Define a Safety Instrumented System (SIS) and explain its independence from normal process control.

• A. It performs safety functions and runs in parallel with the main control system.
• B. It performs safety functions and operates independently from the basic process control to avoid common-mode failures.
• C. It is the primary control system for the process.
• D. It has no relation to safety.

Answer: (B)

At its heart, a Safety Instrumented System is a dedicated safety layer that automatically carries out protective actions when sensors detect a hazardous condition. It’s designed to bring the process to a safe state, such as shutting down equipment, isolating a flow, or triggering relief, to prevent a release or other harm.

Independence from the normal process control is essential. The SIS uses its own sensors, logic solver, and final control elements, often with separate power and software lifecycle processes. This separation prevents common-mode failures—if the regular process control system encounters a fault or disturbance, it won’t compromise the safety actions because the SIS operates on an independent path.

So the best description is that it performs safety functions and runs independently from the basic process control to avoid common-mode failures.