FMEDA â Accurate Product Failure Metrics - Exida

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An FMEDA is an extension of the well proven FMEA technique and can be usedon electrical or mechanical products [GOB03, GOB07].FMEA/FMECAA Failure Modes and Effects Analysis, FMEA, is a structured qualitative analysisof a system, subsystem, process, design or function to identify potential failuremodes, their causes and their effects on (system) operation.The concept and practice of performing a FMEA, has been around in some formsince the 1960’s. The practice was first formalized in 1970s with the developmentof US MIL STD 1629/1629A.In early practice its use was limited to select applications and industries wherecost of failure is particularly high. The primary benefits were to qualitativelyevaluate the safety of a system, determine unacceptable failure modes, identifypotential design improvements, plan maintenance activities and help understandsystem operation in the presence of potential faults.The Failure Modes, Effects and Criticality Analysis, FMECA, was introduced toaddress a primary barrier to effective use of the detailed FMEA results by theaddition of a criticality metric. This allowed users of the analysis to quickly focuson the most important failure modes/effects in terms of consequence but still didnot address the likelihood or probability of the failure mode which is just asimportant in prioritization to drive improvements based on cost / benefitcomparisons.FMEDA DevelopmentThe Failure Modes, Effects and Diagnostic Analysis, FMEDA, technique wasdeveloped in the late 1980’s based in part on a paper in the 1984 RAMSSymposium [COL84]. The FMEDA added two additional pieces of information tothe FMEA analysis process. The first piece of information added in an FMEDA isthe quantitative failure data (failure rates and the distribution of failure modes) forall components being analyzed. The second piece of information added to anFMEDA is the ability of the system or subsystem to detect internal failures viaautomatic on-line diagnostics. This is crucial to achieving and maintainingreliability in increasing complex systems and for systems that may not be fullyexercising all functionality under normal circumstances such as a low demandEmergency Shutdown System, ESD System.There is a clear need for a measurement of automatic diagnostic capability. Thiswas recognized in the late 1980’s [AME87]. In that context the principles andbasic methods for the modern FMEDA were first documented in the bookEvaluating Control System Reliability [Gob92]. The actual term FMEDA was firstFMEDA Development Paper, Revision 1.1 February 19, 2007 Page 2 of 8
used in 1994 [MOR94] and after further refinement the methods were publishedin the late 1990’s [GOB98a, GOBL98b, GOB99]. FMEDA techniques have beenfurther refined during the 2000’s primarily during IEC 61508 preparation work.The key changes have been:1. IEC 61508 Failure Mode Definitions – New Definitions2. Functional Failure Modes3. Mechanical Component UsageWith these changes, the FMEDA technique has matured to become morecomplete and useful.FMEDA – IEC 61508The IEC 61508 standard officially recognizes the FMEDA technique and mostIEC 61508 assessment bodies rely upon FMEDA results to verify that sufficientsafety has been achieved for a particular application. Within the field of functionalsafety, standardized failure modes are also defined which also helped to improvethe ease of performing the FMEDA and interpreting its results.The official approval of IEC 61508, Part 2 in 2000 provided documentation onwhat is expected of a FMEDA and how to use the data in the area of functionalsafety. This has lead to significantly increased use of the FMEDA within therelevant industries and a rapid evolution of the methods and tools with therequired component level failure rate and failure mode data.IEC 61508 use of the FMEDA is focused on determination of two safety integritymeasurements; the dangerous undetected failure rate and a metric known as theSafe Failure Fraction, SFF. The SFF represents the percentage of failures thatare not dangerous and are detected. However, additional quantitative resultsimportant to system level modeling can also be easily derived out of the sameFMEDA and this has driven further evolution of the process and enhanced thevalue of its results while remaining faithful to the original intent of the IEC 61508.Draft versions of future updates of the IEC 61508 standard are now working tocapture some of these advancements.IEC 61508 Failure Mode DefinitionsIEC61508 Part 4 (1998) defines a dangerous failure as a failure which “has thepotential to put the safety-related system in a hazardous or fail-to-function state.”The standard also defines a safe failure as a failure which “does not have thepotential to put the safety-related system in a hazardous or fail-to-function state.”IEC61508 Part 2 further explains a “safe” failure as a failure leading to a safeshut-down or having no impact on the safety integrity of the E/E/PE safetyrelatedsystem.FMEDA Development Paper, Revision 1.1 February 19, 2007 Page 3 of 8
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FMEDA â Accurate Product Failure Metrics - Exida