12
Functional Safety Manual
M310/FSM, Rev AA
January 2012
Horizontal Float Switch
SPECIFICATIONSUseful LifeUSEFUL LIFETIME According to Section 7.4.9.5 of IEC 61508-2, a useful lifetime based on experience
should be assumed.
Although a constant failure rate is assumed by the probabilistic estimation method
(see FMEDA report), this only applies provided that the useful lifetime(1) 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 subsystem itself
and its operating conditions.
This assumption of a constant failure rate is based on the bathtub curve. Therefore, it
is obvious that the PFDAVG 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.
Based on general field failure data and manufactures component data, a useful life
period of approximately 10 to 15 years is expected for the Mobrey level switch.
When plant experience indicates a shorter useful lifetime than indicated here, the
number based on plant experience should be used.
(1) Useful lifetime is a reliability engineering term that describes the operational time interval
where the failure rate of a device is relatively constant. It is not a term which covers product
obsolescence, warranty, or other commercial issues.