Characteristic Values Of Redundant Automation Systems; Basic Concepts - Siemens SIMATIC S7-400H System Manual

Characteristic values of redundant automation
systems
This appendix provides a brief introduction to the characteristic values of redundant
automation systems, and shows the practical effects of redundant configurations, based on a
selection of configurations.
You will find an overview of the MTBF of various SIMATIC products in the SIMATIC FAQ at:
http://support.automation.siemens.com
under entry ID 16818490
A.1

Basic concepts

The quantitative assessment of redundant automation systems is usually based on their
reliability and availability parameters. These are described in detail below.
Reliability
Reliability refers to the capability of technical equipment to fulfill its function during its
operating period. This is usually no longer the case if any of its components fails.
So a commonly used measure for reliability is the MTBF (Mean Time Between Failure). This
can be analyzed statistically based on the parameters of running systems, or by calculating
the failure rates of the components used.
Reliability of modules
The reliability of SIMATIC components is extremely high as a consequence of extensive
quality assurance measures in design and production.
Reliability of automation systems
The use of redundant modules considerably prolongs the MTBF of a system. The
combination of integrated high-quality self-tests and error detection mechanisms of the
S7-400H CPUs allows the detection and localization of virtually all errors.
The MTBF of an S7-400H is determined by the MDT (Mean Down Time) of a system unit.
This time is derived in essence from the error detection time plus the time required to repair
or replace defective modules.
In addition to other measures, a CPU provides a self-test function with an adjustable test
cycle time. The default test cycle time is 90 minutes. This time has an influence on the error
detection time. The repair time usually required for a modular system such as the S7-400H is
4 hours.
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