Regarding the RAMS analysis of the systems, especially for high-availability ones, it is required the quantitative assessment (calculation) of various parameters relating to the maintenance and repair of the system. These parameters contribute to the estimate of the availability of the system and they are considered in security calculations, when it is required the calculation of the probability of failure on demand (PFDG), or per hour (PFHG), of a group of canli majority-logic.
All the operations of maintenance of a system are grouped into two distinct categories, each of which is characterized by parameters that need to be quantified:
  • Preventive maintenance:
    • MTBM: mean time between maintenance.
    • MTTM: mean time to maintenance.
  • Repair:
    • MTTR: mean time to restore.

The calculation is made using the definitions and mathematical expressions contained in IEC 60050-191, IEC 61703 using the data indicated in the technical documentation of the individual components of the system, or, failing them, by using estimated data available in the literature and in special databases.


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