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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum (src_forum@alionscience.com )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Original Message:

Posted by: Destin LeBlanc (Destin.LeBlanc@craneaerospace.com )
Date posted: Fri Dec 12 20:10:30 US/Eastern 2003
Subject: Resistor short circuit failure modes/FMD-97
Message:
In RAC's Failure Mode/Mechanism Distributions 1997 (FMD-97), the failure mode "Shorted" is given for Resistors (I'm specifically looking at "Resistor, Fixed, Wirewound" here). My EEs are telling me this is completely impossible, but every FMEA I've ever seen has considered "Resistor Shorted" as a possible failure mode. Unfortunately, many many web searches on the possible combinations of keywords here have yielded almost no results; I've seen as many people saying shorted resistors are a failure to consider as saying that it is impossible (and I can count them each on one hand). So I'm hoping somebody can shed some more light on this situation. Are the failures listed in FMD-97 only manufacturing defects (ie, not a result of an operational situation)? Is it possible to say for sure that, if you drive too much current through a resistor, it *won't* fail shorted? [I would assume knowledge of the physical construction of resistors would tell us that.] Thanks!


Reply:

Subject: Wirewound Resistor Failure Modes
Reply Posted by: BWD (bdudley@alionscience.com )
Organization: RAC
Date Posted: Wed Dec 17 11:06:36 US/Eastern 2003
Message:
FMD-97 does have 3% short condition. It also has 27% open failure condition. From a failure mode analysis point of view, many wire wound resistors fail due to insulation breakdown. This condition usually results in a short condition that ends in an open circuit or if contamination exists the resistor could “corrode” open. So, the conclusion we have reached is some number of resistors fail in a shorted condition. The FMEA analysis is only looking for effects at higher levels, which is why you must consider the impact of a short condition on the unit even if it is only for a short period of time. If you only consider open conditions you may miss an important side of the analysis and the design. FMD failures are from numerous sources including manufacturing tests, acceptance test, operating tests and field data.


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