The availability of a system measures the ability of the system to be able to perform a required function under given conditions at a given instant, or during a given time interval, assuming that the external resources required are provided.
In most cases the safety of a system is not directly dependent on its availability, however, safety and availability are interdependent in the sense that a failure of one or the other or the bad management of conflicts between the security and availability requirements can hinder the achievement of a reliable system. For this reason often you are asked to calculate the expected availability of the systems, related with safety.
The calculations are carried out in accordance with the definitions and mathematical expressions given in IEC 61703, and, basically, when you calculate the availability, three different indicators are considered:
• Technical availability: This index measures the availability resulting from failure events and the system's ability to perceive such a state, to report it, to face it and solve it; This means that the system takes into account both the repair time and the relevant times for the logistics (awaiting maintenance, waiting for spare parts, testing, etc.).
• Operational Availability: This index takes into account all the "physical" factors that contribute to the unavailability, either induced by the maintenance in its various forms, or induced by the process / project specific characteristics, such as the time required for the "setup".
• Intrinsic Availability: This index does not take into account the logistics of maintenance, but only just the technical time of system repair.
The calculation can be done to determine the instantaneous , average and asymptotic availability