15
57-650 Eclipse
®
SIL Functional Safety Manual
7.5
Report: Lifetime of Critical Components
Although a constant failure rate is assumed by the proba-
bilistic estimation method, 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 therefore 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 (e.g., 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 elec-
tronic components.
Therefore it is obvious that the PFD
AVG
calculation is only
valid for components that have this constant domain and
that the validity of the calculation is limited to the useful
lifetime of each component.
The table below shows which components are contributing
to the dangerous undetected failure rate and therefore to the
PFD
AVG
calculation and what their estimated useful lifetime is.
As there are no aluminum electrolytic capacitors used, the
tantalum electrolytic capacitors are the limiting factors with
regard to the useful lifetime of the system. The tantalum
electrolytic capacitors that are used in the ECLIPSE Enhanced
Model 705 have an estimated useful lifetime of about
50 years. 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 experi-
ences have shown that the useful lifetime often lies within a
range of 8 to 12 years for transmitters.
Useful lifetime of electroytic capacitors contributing to
λλ
du
Type
Useful life at +40° C
Capacitor (electrolytic – Tantalum electrolytic,
solid electrolyte
Approximately 500,000 hours
