Siemens SIMATIC S7-400H - Page 437

Characteristic values of redundant automation systems
A.1 Basic concepts
S7-400H
System Manual, 03/2012, A5E00267695-11
437
The figure below shows the parameters included in the calculation of the MTBF of a system.
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Figure A-2 MTBF
Requirements
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.