Siemens SIMATIC S7-400H System Manual page 314

Characteristic values of redundant automation systems
A.1 Basic concepts
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
● Corrosion
● Vibration and shock
● Electromagnetic interference
● Electrostatic discharge
● RF interference
● Unexpected sequence of events
● Operator error
The CCF factor defines the ratio between the probability of the occurrence of a CCF and the
probability of the occurrence of any other error.
Typical CCF factors range from 2 % to 0.2 % in a system with identical components, and
between 1 % and 0.1 % in a system containing different components.
Within the range stipulated in IEC 61508, a CCF factor between 0.02 % and 5 % is used to
calculate the MTBF.
Figure A-3
Reliability of an S7-400H
The use of redundant modules prolongs the system MTBF by a very large factor. The
integrated high-grade self-test and the test/message functions of the S7-400H CPUs enable
the detection and localization of virtually all errors. The calculated diagnostic coverage is
around 90 %.
The reliability in stand-alone mode is described by the corresponding failure rate. This
corresponds to the reciprocal value of the MTTF (Mean Time To Failure). The MTTF is
equivalent to the MTBF, assuming an infinite repair time MDT. The failure rate of an S7-400H
is calculated according to the SN29500 standard.
The reliability in redundant mode is described by the corresponding failure rate. This
corresponds to the reciprocal value of the MTTF. Those combinations of failed components
which cause a system failure form the minimum sections. The minimum sections are
described individually by the Markov model.
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Common Cause Failure (CCF)
System Manual, 09/2007, A5E00267695-03
S7-400H