Category is how ISO 13849-1 classifies the architecture of a machine’s safety-related control system: how the parts are arranged and what happens when one of them fails. There are five: B, 1, 2, 3 and 4. The standard’s formal name is designated architecture, and you’ll also hear architectural category, safety category, control category, or EN 954-1 category, after the older standard ISO 13849-1 replaced.
Key Points
- Categories B, 1 and 2 are single channel, so one fault can defeat the safety function. Category 2 at least tests the function periodically.
- Categories 3 and 4 are dual channel and survive a single fault, and Category 4 also catches that fault before the next demand. In process terms, that’s the difference between a hardware fault tolerance (HFT) of 0 and an HFT of 1.
- Category caps the performance level (PL) a design can reach, from PLb for B up to PLe for 3 and 4. The mean time to dangerous failure (MTTFd), diagnostic coverage (DC) and common cause failure (CCF) measures decide where under that cap it lands.
Example
An emergency-stop circuit uses two redundant contactors, each monitored so a welded contact is caught before the next demand. A single fault neither defeats the stop function nor goes unnoticed, so the design meets Category 4. With high DC and a good MTTFd behind it, that architecture supports PLe.
See Also: PL, machinery safety, MTTF, CCF
Cited Sources
- ISO 13849-1:2023, Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design
- EU Machinery Regulation (EU) 2023/1230 — EUR-Lex official text