Characteristic values of redundant automation systems
A.1 Basic concepts
CPU 410 Process Automation/CPU 410 SMART
System Manual, 05/2017, A5E31622160-AC
381
Figure A-2 MTBF
This analysis assumes the following conditions:
● The failure rate of all components and all calculations is based on an average
temperature of 40 °C.
● The system installation and configuration is free of errors.
● All replacement parts are available locally, in order to prevent extended repair times due
to missing spare parts. This keeps the component MDT down to a minimum.
● The MDT of individual components is 4 h. The system's MDT is calculated based on the
MDT of the individual components plus the system structure.
● The MTBF of the components meets the following standards:
– SN 29500
This standard is compliant with MIL–HDBK 217–F.
– IEC 60050
– IEC 61709
● The calculations are made using the diagnostic coverage of each component.
● A CCF factor between 0.2% and 2% is assumed, depending on the system configuration.
Common Cause Failure (CCF)
The Common Cause Failure (CCF) is an error which is caused by one or more events which
also lead to an error state on two or more separate channels or components in a system. A
CCF leads to a system failure.
The CCF may be caused by one of the following factors:
● Temperature
● Humidity
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