High Availability Theoretical Basics - Schneider Electric

Consider a common misuse of reliability figures:
Calculation Examples
� Last architecture examination (Distributed Architecture with a Redundant CPU
Rack) has proven a real benefit in implementing a CPU Rack redundancy, with
this architecture a sub-sytem would be elevated to a 99.9 % Reliability, and
almost a 100% Availability.
� The common misuse of this Reliability figure would be to make arguments for the
potential benefit on Reliability and Availability for the whole resulting system.
Note: This example has considered the Sub-System Failure Rate provided by the
reliability Software.
Regarding the Serial System built by the Redundant CPU Racks and the STB Islands,
the worksheet above shows a resulting Reliability (over one year) of 84,06%, the
Standalone System Reliability during the same period of time being 81.95%.
In addition, the worksheet shows a resulting Availability (for a 2 hour MTTR) of
99.96%, the Standalone System Availability (for a 2 hour MTTR) being 99.9957%.
As a result, this data suggests that implementing a CPU Rack Redundancy would
have almost no benefit.
Of course, this is an incorrect conclusion, and the example should suggest a simple
rule: always compare comparable items. If we implement a redundancy on the CPU
Control Rack in order to increase process control core Reliability, and to an extent
Availability, we need to then examine and compare the figures only at this level, as
the entire system has not received an increase in redundancy.
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