Glossary · Functional safety
Category 4 (ISO 13849-1)
Also known as: Cat. 4
German: Kategorie 4
In ISO 13849-1, Category 4 is a redundant architecture in which a single fault does not lead to the loss of the safety function and is detected at or before the next demand. If that is not possible, an accumulation of undetected faults must not lead to the loss of the safety function.
- Functional safety
- Systems engineering
In one sentence
Category 4 in ISO 13849-1 is a redundant architecture with high diagnostic coverage in which faults are detected before the next demand.
Example
A press safety control uses two channels with high MTTFd, continuous cross-monitoring of valves and contactors, and diagnostics that block the next stroke as soon as a discrepancy occurs.
How it applies
- Architecture: Category 4 requires high MTTFd in each channel, high average diagnostic coverage and measures against common-cause failure.
- Achievable level: Category 4 is intended for PL e, the highest Performance Level (PL).
- Engineering: Monitoring of every output element, for example via positively guided feedback contacts or valve position sensing, is usually needed to reach the required diagnostic coverage.
- Documentation: Document the diagnostic measures and their test rates, and describe the fault reactions in the operating instructions. The documentation team should explain that a detected fault blocks restart until it is corrected, so that users do not treat it as a nuisance.
Category 4 vs. PL e
A category describes the architecture, a PL describes the achieved reliability. Category 4 is the usual route to Performance Level e (PLe), but Category 3 with high MTTFd and medium diagnostic coverage can also achieve PL e. Using Category 4 components alone does not make a function PL e.