01-02-2013, 12:00 PM
Safety analysis and standards
1Safety analysis.ppt (Size: 849 KB / Downloads: 42)
Failure Mode and Effects Analysis (FMEA)
Analysis method to identify component failures which have significant consequences affecting the system operation in the application considered.® identify faults (component failures) that lead to system failures.
FMEA: Purpose (overall)
There are different reasons why an FMEA can be performed:
Evaluation of effects and sequences of events caused by each identified item failure mode(® get to know the system better)
Determination of the significance or criticality of each failure mode as to the system’s correct function or performance and the impact on the availability and/or safety of the related process(® identify weak spots)
Classification of identified failure modes according to their detectability, diagnosability, testability, item replaceability and operating provisions (tests, repair, maintenance, logistics etc.)(® take the necessary precautions)
Estimation of measures of the significance and probability of failure(® demonstrate level of availability/safety to user or certification agency)
FMEA: Critical decisions
Depending on the exact purpose of the analysis, several decisions have to be made:
For what purpose is it performed (find weak spots « demonstrate safety to certification agency, demonstrate safety « compute availability)
When is the analysis performed (e.g. before « after detailed design)?
What is the system (highest level considered), where are the boundaries to the external world (that is assumed fault-free)?
Which components are analyzed (lowest level considered)?
Which failure modes are considered (electrical, mechanical, hydraulic, design faults, human/operation errors)?
Are secondary and higher-order effects considered (i.e. one fault causing a second fault which then causes a system failure etc.)?
By whom is the analysis performed (designer, who knows system best « third party, which is unbiased and brings in an independent view)?
FMEA and FMECA
FMEA only provides qualitative analysis (cause effect chain).
FMECA (failure mode, effects and criticality analysis) also provides (limited) quantitative information.
each basic failure mode is assigned a failure probability and a failure criticality
if based on the result of the FMECA the system is to be improved (to make it more dependable) the failure modes with the highest probability leading to failures with the highest criticality are considered first.
Coffee machine example:
If the coffee machine is damaged, this is more critical than if the coffee machine is OK and no coffee can be produced temporarily
If the water has to be refilled every 20 cups and the coffee has to be refilled every 2 cups, the failure mode “coffee bean container too full” is more probable than “water tank too full”.