A function that is demanded rarely is judged by how likely it is to be failed when a demand comes (PFDavg). A function that is demanded often, or acts continuously, is judged by how often it fails dangerously: the average frequency of dangerous failure per hour (PFH).
Choosing the mode
The usual boundary is a demand rate of once per year: above it, or more than twice the proof test frequency, treat the function as high demand. The reasoning: if demands come more often than proof tests, a hidden failure is found by the next demand (with the hazard) rather than by the test, so the test interval no longer protects you.
Examples of high demand functions in process plants: interlocks that act on every batch step, permissives used during every start-up, functions acting as the normal control of a hazardous step, machine guarding.
SIL bands for PFH
SIL 1 from 10⁻⁶ to 10⁻⁵ per hour, SIL 2 from 10⁻⁷ to 10⁻⁶, SIL 3 from 10⁻⁸ to 10⁻⁷, SIL 4 from 10⁻⁹ to 10⁻⁸.
The arithmetic for a single channel
Without diagnostics that bring the process to a safe state, every dangerous undetected failure leads to the hazard at the next demand, so to a first approximation
PFH ≈ λDU
For redundant groups the independent part becomes small and the common cause part β·λDU usually dominates.
Worked example
A permissive with one position switch and one valve, overall λDU = 5 × 10⁻⁷ per hour: PFH ≈ 5 × 10⁻⁷ per hour, SIL 2 band. The same hardware in low demand with a yearly test would show PFDavg ≈ 2.2 × 10⁻³, also SIL 2; but the meaning is different, and the proof test interval does not appear in the high demand result.
In First Out AI
Set the demand mode to "High demand (PFH)" (Pro). PFH is calculated for 1oo1, 1oo2, 2oo2, 1oo3 and 2oo3. Fill in "Expected demand rate" in the specification details so the SRS states why the mode was chosen.
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.