What is reliability

Reliability is the ability of a component or system to perform its required functions, at the specified levels of performance, without failure, in the specified environment and operational loads, for the time required. Reliable products will have less faults within its specified lifetime and thus reduced operational costs. A reliable product will also have improved safety if severe consequences occur upon failures. It is important when discussing reliability to understand the difference between component reliability, system reliability and availability. This page aims to give a practical introduction to the basics of reliability theory.

Component reliability

Component reliability is the reliability of a single component. A component could range from a small part to larger assemblies. The reliability is always related to the component functional outputs. Component reliability can only be improved by changed design, material selection, surface treatments and operational conditions. 

Mathematical models for component reliability will not directly improve the reliability of the component itself, but can be used for aid in the design optimisation to predict the reliability of existing components. For example, what is the probability that a component will survive 10 years in operation. Such quantitative reliability estimates require access to realistic component experience data. The data should include a large number of units operated in a relevant environment and for a long period of time (if long time estimates are required). Most reliability estimation techniques also need a realistic mathematical model that describes the experience data. Typical expressions are:

The lifetime exposure of a component is normally given in years or hours, but could also be on/off cycles (switch), km (car), rotations (shaft) and load cycles (bolt).

System reliability

System reliability is the reliability of a group of several components. System reliability dependents on component reliability and how components are configured as a logical structure. The basic system reliability structures are:

  • Series structure (NooN)
  • Parallel structure (1ooN)
  • K out of N structure (KooN)

Note that KooN refers to a system that is functioning as long as K out of a total of N component parts of a system are functioning. A system is often configured as a combination of the above basic structures. System reliability can be optimised by calculations.


Availability is the probability that the component or system will be in an operational state at a given point of time. Availability is thus a function of reliability, maintainability and maintenance support. Availability is an indicator for component or system dependability and is often used as a measure for performance with regards to production (production availability) and safety (safety availability).

Updated: 07.01.2013

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