The U. S. standard ANSI/ISA-S84.01 and the international standard IEC-61511 require the assignment of a Safety Integrity Level for all instances where an automated, instrumented safeguard is utilized as protection from a hazardous event. The assigned SIL is based on the risk associated with the hazardous event and dictates how robust the design of the instrumented safeguard must be.

The process for addressing the Safety Integrity Level requirements of ISA-S84.01 consists of three general steps:

· Select appropriate SIL (SIL Assignment)

· Design SIS to meet target SIL (SIS Design)

· Perform availability calculations to verify system integrity (SIL Verification)

This procedure is concerned with the first of these steps. The second and third steps are addressed in Practice  Safety Integrity Level (SIL) Verification. Additionally.

The SIL Assignment normally is done in a team-based review meeting. There, the team reviews all identified hazards, determines which ones use (or need to use) a SIF as a safeguard, and then applies risk ranking criteria to determine the required SIL for each case.