Beware: Availability can drop below steady state!
Most system availability analyses relate to steady state i.e. the availability that the system converges to as operation time increases. Standard equations for calculating availability relate to the steady state (e.g. EN 61078).
Point availability is the expected system availability at a specific time point. While steady state is a good indicator for the system behavior, it is important to ask:
Are there cases where the system point availability drops below the steady state availability?
The surprising answer is YES!
This means that a steady state analysis may provide an over optimistic result!
We used BQR’s RBD Monte Carlo simulation software to simulate various systems and identify cases where point availability drops below steady state:
For simplicity a constant repair rate is assumed for all the following examples.
Block with non exponential failure distribution:
A single block with a Weibull failure distribution can have a point availability that drops below the steady state: