Iso 13849 Safety Metrics Computation - ST STM32F2 Series User Manual

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A.1.2

ISO 13849 safety metrics computation

Appendix C of ISO 13849 presents tables of standardized MTTFd for the various electric or electronics
components. However, table C.3 in ISO 13849 points to ICs manufacturer's data while attempting to classify
MTTFd for programmable ICs. As a consequence, safety analysis results of this Safety Manual can be re-mapped
in ISO 13849 domain, because even computed for IEC 61508 they are definitely more and more accurate in the
definition of dangerous failures identification.
When for a certain component PFH << 1 we can assume that MTTFd = 1 / PFH [years].
From the reliability theory, MTTF (the inverse of λ and PFH) is a metric applicable only to not reparable systems.
Nowadays it is a common practice to use MTBF also for not reparable systems where MTBF has to be
UM1845 - Rev 4
Figure 7. Block diagram for ISO 13849 Cat. 2
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interconnecting means
input device, e.g. sensor
logic
output device, e.g. main contactor
monitoring
test equipment
output of TE
Figure 8. Block diagram for ISO 13849 Cat. 3 and Cat. 4
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interconnecting means
cross monitoring
input device, e.g. sensor
logic
monitoring
output device, e.g. main contactor
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UM1845
ISO 13849-1 / ISO 13849-2
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