In certain conditions, there are techniques for solving these problems. Detection is so an assessment of the capability of the design review to detect a potential cause or mechanism or design weakness. In repairable systems where the system is unavailable for the time necessary to effect repairs or maintenance, availability implies that the system will not be functioning for a given time interval. The functioning of apparatus must be guaranteed in the intermediate area defined by the normal climatic limits see Fig. The above considerations are the basis for implementing screening tests, whose goal is to demonstrate the percentage of components that are intrinsically weak, that is the elements which fall into the area of premature failures of the bathtub curve, as discussed in Chapter 2. This probability is depicted in figure 2. To this aim the system is assumed as a combination of elementary devices subsystems and elements - that follow a well-defined functional structure.
Step 11 Recommendations Recommendations are made reporting useful observations to clarify for example, eventual aspects not completely analyzed, unusual conditions, the effects of fail-ures of redundant elements, critical aspects of the project, references to other data for analysis of sequential failures and all observations upon completing the analy-sis. Failure mode Failure mode Definition Remote Low Moderate Description Failure is unlikely Relatively few failures Occasional failures Frequency 0. In order to define dependability, it is necessary to clarify first the concepts of service and user. From the analysis of Eq. Parameter D should be now evaluated. Such terms are defined in this Chapter.
As stated previously, it allows evaluating the probability of finding the system in a determined state depending only on the probability of exiting from the preceding state and not on the history of the system. To calculate the reliability of this configuration we use binomial distribution hypothesizing that the generic element of the system can assume only two condi- tions: correct functioning or in failure. On the basis of this concept, it is evident that dependability gives a general description of the item - a compo- nent, an equipment, a system, and so on- in non-quantitative terms. Considering the example of a system with two independent elements connected in parallel configuration and assuming a failure rate constant equal to λ1 and λ2, from Eq. Here a brief overview of this law can be very useful. Intermittent failure, characterized by a succession, generally casual, of periods of normal operations with periods of failure breakdowns, without any maintenance operations carried out on the device. It can also verify that the solutions adopted are efficient.
The axiomatic definition is the foundation of the mathematical theory of probabili- ty. A common element which characterizes these databases is the hypothesis of components with a constant fail-ure rate. As such, it is a method used in the analysis of the reliability and safety of a device and furnishes the best results if used from the initial stages of its design. The problem with the statistical analysis adopted by this handbook is that the data comes from non-homogeneous populations. This classification is carried out taking into account various aspects: the nature of the system under examination, performance and functional characteristics of the system, contractual and legal requirements, especially in regard to operator safety, and finally, guarantee requirements.
Obviously, this increase is always positive with the increase in the number of elements in active redundancy, but, as it was logical to expect, always smaller and however, not such as to justify cost in design review. Maintainability is the probability M t that a malfunctioning system can be restored to its correct functioning within time t. In this paragraph, we are trying to give, as much as possible, an exhaustive vision of the essential terminology. Then, given the possibility of redundancy, identify the configurations which permit the system to function in accordance with its design specifications. In practice, these hypotheses imply that from the 8 possible states suppos-ing that all components are repairable , only 6 are now possible: State S0: initial state where all devices are functioning. In fact, values for failure rates are given for many devices in the fail-ure rate handbooks. In fact, the series-parallel con- figuration in which the entities E1, E2, E3 of the superior path are located in series among themselves and in parallel with series E4, E5 of the inferior branch.
Unfortunately, due to collection and organization me-thods utilized, the data is not always reliable. For instance, the following situation can be analyzed. Given the criticality the device, it is furthermore necessary to monitor the supply voltage by means of a supervisor circuit. This gave rise to the origin of handbooks to be used as support for the project. The intersection is here denoted with E1 E2.
In other words, the model defines states of the system as those combinations that consent to the func-tioning or non-functioning of the system. Generally, three different operating conditions can be identified: continuously active, standby and intermittent activity components in standby are normally passive. It is evident that in order to carry out a correct quantitative analysis of a fault tree it is necessary to know the calculation of the probability and the Boolean algebra, the study of which is not treated here. Consequently, in order to deal with the problem of dependability, we need to know what impediments may arise and the technolo-gy to avoid the consequences. Lorenzo PerettoAlma Mater Studiorum - Universit di BolognaDipartimento di Ingegneria ElettricaViale Risorgimento, 2I - 40136 BolognaItaly Dr. In this case, in fact, the distribution is a simple table of the possible values associated to the probability of each value. The attention to dedicate to an event depends first of all on the effect it may have on the safety of personnel, on the damage it can cause with subsequent losses and its effect on the availability of service.
These are two examples where the measurements are limited to discrete points on the axis line. The evaluation of the behavior of a system can be done in the following ways: Experimental empirical : a prototype of the system is used and the parame-ters are estimated by means of statistical data. Finally, property 3 implies that if two events have no common outcomes the relative frequency of outcomes in E1 E2 is the sum of the relative frequencies of the outcomes in the events. This event however is conditioned by the fact that the system did not fails before time t. Phase 2 Controls and initial measurements: This phase ascertains that all com-ponents to be tested are functioning correctly conformity assessment. In all reliability handbooks, the operating environment is identified by means of the E factor Environmental Factor. As seen in Chapter 1, reliability is defined as the probability that a device performs a specific function up to a specific time interval, in pre-established conditions of use.