Glossary · Functional safety
SIL 3
Also known as: safety integrity level 3
German: SIL 3
In functional safety under IEC 61508, SIL 3 is the third of four safety integrity levels. It corresponds to an average probability of failure on demand from 10^-4 to less than 10^-3 in low-demand mode, or a probability of dangerous failure per hour from 10^-8 to less than 10^-7 in high-demand or continuous mode.
- Functional safety
- Verification
In one sentence
SIL 3 is the second-highest IEC 61508 safety integrity level and the highest in IEC 62061, requiring very low failure probability.
Example
A high-integrity pressure protection system on a gas pipeline is specified as SIL 3 and uses redundant transmitters, a certified logic solver and two shutdown valves in series.
How it applies
- Process industry: SIL 3 functions are demanding. They usually require redundancy, a certified logic solver, rigorous proof testing and strong management of systematic failures.
- Machinery: SIL 3 is the highest level in IEC 62061.
- Components: A device marked SIL 3 capable has the systematic capability and failure data to be used in SIL 3 functions under the conditions of its safety manual. The function still needs its own SIL verification.
- Documentation: Quote SIL per safety function, with the mode of operation and the standard applied. The documentation team should avoid generic claims such as a SIL 3 machine or a SIL 3 plant.
SIL 3 vs. PL e
In high-demand mode, the PFH range of SIL 3 matches that of Performance Level e (PLe). They are not interchangeable labels, because the standards use different methods and requirements.