In the Risk Analysis it is fixed a tolerable rate of dangerous situations (tolerable hazard rate: THR) for each component, apparatus, physically independent system.

From the point of view of safety engineering THR constitutes the objective of the safety integrity levels (SIL), both as regards the random malfunctions, both for those systematic.

The conformity of a component, equipment or system, to a certain THR, if one considers only the fraction stochastic malfunctions, is obtained by calculating the probability of failure of that component or system. This calculation is performed in accordance with the requirements specified in IEC 61508-3 and IEC 61508-6 standards and according to the information supplied in CELENC EN 50129 .

To perform the calculation of the probability of failure of the various blocks that constitute a component, apparatus, system, the techniques given in IEC  61508-6 are used, using all available data for each individual element:

• Failure Rate: (total λ, fraction of dangerous failures λD, fraction of dangerous failures detected λDD, fraction of failures not detected λDU, etc.);

• diagnostic coverage (DC);

• fraction of common mode failures (with a common cause) not recognized (β);

• fraction of common mode failures detected (βD);

• mean time to recovery (MTTR);

• interval between periodic tests (T1);

• interval between applications (T2).

If not all the necessary data are available, some of them can be calculated (for example by referring to what is stated in the same IEC 61508-6 ). Alternatively, the data contained in appropriate date basis, or data estimated with caution margins available in the literature may be used.
The overall failure rate of a device or system is carried out using various methods such as reliability block diagram (RBD), the fault tree analysis (FTA), the Weibull analysis, and so on. In more complex cases, special software that can be purchased on the market, are used.


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