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 
- cf.  (PDF file; 2.72 MB), BfS: Methods for Probabilistic Safety Analysis for Nuclear Power Plants, Dec. 1996, BfS-KT-16-97 , Chap. 188.8.131.52. and Appendix C1.1.
- 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).
- VDI 4001, sheet 2 and VDI 4010, sheet 1.
- VDI 4001, sheet 2 and VDI 4010, sheet 1 and IEV 191.