Characteristic values of redundant automation systems
A.2 Comparison of MTBF for selected configurations
CPU 410-5H Process Automation/CPU 410 SMART
System Manual, 10/2013, A5E32631667-AA
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The availability of a system is expressed as a percentage. It is defined by the mean time
between failure (MTBF) and the mean time to repair MTTR (MDT). The availability of a two-
channel (1-out-of-2) fault-tolerant system can be calculated using the following formula:
Figure A-4 Availability
Comparison of MTBF for selected configurations
The following sections compare systems with a centralized and distributed I/Os.
The following framework conditions are set for the calculation.
● MDT (Mean Down Time) 4 hours
● Ambient temperature 40 degrees
● Buffer voltage is safeguarded
System configurations with redundant CPU 410-5H
The following system with one CPU (e.g., CPU 410-5H PN/DP) in stand-alone operation
serves as the basis for calculating a reference factor that defines the multiple of the system
MTBF of other systems with centralized I/O compared with the base line.
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