In message <[email protected]>, dated Tue, 26 Jun 2007, Scott Xe <[email protected]> writes:
>Recently, we have a returned multi-functional audio video product >having such symptom: part of cabinet melted and the pcb around the >power rectifiers area burned out. I tried to locate the possible >causes of catching fire. However, under such circumstances, I have no >idea how to start the investigation. First, is it certain that the overheating came from inside the product? Could the damage have been caused by an external source, such as a decorative candle? You have to examine the circuit diagram (schematic) and, very carefully, the faulty product. Is there any sign of an incorrect component having been fitted? If so, what effect would that have? (It may be incorrect but could not cause severe overheating.) If you do not find an incorrect component, look again at the circuit diagram. Can you see any single fault that could cause overheating? For example, if one diode in a bridge rectifier fails short-circuit, but no other diode fails as a result, what happens? (When the product was originally tested for safety, the consequences of short-circuiting one diode may have been different.) Could there have been one fault that went undetected, followed by a second, unrelated fault several days later? IEC 60065 and other safety standards do not cover two or more *unrelated* faults, because the probability is very low. But very low probability events DO happen. You have to use detective skills to find possible causes. Then you may have to simulate possible faults on a new sample, to see if you can reproduce the severe overheating. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk There are benefits from being irrational - just ask the square root of 2. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
