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

SIL 1

Also known as: safety integrity level 1

German: SIL 1

In functional safety under IEC 61508, SIL 1 is the lowest of four safety integrity levels. It corresponds to an average probability of dangerous failure on demand (PFDavg) from 10^-2 to less than 10^-1 in low-demand mode, or an average frequency of dangerous failure (PFH) from 10^-6 to less than 10^-5 per hour in high-demand or continuous mode.

  • Functional safety
  • Verification

In one sentence

SIL 1 is the lowest IEC 61508 safety integrity level: PFDavg 10^-2 to <10^-1 on demand, or PFH 10^-6 to <10^-5 per hour.

Example

A high-level alarm function on a non-toxic storage tank, which trips the feed pump, is specified as SIL 1 in low-demand mode, with a single level switch, a safety relay and the pump contactor.

How it applies

  • Target bands: In low-demand mode, SIL 1 means a PFDavg from 10^-2 to below 10^-1, that is a risk reduction factor from more than 10 up to 100. In high-demand or continuous mode, it means a PFH from 10^-6 to below 10^-5 per hour. Which band applies depends on the Demand mode of the function.
  • Design: SIL 1 functions can often be built with a single channel (a Hardware fault tolerance (HFT) of 0), provided the Safe failure fraction (SFF) or the route chosen under IEC 61508 or IEC 61511 allows it. The elements need at least systematic capability SC 1, and the proof test interval must match the PFDavg calculation.
  • Sectors: IEC 62061 uses SIL 1 for machinery safety functions with low risk; in process plants, many Safety instrumented function (SIF) loops are SIL 1. A SIL 1 function is still a safety function with its full lifecycle: specification, verification, validation, proof testing and management of change.
  • Documentation: State the SIL per safety function, together with the mode of operation and the standard applied. A component rated for SIL 1 or higher only states what the component can support; the function needs its own SIL verification.

SIL 1 vs. PL b and PL c

In high-demand mode, the PFH band of SIL 1 covers the ranges of Performance Level (PL) b and c under ISO 13849-1. The scales overlap numerically, but the standards determine them with different methods and requirements, so a PL can't simply be relabeled as a SIL or the other way round.

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.

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