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SIF design

Safety alarms and operator response as a protection layer

When an alarm with operator action can be credited, the time budget that decides it, and how to specify it.

2 min read · First Out AI knowledge library

An alarm followed by a defined operator action can be an independent protection layer, but only if the operator can realistically detect, decide and act in time.

Conditions

  • The alarm is independent of the initiating cause (separate sensor and preferably separate alarm system).
  • The action is specific, trained and written in a procedure.
  • The time from alarm to the point of no return is enough for detection, diagnosis and action, with margin; many companies require at least 10 to 20 minutes for credit.
  • The alarm is prioritised and not lost in a flood.
  • The alarm and the action are tested.

In LOPA

Credit at most one operator response per scenario under most company rules, and none when the operator's error is the initiating cause.

In First Out AI

Add the alarm as an IPL row with its PFD and the evidence in "Independence, evidence". The AI asks about the time available when it reviews the scenario.

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.