Last edit: 18/04/2023
THE DOUBT:
The terms FMEA and FMECA may be common among component manufacturers, however the term FMEDA is only known in case the manufacturer has safety components in its portfolio. What do the different acronym mean and what are the differences?
CONSIDERATIONS:
Failure Modes and Effect Analysis (FMEA) is a systematic procedure for the analysis of a system or of a component to identify the potential failure modes, their causes and effects on system performance (performance of the immediate assembly and the entire system or a process).
FMEA is considered to be a method to identify the severity of potential failure modes and to provide an input to mitigating measures to reduce risk. In some applications however, FMEA also includes an estimation of the probability of occurrence of the failure modes. This enhances the analysis by providing a measure of the failure mode’s likelihood. In this respect, the FMEA is very similar to the FMEDA.
Application of FMEA is preceded by a hierarchical decomposition of the system into its more basic elements. It is useful to employ simple block diagrams to illustrate this decomposition. The analysis then starts with lowest level elements. A failure mode effect at a lower level may then become a failure cause of a failure mode of an item in the next higher level. The analysis proceeds in a bottom-up fashion until the end effect on the system is identified.
FMECA (Failure Modes, Effects and Criticality Analysis), instead, is an extension to the FMEA to include a means of ranking the severity of the failure modes to allow prioritization of countermeasures. This is done by combining the severity measure and frequency of occurrence to produce a metric called criticality.
The Failure Modes, Effects, and Diagnostic Analysis (FMEDA) is used to calculate the product random failures: it is an extension of the classic FMEA procedure. The technique was first developed for electronic devices but it is now used for mechanical and electro-mechanical devices as well. The FMEDA results are the different failure rates used in Functional Safety:
- λSD: Safe Detected failure rate
- λSU: Safe Undetected failure rate
- λDD: Dangerous Detected failure rate
- λDU: Dangerous Undetected failure rate
- λNE: No Effect failure rate
A FMEDA is done by examining each component in a product and, for each one, the effect of a random failure on the product is analysed. Questions asked are: will a failure in a specific resistor cause the product to fail safe, fail dangerous or lose calibration? If the serial communication line from the A/D to the microprocessor gets shorted, how does the product respond? If this spring fractures, does that cause a dangerous or a safe failure? In this way, the failure rate of each component is analysed and the various groups are added.
The end result is therefore a product specific set of failure data that includes failure rates for each failure mode: failure rates that are detected and undetected by diagnostics, Safe Failure Fraction calculations and, often, an explanation on how to use the numbers for safety verification calculations.
A FMEDA is sometimes done by the product manufacturer but, typically, it is done by third parties.
It should be emphasized that a FMEDA provides failure rates, failure modes and diagnostic coverage effectiveness for random hardware failures. It does not include failure rates due to “systematic” causes, including incorrect installation, inadvertent damage, incorrect calibration or any other human error.
CONCLUSIONS:
Of the three methods analysed in this article, FMEDA is the one used to calculate the different Failure rates of components used in Low demand mode Safety Instrumented Systems. Those failure rates are needed if Route 1H, according to IEC 61508-2, is used to assess the reliability level of a SIF.