3.8 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 failure; average amount of time between two successive error
events, repair time excluded
● MTTR = mean time to repair; average amount of time between repair events
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
– Spare parts warehousing
– Repairs in runtime or repairs without downtime
With "repairs in runtime", no repair time is needed in the system to correct unscheduled
operation disruptions.
Basics of high availability
3.8 Definition of availability
High Availability Process Control Systems (V9.0)
30 Function Manual, 05/2017, A5E39221836-AA
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