Within the output port for a reliability element there are three outputs provided in a folder named Results:
Note that if the element is being modeled as a system with child elements, these outputs are contained within a “Properties” folder when viewing the output port for the element:
Current_Operational Availability: This is the fraction of time the element was operating over a specified time period immediately previous to the current time. The specified time period is approximately 2% of the simulation duration.
Current_Inherent Availability: This is the fraction of time the element was operable over a specified time period immediately previous to the current time. A component that is turned off or has unmet external requirements, but has not failed would be considered operable. Hence, the Inherent Availability is always greater than or equal to the Operational Availability. The specified time period is approximately 2% of the simulation duration.
Reliability: This value is 1 if the element has not failed, and 0 if it has failed. Unlike the availability results, no averaging period is required. At any point in time, the element has either failed or has not failed.
Note: These two availability results
represent current (or, conceptually, instantaneous) values. They are
different from and should not be confused with the availability results that are
presented on the Results tab of Reliability elements, which represent the
availability over the entire duration of the simulation.
When viewing the current availability results, you must interpret them with caution. This is because they represent the "instantaneous" value of the statistic at any given point in time (as opposed to the cumulative value of the statistic). It is computed by dividing time into "slices" or "windows" for a time period just previous to the plot point (with a length of approximately 2% of the duration). Therefore, the plot is most useful if a large number of Monte Carlo realizations have been run, and you are viewing probability histories. Viewing single realizations is of limited value.
Similarly, if you were to plot the distribution of one of the availability outputs and note the mean, you would see that the result would most likely be different from the result shown in the Summary section of the Reliability element’s Result tab. This is because the distribution results would be based on an instantaneous metric (the availability for the last 2% of the simulation duration), while the Summary results are based on a cumulative metric (the availability over the entire simulation).
These considerations do not apply for the Reliability output, as no averaging period is required: at any point in time, the element has either failed or has not failed.
Learn more about: