On 7/20/2007, John Woodgate wrote:
That's the point: if they type is changed to metal film or even wirewound it won't open quickly, there WILL be heating, and maybe a great deal of it. I have experienced this very thing first-hand. Many years ago (more than I care to admit) when I worked at Bell Labs, we were developing a new telephone product. We carefully selected a specific fusible, flameproof resistor for use in the ring detection circuit because we knew that resistor would get overstressed in the "overvoltage" test that is now part of UL 60950 (but not EN 60950). That test involves placing up to 600 VRMS of 50/60 Hz on the phone line, with various current limiting impedances selected to induce maximum heating. We got back the latest revision of our circuit board and a technician quickly assembled five units for internal testing. The special fusible resistor was not in stock so he substituted a common carbon composition resistor. At the time we were just doing functional tests and did not plan to subject those units to overvoltage testing. As luck would have it, one of those five prototypes did find its way into the overvoltage testing, and the results were interesting. When one of the long-term overvoltage conditions was applied, there was a brief current surge and a puff of smoke, and then the current went to near-zero and all was calm. Since this was a 30 minute test, the safety engineer figured it was okay to go get a cup of coffee. When he returned, he found smoke pouring out the door to the lab. Inside, he found the entire telephone completely engulfed in open flames. This rather spectacular failure was because a common carbon composition resistor had been substituted for a fusible flameproof part. The behavior of a resistor under overstress conditions depends heavily on how the resistor is constructed. In general, carbon film and metal film resistors will fail with little heat, because the film coating is thin and it just burns away. Incidentally, these resistors are typically also the least surge tolerant, for the same reason. On the other hand, a carbon composition resistor (the type that caused the above referenced fire) has a solid core of carbon material. Under severe overheating conditions, this type of resistor tends to crack in the middle, but the two ends of the carbon composition resistive element then begin to function much like the carbon rods that are used for arc welding. A sustained arc forms in the gap and the amount of heat generated is extraordinary. On a related note, when AT&T first started self-imposing the overvoltage test, a lot of work was done to develop a suitable surge tolerant fusible element to go in series with the phone line. The initial solution that was used for several years was in fact a carbon composition resistor, but it was installed with a spring attached to one end that would physically separate the two halves of the resistor when it cracked, so that arcing would be prevented. We called it the "snapper resistor." This rather odd solution was used for several years until suitable surge tolerant conventional fuses were developed. In summary, I agree with others who have suggested that if you are depending on a resistor to fail in a certain way when overheated, you should only use a part that is specifically designed and specified to have the desired behavior. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) [email protected] http://www.randolph-telecom.com <http://www.randolph-telecom.com/> ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________ - ---------------------------------------------------------------- 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
