Glossary Updates12 new terms added to the glossaries · October 2, 2026, 22:44 CEST
AI TechDocKnowledge

Glossary · Functional safety

SIL 4

Also known as: safety integrity level 4

German: SIL 4

In functional safety under IEC 61508, SIL 4 is the highest of four safety integrity levels. It corresponds to an average probability of dangerous failure on demand (PFDavg) from 10^-5 to less than 10^-4 in low-demand mode, or an average frequency of dangerous failure (PFH) from 10^-9 to less than 10^-8 per hour in high-demand or continuous mode.

  • Functional safety
  • Verification

In one sentence

SIL 4 is the highest IEC 61508 safety integrity level: PFDavg 10^-5 to <10^-4 or PFH 10^-9 to <10^-8 per hour; not used in IEC 62061.

Example

A railway interlocking that prevents conflicting routes is developed to SIL 4 under the railway signaling standards, which build on the IEC 61508 concept of safety integrity levels.

How it applies

  • Target bands: In low-demand mode, SIL 4 means a PFDavg from 10^-5 to below 10^-4, a risk reduction factor from more than 10,000 up to 100,000. In high-demand or continuous mode, it means a PFH from 10^-9 to below 10^-8 per hour.
  • Machinery: IEC 62061 goes up to SIL 3 only; SIL 4 is not used for machinery safety functions. If a risk assessment seems to call for more than SIL 3, the machine design or additional protective measures have to be reconsidered.
  • Process industry: IEC 61511 treats SIL 4 safety instrumented functions as exceptional. A SIL 4 requirement is a signal to reduce the risk by other means first, such as inherently safer process design or independent protection layers; where it remains, IEC 61511 points to IEC 61508 for the design.
  • Where it is used: SIL 4 appears mainly in sectors with their own standards built on the IEC 61508 concept, notably railway signaling. It demands extreme rigor: high hardware fault tolerance, very high diagnostic coverage, systematic capability SC 4 and extensive independent assessment.
  • Documentation: A SIL 4 claim must always name the standard and the safety function it refers to. Component certificates up to SIL 4 describe what a component can support; they don't make a machine, plant or function SIL 4.

SIL 4 vs. PL e

There is no Performance Level that corresponds to SIL 4. Performance Level e (PLe), the highest level of ISO 13849-1, matches the PFH band of SIL 3.

By knowledge.aitechdoc.world · Published September 28, 2026 · Last reviewed

Source: IEC 61508-1, Functional safety of electrical/electronic/programmable electronic safety-related systems — Part 1: General requirements

Definitions follow the cited standards and specifications. Where a source is a copyrighted publication, such as an ISO, IEC or EN standard, the definition is a close paraphrase, not a verbatim quotation, so as not to infringe copyright. We recommend reading the original publication. The sections “How it applies” are editorial commentary by AI TechDoc Blog and are not part of any standard.

Seen a mistake? Send us a note!