Characteristic values of redundant automation systems
A.2 Comparison of MTBF for selected configurations
S7-400H
440 System Manual, 03/2012, A5E00267695-11
A.2 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
A.2.1 System configurations with redundant CPU 417-5H
The following system with one CPU (e.g. 417-5H) operating in stand-alone mode forms the
basis for calculation of a reference factor, which defines the multiple of the system MTBF of
other systems with centralized I/Os compared to the base line.
Fault-tolerant CPU in stand-alone mode
Fault-tolerant CPU in stand-alone mode (e.g. CPU 417–5H) Factor
85UDFN
36$
&38+
1
Redundant CPUs in different racks
Redundant CPU 417–5H in divided rack, CCF = 2% Factor
5DFN85+
[ILEHURSWLFFDEOHV
36$
&38+
36$
&38+
approx. 20
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