not available

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The non-availability is a reliability parameter and is used in the reliability model - for example the fault tree - for so-called stand-by components . The stand-by components include: B. the emergency diesel in the event of a power failure , the fire alarm and the fire pump in the event of a fire. When required, the component should come into operation, which is then either available or not available.

Examples of the unavailability sizes of technical components according to:

  • Automatic fire detector: up to
  • Stationary fire pump: up to

The unavailability is the complement to the availability . Both quantities are defined as qualitative as well as quantitative properties of a component:

Unavailability : "Ability or probability to find a unit in a non-functional state at a given point in time."

The combination of the probability quantities of availability (v) and unavailability (n) results from the following equation:


  • VDI 4001, Part 2, Terminology of Reliability, July 2006
  • VDI 4010, sheet 1, overview of reliability data systems (ZDS), March 1997
  • IEV 191-02-05, German online edition of the IEV [2]

Individual evidence

  1. cf. [1] (PDF file; 2.72 MB), BfS: Methods for Probabilistic Safety Analysis for Nuclear Power Plants, Dec. 1996, BfS-KT-16-97 , Chap. and Appendix C1.1.
  2. a b c data for the quantification of event flow charts and fault trees, March 1997, BfS-KT-18/97, chap. 2.1.4 and 6.3. (Unavailabilities for fire protection equipment).
  3. VDI 4001, sheet 2 and VDI 4010, sheet 1.
  4. VDI 4001, sheet 2 and VDI 4010, sheet 1 and IEV 191.