FMEDA is a reliability assessment technique that examines potential failure modes within a system and its equipment in order to determine how these failures may affect the safety performance of equipment or subsystems.
What FMEDA aims at
FMEDA consists of two separate analyses, Failure Mode and Effects Analysis (FMEA) and Diagnostic Analysis (DA).
This analysis is used to determine the diagnostic coverage and safety fault fraction (SFF) and must include all components, including electrical, electronic, electromechanical, mechanical, etc., necessary to enable the element to process the safety function(s) required by the safety system.
Each potential failure mode is classified according to its impact on mission success.
All possible dangerous failure modes that lead to an unsafe state, prevent a safe response when such response is required, or otherwise compromise the safety integrity of the safety systems must be considered for each of the components.
Diagnostic coverage is defined as the fraction of dangerous faults detected by automated online diagnostic tests. The fraction of dangerous faults is calculated using the percentage of dangerous faults associated with detected dangerous faults divided by the total percentage of dangerous faults.
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An example of FMEDA
FMEDA is based on the different approaches highlighted in IEC and MIL standards but oriented to provide the necessary information for a SIL reliability assessment, according to clause 7.4.5 and Route 1H of IEC 61508-2.
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Further, the analysis includes the Failure Rate (Estimated), i.e. the resulting failure rate value obtained by apportioning failure mode and effect probability. This value is used to define the resulting failure rates by assessing the effect or the failure rate in relation to the expected safety function for the whole assembly and, consequently, of each device that composes it.