Reliability definition
The most current reliability definition is, the (probability of) capability of an entity to perform a required function, in a defined environment and during a defined period of time.
The goal objective
The goal objective in the design of a system, is that it will be able to perform the functions for which it has been designed, without thinking about the duration of its use.
However, the electrical/electronic systems are designed to based on components which have a limited life. It is therefore important to predict (statistically) the behavior of each of these components to obtain:
- A better antecipation of maintenance phases,
- A better control of the safety of this system,
And so, to optimize the best costs in the life cycle of the system.
It’s possible to speak about reliability prediction, the reliability of a system established before the use phase of the system, and of operational reliability, the reliability actually obtained during its operation.
The aim is, of course, to have a reliability prediction as close as possible of the operational reliability.
Methods to define the reliability of a electrical/electronic system
Define the system reliability means calculate the period of operation of each component until the moment of which the system will no longer fulfill its function. This duration is called MTTF (Mean Time To Failure).
To Define this period, it’ would be necessary to use suficient informations about the components of the system based on the Return Of Experience (ROE). This Return Of Experience may be created based on:
- A sufficient period of exploration of the behavior of components,
- Environmental conditions (Climate, vibration, temperature, EMC),
- The sufficient solicitation of these components.
In order to make this return of experience exploitable by the designers of electrical/electronic systems, the compilations were created in the 1960-70, in order to collect all the return of experience in the form of behavioral models for each of the components.
The first collection of reliability that have been created was the “American Military Handbook 217 » (MIL-HDBK-217). This first compilation has been established in 1962 by the American army, and the latest version of this collection dates from 1991. The first french compendium of reliability that have been created was the “Compendium of Reliability” (RDF 70). The compendium has been prepared from a return of experience of France Télécom, and now, this collection is a standard called UTE C 80-810. From these two collections the RAMS guide was born, which incorporates the models of the collection HDBK-217 and RDF 2000, and is enriched by the return of experience of a consortium of French manufacturers.
Thus, all of these behavioral models, based on the return of experience, has been created to facilitate the statistical calculation of reliability of a designed system with a basis on electrical components, but must evolve continuously to be able to integrate always more return of experience and components.