turck 100000703, Руководство по безопасности

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V01.0 | 2019/09
5.2 Assumptions
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Failure rates are constant for 10 years, wear out mechanisms are not included
■
Propagation of failures is not relevant
■
External power supply failure rates are not included
■
All components that are not part of the safety function and cannot influence the safety func-
tion (feedback immune) are excluded.
5.3 FMEDA results
The following safety characteristic are the results of the FMEDA.
λ
SD
λ
SU
λ
DD
λ
DU
No effect SFF DC
0 FIT 0 FIT 316 FIT 31 FIT 526 FIT 91 % 91 %
The stated Safe Failure Fraction (SFF) is for reference only. The complete subsystem will need to
be evaluated to determine the overall SFF.
The failure rates used in this analysis are the basic failure rates from the Siemens standard
SN 29500 based on the average ambient temperature of components of 40 °C.
“No effect” is a failure mode of a component that plays part in implementing the safety func-
tion but is neither a safe nor a dangerous failure. According to IEC 62061, it would be possible
to classify the “No effect” failures as “Safe Undetected” failures. Not doing so represents the
worst-case.
5.4 Examples for using the results
The following sections provide information about the probability of failures in high demand
and low demand mode according to IEC 61508.
5.4.1 Probability of dangerous failure per hour (High Demand mode)
The PFH values are based on a worst-case diagnostic test rate and a reaction time of 50 ms. The
ratio of the diagnostic test rate to the demand rate shall equal or exceed 100.
PFH
3,0945 E-08 1/h
5.4.2 Average probability of dangerous failure on demand (Low Demand mode)
With the FMEDA results and the values specified in the following table the average frequency of
dangerous failure can be calculated exemplarily:
T1 8760 h
MTTR 24 h
PFDavg
1,44 E-04