What is the difference between a category 3 and category 4 safety system?

The difference between Category 3 and Category 4 safety systems under ISO 13849-1 or the older EN 954-1 relates to fault tolerance and diagnostic coverage. Category 3 requires that the safety system be designed so that a single fault does not lead to loss of the safety function, and that single faults are detected where reasonably practicable, using dual channels and monitoring. However, an accumulation of undetected faults could lead to loss of safety function. Category 4 is the highest category, requiring that a single fault does not lead to loss of safety function, that single faults are detected at or before next demand, and that an accumulation of faults does not lead to loss of safety function. In practice, Category 4 systems have higher diagnostic coverage, more robust monitoring, and are designed to detect all possible faults, often requiring more redundancy and continuous self-checking. Category 4 is used for higher risk applications with high severity and frequent exposure, while Category 3 is used for medium to high risk where some fault accumulation is tolerable but single fault tolerance is still needed.