Quantifying the failure probability of a protective device is not sufficient to fulfill a Safety Integrity Level. Primary measures for the prevention and control of faults must be implemented to this end. The relevant standard requires, first and foremost, the application of a special quality management system (Functional Safety Management System).
In addition, failure control by means such as redundancy, fail-safe behavior, and fault detection (diagnostics) are mandatory. The extent to which these measures need to be applied depends on the targeted SIL.