Defining Safety Functions

Abstract

Safety functions are among the most important considerations in modern safety applications and are specifically prescribed in the functional safety area. Various functional safety standards provide steps that are necessary to be followed in order to define safety functions correctly. The process starts from the conceptualization of the system, in which the system scope is defined including system definition, system delineation, the definition of equipment under control (EUC), and EUC control system (ECS). The previous step is necessary to perform hazard and risk analysis, in which hazards, hazardous events, and situations are identified and related to faults, reasonably foreseeable misuse, or malicious actions. Each hazard is evaluated according to the standard applicable to the analyzed system, and its risk is quantified and classified according to that standard. Finally, each hazard needs to be addressed by one or more safety functions, which shall prevent or mitigate one of the aspects of the hazard (e.g., probability, exposure). Safety functions are designed to monitor (measure, evaluate) aspects of interest to the actuation of the hazard and to intervene in case monitored values are observed to have undesired values. Risk classification is transferred to the safety integrity requirement of the safety functions, which need to be compliant with this requirement in terms of their definition, design, implementation, and verification standpoints. In this chapter, an exemplary process is carried out concerning the requirements of functional safety for machinery, having in mind standards IEC 62061 and ISO 13849.