Appendix 2: Impact Of Lifetime Of Critical Components On The Failure Rate - turck IM1-12-R Handbuch

Anhang – EXIDA-Report FMEDA Turck 04/07-14 R002

Appendix 2: Impact of lifetime of critical components on the failure rate

Although a constant failure rate is assumed by the probabilistic estimation method (see section
4.2.3) this only applies provided that the useful lifetime of components is not exceeded. Beyond
their useful lifetime, the result of the probabilistic calculation method is meaningless, as the
probability of failure significantly increases with time. The useful lifetime is highly dependent on
the component itself and its operating conditions – temperature in particular (for example,
electrolyte capacitors can be very sensitive).
This assumption of a constant failure rate is based on the bathtub curve, which shows the
typical behavior for electronic components.
Therefore it is obvious that the PFD calculation is only valid for components which have this
AVG
constant domain and that the validity of the calculation is limited to the useful lifetime of each
component.
It is assumed that early failures are detected to a huge percentage during the installation period
and therefore the assumption of a constant failure rate during the useful lifetime is valid.
The circuits of the Isolating Switching Amplifiers IM1-**(Ex)-* and MK13-R-Ex0 do not contain
any electrolytic capacitors or other components that are contributing to the dangerous
undetected failure rate. Therefore there is no limiting factor with regard to the useful lifetime of
the system.
However, according to section 7.4.7.4 of IEC 61508-2, a useful lifetime, based on experience,
should be assumed. According to section 7.4.7.4 note 3 of IEC 61508-2 experience has shown
that the useful lifetime often lies within a range of 8 to 12 years.
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