SIL belongs to the function

A certified component does not certify the whole protection function. A safety instrumented function brings together sensing, logic and final elements, with the requirements needed to achieve a safe process state. The required integrity comes from risk assessment and the allocation of safeguards; it should not be selected as though it were a product category.

IEC 61511 addresses the lifecycle of safety instrumented systems in the process industry. Specification, design, installation, operation and maintenance belong to a connected process. A device certificate provides information to consider, but it does not answer every question about the complete function.

Imagine a safeguard intended to stop a feed: does the sensor detect the specified condition? Does the logic receive the right information? Does the final element bring the process to the defined state? These questions need to be considered for the actual scenario, with appropriate competence and information.

Before discussing a SIL target, I would clarify the event to prevent, the safe state and the time available to act. I would then keep the conversation open about verification, maintenance and changes over time. Does your specification describe the function as clearly as its required integrity?