Homi Ahmadi (Compliance Engineering 18, no. 2 (2001): 54–61), the author forgot to reference Clause 2.10.5.3 (CAN/CSA-C22.2 No. 60950-00) on printed circuit boards. The distance between two adjacent tracks (inner layers) is treated as distance through insulation. This is important when planning the interlayer tracks on a multilayer board. John
Reynolds Ahmadi replies: This gentleman is correct about Clause 2.10.5.3, and I appreciate his feedback. However, one has to remember that almost all technical articles can become too long and bore the reader. In my article, I tried to give a brief overview and provide some practical information. We can show and explain every single possible situation, but where do we draw the line? There is enough information about creepage and clearance to fill a book. Perhaps in the future, I can try to cover more of the topic. Homi
Ahmadi Y Capacitors in Hipot Testing To the Editor: In the article "Understanding the How and Why of Electrical Product Safety Testing" (Compliance Engineering Annual Reference Guide 18, no. 3 (2001): 192–197), the author Homi Ahmadi refers to UL allowing the disconnection of Y capacitors prior to ac hipot testing. Can the author refer me to the UL document that authenticates this? Ed
Rodriguez Ahmadi replies: Some internal test procedures and even some test houses allow Y capacitors to be removed prior to hipot tests. Normally, this is allowed during the type test and not during the production-line test. One of the reasons engineers give for this is that Y capacitors are already tested and approved for high voltages and therefore do not need to be tested again. Also, the addition of Y capacitors would increase the leakage current, which in turn may set the hipot tester to trip. The removal of Y capacitors prior to hipot testing is not clearly stated in the standard. However, UL 1950, Clause 5.3.2, Note 2 states, "where there are capacitors across the insulation under test...it is recommended that DC test voltages are used." In addition, the last paragraph of UL 1950, Clause 5.3.2 states, "For equipment incorporating both reinforced insulation and lower-grade insulation, care is taken that the voltage applied to the reinforced insulation does not overstress basic or supplementary insulation." Normally, for production-line testing, the hipot test is conducted between the primary and the chassis, and therefore, only basic insulation is tested. The test voltage for basic insulation is generally around 1500 V ac. There are many reasons why a product fails the hipot test. One of the most common failures is the lack of adequate creepage or clearance distances, or insufficient distance through insulation. When hipot testing a product for reinforced insulation, it is important to either use a dc test voltage or remove any decoupling capacitors used between the secondary return and the chassis. Homi Ahmadi Medical Standards News To the Editor: I am writing to make you aware of some inaccuracies in "Stricter Requirements in Revised Standard for Medical EMC" (in Newsline, Compliance Engineering 18, no. 8 (2001): 24–25). In general, the article makes overstatements or misstatements regarding the application of EN 60601-1-2 that could cause those manufacturers who depend only on test houses to take unnecessary and costly action. Although I did not participate in the working group within IEC/SC 62A that wrote the article, I am involved in the subcommittee and others under Technical Committee 62. After a quick review of the article, I found the following inaccuracies. The list of conditions pertaining to performance, while accurate, fails to mention that they (and all requirements pertaining to performance) apply exclusively to "essential performance" (those aspects of the equipment necessary to maintain residual risk within acceptable limits). This means that all immunity requirements apply exclusively to safety-related performance—a critical omission. The article fails to mention the new emissions requirements that could be important to some manufacturers (magnetic fields, extended bandwidth for testing, and harmonic distortion common to switch-mode power supplies). The stated publication date of the European Norm is in conflict with the European Committee for Electrotechnical Standardization's (CENELEC's) published schedule, which calls for August 2002, not February. Please check CENELEC's Web site at http://www.cenelec.org under the EN 60601-1 project. I am uncertain whom the author is using as the authority that set the 10-month deadline for application of the IEC 60601-1-2 standard because there is no such authority in the IEC. These standards are voluntary. I'm sure you would agree that no one is served by misleading people regarding the use of standards, especially when they can play a crucial role in demonstrating regulatory compliance (even though their use for this purpose is not mandatory). The IEC 60601-1 series of standards is, however, a valuable tool for manufacturers in ensuring the safety of patients and users when applied as intended. I am certain that the author did not intend to mislead your readers and hope to see clarification provided in the near future. Mike
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