System-Wide Availability Analyses - Siemens SIMATIC PCS 7 Function Manual

Basics of Fault Tolerance

2.2 System-wide availability analyses

2.2 System-wide availability analyses
Introduction
Availability must be analyzed globally for the system as a whole. Based on the degree of
availability needed, each system level, each system and each component within a level should
be evaluated. It is important to know the importance of each of these for the availability
requirements as well as the ways and means that the required availability will be achieved.
Avoiding repair time
In many industrial processes, it is not enough to simply correct the failure of a component and
then continue the process. The repair has to be made without interruption to the continuing
production process. The repair time can be considerably reduced by keeping replacement
parts in stock on site. The use of fault-tolerant components in the process control system
enables you to correct the cause of the system or component failure in runtime. The function
of the component is retained if no fault occurs in the remaining active (redundant) components
during the time a failed counterpart component is being repaired. That is, the plant continues
operation without disruption.
Avoiding impermissible signal edge transitions
A reserve system with connected backup I/O may not cause an impermissible signal edge
transition when a change occurs in the operating state (power on or off) or operating mode
(master or slave).
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Fault-tolerant Process Control Systems (V8.1)
Function Manual, 11/2014, A5E34878832-AA