Joe, It is quite common for local filtering consisting of an inductor followed by a capacitor to be added in the logic-level supply rail following an on-board DC:DC converter when powering sensitive ICs. Often there are many such filters on each card and it is not feasible to fuse each of them individually. If you short, or partially short (low resistance) the output capacitor then you can reach a condition where the DC:DC converter keeps pumping power into the fault and either the inductor fails or the output tracks burn up. [Of course, this is just one example of many where components are placed in series with the supply line.] I must say that my approach has been to consider that provided that the inductor open circuits cleanly (perhaps some smoke, but no fire and no charring of the pcb on which the inductor is mounted) then that is OK safety-wise. Personally, I would not be happy if an inner-layer trace were to burn up for both safety reasons and board re-use reasons. I'm none too keen on surface layers burning up either. A possible safety problem with inner traces opening is that this failure could damage the safety insulation (e.g. by charring or de-lamination) between other traces on that pcb. This of course would vary from board to board and may not have been an issue for Joe's board. Of course, I am also open to flaming but to an extent that's how we all learn. 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? So, having put on my Nomex clothing I await a reply! Richard Hughes
-----Original Message----- 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.
