Category 3 in ISO 13849-1: Behaviour vs. Hardware Fault Tolerance

WHAT MANY ENGINEERS GET WRONG ABOUT FUNCTIONAL SAFETY In functional safety discussions, Category 3 (CAT3) is often oversimplified—or worse, misunderstood. A common assumption is that CAT3 directly implies a specific hardware architecture or Hardware Fault Tolerance (HFT) level. In reality, this interpretation can lead to incorrect design decisions and flawed safety assessments. This paper revisits the fundamental intent of ISO 13849-1 and clarifies a key principle.