Glossary · Functional safety
SIL 2
Also known as: safety integrity level 2
German: SIL 2
In functional safety under IEC 61508, SIL 2 is the second of four safety integrity levels. It corresponds to an average probability of dangerous failure on demand (PFDavg) from 10^-3 to less than 10^-2 in low-demand mode, or an average frequency of dangerous failure (PFH) from 10^-7 to less than 10^-6 per hour in high-demand or continuous mode.
- Functional safety
- Verification
In one sentence
SIL 2 is the second IEC 61508 safety integrity level: PFDavg 10^-3 to <10^-2 on demand, or PFH 10^-7 to <10^-6 per hour.
Example
The guard interlocking of a robot cell is specified as SIL 2 under IEC 62061 and realized with a coded safety switch, a safety PLC and two contactors monitored by feedback contacts.
How it applies
- Target bands: In low-demand mode, SIL 2 means a PFDavg from 10^-3 to below 10^-2, a risk reduction factor from more than 100 up to 1,000. In high-demand or continuous mode, it means a PFH from 10^-7 to below 10^-6 per hour. Which band applies depends on the Demand mode of the function.
- Design: SIL 2 functions often use redundancy or high diagnostic coverage, depending on the Hardware fault tolerance (HFT) and Safe failure fraction (SFF) of the subsystems. All elements need at least systematic capability SC 2, or must be combined in a way that the standard accepts for SC 2.
- Sectors: SIL 2 is common for machinery safety functions under IEC 62061, such as guard interlocking or safe speed monitoring, and for many Safety instrumented function (SIF) loops in process plants.
- Documentation: State the SIL per safety function, with the mode of operation and the standard applied. A "SIL 2 capable" or "SIL 3 capable" certificate of a sensor or logic solver describes the component under the conditions of its safety manual; the function reaches SIL 2 only after its own SIL verification and validation. Avoid statements such as "a SIL 2 machine".
SIL 2 vs. PL d
In high-demand mode, the PFH band of SIL 2 matches that of Performance Level (PL) d under ISO 13849-1. The labels are not interchangeable: the standards use different methods for architecture, diagnostics and systematic failures.