Dear Rich, Thank you. So in reality no way to avoid abnormal testing, since on transformer we always measure >42,4Vpk.
Best regards, Bostjan From: Richard Nute [mailto:ri...@ieee.org] Sent: Thursday, January 15, 2015 1:52 AM To: Boštjan Glavič; EMC-PSTC@LISTSERV.IEEE.ORG Subject: RE: [PSES] safety distances in DC/DC Dear Boštjan: a) If internal distances input to output comply with functional insulation only, fault condition tests need to be performed on functional insulation (including transformer) in order to be sure that output remains SELV. In addition, transformer needs to pass electric strength test for basic insulation If the results of the fault test are that the output remains SELV, then basic insulation is not required. The test proves that the failure of the functional insulation does not result in a non-SELV output. If the results of the fault test are that the output exceeds SELV, then the insulation (between the 88 volts peak circuit and output) must be comprised of BOTH basic and supplementary insulation. IEC 60950-1 does not contemplate a SELV-to-SELV dc-dc converter that generates greater than SELV in the converter circuits and impressed on the isolation transformer and isolation insulation. That said, the dielectric strength test voltage (for 88 volts peak primary to the secondary) would be taken from Table 5, Part 2. Vb, for double or reinforced insulation, would be 1257 volts rms! a) If internal distances input to output comply with basic insulation, there is no need for fault condition tests. No. The insulation from the 88 volts peak circuits to the secondary (output) must have double or reinforced insulation. Clearances are specified in Table 2M, and would be 1.4 mm for reinforced insulation. Creepage distances are related to the r.m.s. voltage of the primary, 2.10.2.2. The r.m.s. of 88 volts peak is 62.2 or less, depending on waveshape. Creepage distances are specfied in Table 2N and would be 0.1 or 0.16 depending on material. But, a creepage (physically) cannot be less than a clearance, so the clearance requirement of 1.4 mm applies. Note that some authorities would require isolation of the input from the 88 volts peak circuits. A fault test will prove that this cannot happen and that only functional insulation is required. (The law of conservation of energy.) a) Since measured working voltage 88Vpk is above TNV-2 limit it is considered as hazardous secondary voltage and we need reinforced insulation in order to avoid fault condition tests. Yes. But, if fault tests prove that the output remains SELV, then there is no requirement for reinforced insulation. Good luck, and best regards, Rich - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
