Definition Of Availability - Siemens SIMATIC PCS 7 Function Manual

2.8

Definition of availability

Definitions
Availability is usually defined as follows:
Quotient of MTBF and (MTBF + MTTR)
or in short form
actual operating condition / nominal operating condition.
Whereby:
● MTBF = mean time between two successive error events, repair time excluded
● MTTR = mean time to repair
Increasing the basic availability
Based on this definition, the basic availability of a standard component or a standard system
can be increased by the following:
● Reduction of error frequency
● Decreasing the period necessary for repairs
With "repairs during ongoing operation", no repair time is needed in the system to correct
unscheduled operation disruptions.
2.9 Definition of the standby modes
Introduction
The availability of a system can be increased by additional components in the system (standby
components). The operating mode of these components distinguishes them from the
components that are active in process mode.
Fault-tolerant Process Control Systems (V8.0)
Function Manual, 03/2012, A5E02779471-02
A variety of measures can reduce the repair time:
– Proximity to customer service
– Replacement parts warehousing
– Repairs in runtime or repairs without downtime
Basics of Fault Tolerance
2.9 Definition of the standby modes
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