Definition Of Availability - Siemens SIMATIC PCS 7 Function Manual

Basics of Fault Tolerance

2.8 Definition of availability

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
A variety of measures can reduce the repair time:
– Proximity to customer service
– Replacement parts warehousing
– Repairs in runtime or repairs without downtime
With "repairs during ongoing operation", no repair time is needed in the system to correct
unscheduled operation disruptions.
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Fault-tolerant Process Control Systems (V8.1)
Function Manual, 11/2014, A5E34878832-AA