A Safety Instrumented System (SIS) is defined as:

The definition of a Safety Instrumented System (SIS) emphasizes its independence and separation from other control systems. An SIS is specifically designed to take action to prevent a hazardous event or to mitigate its consequences when predetermined safety limits are violated. The components of an SIS, which include sensors, logic solvers, and final control elements, operate autonomously to ensure that safety functions are executed independently of other systems.

This independent operation is crucial because it minimizes the risk of common cause failures, ensuring that a safety function can still be performed even if other interconnected systems fail. Therefore, the characterization of an SIS as a separate and independent system aligns precisely with its role in managing safety risks.

In contrast, the other options describe systems or scenarios that do not align with these core principles of an SIS. For example, a system that requires human intervention is more reliant on user actions and decision-making, which could introduce variability and potential errors, thereby undermining the automatic fail-safe capacity that an SIS is designed to provide.