Glossary · Functional safety
Category 3 (ISO 13849-1)
Also known as: Cat. 3
German: Kategorie 3
In ISO 13849-1, Category 3 is a redundant architecture in which a single fault does not lead to the loss of the safety function, and the single fault is detected whenever reasonably practicable. An accumulation of undetected faults can still lead to the loss of the safety function.
- Functional safety
- Systems engineering
In one sentence
Category 3 in ISO 13849-1 is a two-channel architecture: a single fault does not cause loss of the safety function, but accumulated faults can.
Example
A guard door uses two interlocking switches read by a safety relay that monitors both channels and two contactors with mirror contacts; one welded contactor is detected at the next cycle.
How it applies
- Architecture: Category 3 uses two channels with cross-monitoring. It requires well-tried safety principles, average diagnostic coverage of at least low and measures against Common-cause failure (CCF).
- Achievable level: Depending on MTTFd and diagnostic coverage, Category 3 can reach up to PL e.
- Fault masking: When several devices are connected in series on the same channels, faults may mask each other and reduce the effective diagnostic coverage. ISO TR 24119 addresses this for interlocking devices.
- Documentation: The calculation report should show both channels, their MTTFd, the DC assumptions and the CCF score. The documentation team should state which faults the system detects and what the operator must do after a detected fault.
Category 3 vs. Category 4
Both are redundant. Category 3 accepts that an accumulation of undetected faults can lead to loss. Category 4 (ISO 13849-1) requires high diagnostic coverage and that faults are detected at or before the next demand, or that accumulated undetected faults cannot lead to loss.