What components form a Safety Instrumented System (SIS) according to IEC61511?

• A. Sensors, actuators, and alarm systems
• B. Sensors, logic solvers, and final elements
• C. Actuators, switches, and control panels
• D. Only logic solvers and control panels

Answer: (B)

A Safety Instrumented System (SIS), as defined by IEC 61511, is composed of three core components: sensors, logic solvers, and final elements.

1. Sensors are critical as they detect hazardous conditions or changes in the process variable that may indicate a potential unsafe situation. They provide the necessary input to the logic solver and play a vital role in the system's safety.

2. Logic Solvers take the input from the sensors and evaluate it according to the programmed logic. The logic solver processes this information to determine whether a safe state needs to be actuated, serving as the brain of the SIS.

3. Final Elements act based on the decisions made by the logic solvers. They typically include devices like valves or actuators that can either shut down the process or implement safety actions to mitigate risks.

The integration of these components ensures that an SIS can effectively monitor, assess, and respond to unsafe situations, thus maintaining safety integrity in industrial environments. In contrast, options that suggest alternative combinations do not align fully with this standard definition, as they omit critical components or include elements not defined as part of an SIS in the context of IEC 61511.