Appendix 2: Impact Of Lifetime Of Critical Components On The Failure Rate - turck IM35-11Ex-Hi/24VDC Bedienungsanleitung

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
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.
Table 13 shows which components are contributing to the dangerous undetected failure rate
and therefore to the PFD
Table 13: Useful lifetime of components contributing to λ λ λ λ
Type
Capacitor (electrolytic) -
Tantalum electrolytic, solid
electrolyte
As there are no aluminium electrolytic capacitors used the only limiting factor are the Tantalum
electrolytic capacitors with regard to the useful lifetime of the system, which have a useful
lifetime of about 57 years.
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.
exida.com
© GmbH
Stephan Aschenbrenner
more@turck.com • www.turck.com • 2015/04
Anhang – EXIDA-Report FMEDA Turck 04/07-14 R001
calculation is only valid for components which have this
AVG
calculation and what their estimated useful lifetime is.
AVG
Name
C38 (IM33-**(Ex)-Hi/24VDC)
du
Useful life
Appr. 500 000 hours
TURCK 04-07-14 R001 V2R0.doc; February 8, 2013
Page 25 of 25
39