What follows is our experience for SIL4 railway safety application. We are dealing with functional safety. When performing FMEAs, the relevant standard EN50129 (see Appendix C) allows not to consider shorts or decrease of resistance if:
- the resistor is operated well within its nominal dissipation and (over)voltage ratings (i.e. no failure due to overheating or discharge can occur); - the distance between terminals and PCB pads satisfy double insulation clearances/creepages. That is true for thin film resistors. For thick film resistors, there are some studies showing the possibility of shorts due to dendritic growth of Ag or Cu, however this could occur only in very chemically aggressive environment (which is not the case in our applications). So, after deep analysis of more than 30 different studies made by several universities or research centres, at the time we have concluded that we can exclude short circuit failure and even decrease of resistance value also for thick film, for SMD cases larger than 0805 (for smaller cases pads are so close that it would be difficult to exclude external shorts). Regards, Francesco Campedelli Da: Charlie Blackham [mailto:char...@sulisconsultants.com] Inviato: lunedì 17 novembre 2014 10:40 A: EMC-PSTC@LISTSERV.IEEE.ORG Oggetto: [PSES] Surface mount resistors failing short circuit All Have a discussion with an NRTL regarding fault testing on the charge/discharge circuit for a 65mAh 3.1V coin cell battery in product being assessed against 60950-1. The NRTL insist on short-circuiting the single 560R current limiting resistor and I am unable to find any evidence that thick-film or thin-film surface mount resistors can fail short-circuit as they appear to fail open-circuit Now, we could add a 2nd series resistor, but that would require a board change - can SMT resistors fail this way, or are the NRTL being over-enthusiastic (unreasonable) Thanks Charlie Charlie Blackham Sulis Consultants Ltd Tel: +44 (0)7946 624317 LinkedIn: uk.linkedin.com/in/charlieblackham/<http://uk.linkedin.com/in/charlieblackham/> Web: www.sulisconsultants.com<http://www.sulisconsultants.com/> Registered in England and Wales, number 05466247 - ---------------------------------------------------------------- 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<mailto: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)<http://www.ieee-pses.org/list.html> List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org<mailto:sdoug...@ieee.org>> Mike Cantwell <mcantw...@ieee.org<mailto:mcantw...@ieee.org>> For policy questions, send mail to: Jim Bacher <j.bac...@ieee.org<mailto:j.bac...@ieee.org>> David Heald <dhe...@gmail.com<mailto:dhe...@gmail.com>> ________________________________ CONFIDENTIALITY : This e-mail and any attachments are confidential and may be privileged. If you are not a named recipient, please notify the sender immediately and do not disclose the contents to another person, use it for any purpose or store or copy the information in any medium. - ---------------------------------------------------------------- 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>
