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Glossary · Functional safety

Demand mode

Also known as: Mode of operation (IEC 61508), Low demand mode, High demand mode, Continuous mode, Low-demand mode, High-demand mode

German: Betriebsart mit niedriger oder hoher Anforderungsrate

In functional safety under IEC 61508, the demand mode or mode of operation describes how often a safety function is called upon: in low-demand mode no more than once per year, in high-demand mode more than once per year, and in continuous mode as part of normal operation; it decides whether the SIL target is expressed as PFDavg or as PFH.

  • Functional safety
  • Verification

In one sentence

The demand mode says how often a safety function is needed: low demand (≤ once a year) uses PFDavg, high demand and continuous use PFH.

Example

A pressure relief SIF on a reactor is expected to trip once in ten years and is assessed in low-demand mode with PFDavg; the two-hand control of a press is used every cycle and is assessed in high-demand mode with PFH.

How it applies

  • Three modes: In low-demand mode, the safety function is performed only on demand, to bring the equipment into a safe state, and demands occur no more than once per year. In high-demand mode, demands occur more than once per year. In continuous mode, the safety function keeps the equipment in a safe state as part of normal operation, for example a safe speed limit that is always active.
  • Failure measure: The mode decides the target. Low-demand functions are specified and verified with the Average probability of dangerous failure on demand (PFDavg); high-demand and continuous functions with the average frequency of dangerous failure per hour (see Probability of dangerous failure per hour (PFHd)). The same SIL therefore means different numbers in different modes.
  • Sectors: Process safety under IEC 61511 deals mostly with low-demand safety instrumented functions, whose PFDavg depends heavily on the proof test interval. Machinery safety under IEC 62061 and ISO 13849-1 deals with high-demand and continuous functions and works with PFH.
  • Borderline cases: A function near one demand per year, or one whose demand rate is close to its proof test frequency, needs care: the low-demand calculation may overstate its integrity. Justify the chosen mode with the expected demand rate.
  • Documentation: State the mode of operation for every safety function in the safety requirements specification and repeat it wherever a SIL is quoted. A SIL without its mode and failure measure is incomplete.

Low-demand mode vs. high-demand mode

In low-demand mode, a hidden dangerous failure matters only if a demand arrives before the next proof test finds it; that is why PFDavg depends on the proof test interval. In high-demand mode, a demand is almost certain to follow soon after a dangerous failure, so the failure rate itself, expressed as PFH, is the relevant measure.

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

Source: IEC 61508-4, Functional safety of electrical/electronic/programmable electronic safety-related systems — Part 4: Definitions and abbreviations

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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