Functional Safety
What is Demand Mode in Safety Instrumented Function Design?
Vinit Pandey · Published 4 August 2026
In short: Demand mode describes how frequently a Safety Instrumented Function is called upon to act; low-demand mode functions are assessed using average probability of failure on demand (PFDavg), while high-demand or continuous mode functions are assessed using probability of dangerous failure per hour.
Key takeaways
- Low-demand mode applies where a SIF is called on no more than roughly once per year
- Low-demand functions are evaluated against average probability of failure on demand (PFDavg)
- High-demand or continuous mode functions are evaluated against failure probability per hour instead
- Misclassifying demand mode can lead to using the wrong SIL verification calculation entirely
IEC 61511 distinguishes between low-demand mode, where a safety instrumented function is called upon no more than roughly once per year, and high-demand or continuous mode, where demands occur more frequently — this distinction changes which reliability metric and calculation approach applies.
Low-demand mode functions, the more common case in batch and continuous process industry applications, are evaluated against a target average probability of failure on demand (PFDavg), reflecting the function's average unavailability between proof tests.
High-demand or continuous mode functions are instead evaluated against a target probability of dangerous failure per hour, since these functions may need to act correctly at any moment rather than only occasionally in response to a rare demand.
Misclassifying a function's demand mode can lead to using the wrong SIL verification calculation entirely, understating or overstating the reliability actually required — making correct demand mode classification an early and consequential step in SIL assessment.