All, I agree with the concept that the short circuit is not always worst case. I have seen many ITE power supplies shut down with a sc, but an output overload lets the smoke out.
One problem with a trace opening is the reliability of that opening in a safe manner. When a certified fuse blows, we can generally trust it will blow in the same manner under a sc or overload in the field, due to the tight manufacturing constraints and agency testing on certified fuses. When a trace burns out, do we know how reliable it is? Repeated tests to verify the reliability may be in order. Whether the standard requires it (I haven't researched it), I would consider it due diligence. Sam Davis Regulatory Engineer Professional Testing Inc. (512)244-3371 x112 www.ptitest.com -----Original Message----- All - In consideration of the proliferation of SMPS in electronic equipment, it is not unrealistic to expect a simple short-circuit might not meet the intent of GR-1089. While many linear supplies will run indefinitely under sc, most SMPS will go into hysteresis or shut down completely under a solid sc. A reasonable test condition is just below the current limit of the SMPS output. While I also dislike a PWB trace opening, we are considering a fault condition and the compliance criteria are related to failing in a safe manner. As well, once a trace opens, it is most likely the assembly/subassembly will be discarded, rather than reused, since it will not function correctly and not be considerable repairable. Regards, Peter L. Tarver, PE Product Safety Manager Sanmina-SCI Homologation Services San Jose, CA peter.tar...@sanmina-sci.com Richard Hughes wrote: Perhaps some will take issue with me in applying a partial short in addition to a dead short. In my defence I will say that I work for a manufacturer and not a test lab and the reality is that failed capacitors are unlikely to have a zero ohm impedance when they fail in real life. The question then becomes one of how far should one go to minimise product liability? Richard Hughes In a message dated 11/27/2002, Joe Randolph writes: The only remaining gray area is whether the failure of a component or circuit trace is acceptable *provided* that it is located on the same circuit card where the short was induced, *and* that no safety hazard resulted. My interpretation is that this would be acceptable. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
