The Battle of the Failure RatesThursday, May 17, 2012
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Site Specific vs. Product Specific
During a field failure analysis study, it was discovered that the calculated failure rate of a specific device varied by more than 4X depending on installation site. Particularly, it was one specific site with a high failure rate. While any reliability engineer would say of course, the environment is different; stress factors are different. But in this study the environments were almost identical! What is going on here?
An engineer visited the site with the high failure rates, and one member of the maintenance team demonstrated the test process. A DC input module was stimulated with a 115 VAC power plug. As one might expect, every module tested failed, often with a puff of black smoke. There is a lesson to be learned here: Failures are both product specific and site specific.
Recently, during a technical conference in Norway, a speaker was comparing exida FMEDA failure rates with OREDA failure rates. The OREDA numbers were higher. It was stated during the debate that OREDA failure data contains a “majority” of failures due to maintenance faults. Wow, no wonder those failure rates are higher!
FMEDA failure rates are designed to be product specific. Site specific failures need to be modeled separately. Why should a site with a good safety culture and quality maintenance be penalized with an “average” rate that accounts for all the bad practices of others? The problem is getting that portion of the data. This is modeled in the exSILentia software with a Maintenance Capability factor. This influences a number of variables in the calculation including probability of successful repair, probability of successful proof test, etc. This separation clearly allows much better optimization and far superior PFD modeling.