A system or device is deemed to be safety-related if it provides functions which significantly reduce the risk of a hazard, and in combination with other risk reduction measures, reduces the overall risk to a tolerable level, or if it is required to function to maintain or achieve a safe state for the equipment under control.
These functions are known as the safety functions of the system or device and are the ability to prevent initiation of a hazard or detect the onset of a hazard, and to take the necessary actions to terminate the hazardous event, achieve a safe state, or mitigate the consequences of a hazard.
All elements of the system that are required to perform the safety function, including utilities, are safety related, and should be considered part of the safety-related system (SRS). A typical example of a SRS is the Safety Instrumented System (SIS), which also goes by names such as Safety Interlock Systems, Emergency Shutdown (ESD) Systems, or Safety Shutdown Systems (SSS).
Part of the control function of a process control system (PCS) can be safety-related. A PCS can be considered a SRS if it meets the requirements stated above. This will be further discussed in the next Section. However, those which are not considered safety related should still be designed, installed, operated and maintained so that their failure does not place a rate demand in the protective system which was not anticipated in its design.
Finally, it must be emphasized that the use of SRSs does not circumvent the need for a hierarchical approach to risk reduction measures such as inherent safety, and careful consideration of prevention of common mode failures by use of diverse technology and functionality (such as relief valves), independent utilities and maintenance and test procedures, physical separation, and external risk reduction (such as bunds). Measures should favor simple technological solutions rather than complex ones.