Failure rate evaluation method for HW architecture derived from functional safety standards (ISO 19014, ISO 25119, IEC 61508)

An important issue concerning the use of safety standards in the design of a complex system is the proper evaluation of the risks. The risk is correlated to the probability of failure, then to the failure rate, and it plays a fundamental role in the safety assessment process. This paper focuses on the failure rate evaluation methods adopted by the following functional safety standards: ISO 19014, ISO 25119, and IEC 61508. All of these standards are applicable, respectively, in the earth-moving machinery, in the tractors and machinery for agriculture and forestry, and in the industrial automation fields. As proposed by the standards, the failure rate evaluation depends on the conventional parameters like Mean Time To Failure, Diagnostic Coverage level, and Hardware Categories. Nevertheless, assuming the same value of these parameters, the results obtained adopting the suggested methods from different standards are not always the same. The aim of this paper is to give an analytical approach to calculate, on a common basis, the failure rate starting from a certain set of input parameters. In addition, the approximations are analysed in order to identify the correct assumptions for the proper application of all these “not exact” values in each application field.