SIS functional safety requirements:

The IEC 61511 standard specifies requirements that must be sufficient to design the SIS and include the following:

  • A description of all the necessary SIFs to achieve the required functional safety.
  • Requirements to identify and take account of common cause failures
  • A definition of the safe state of the process for each identified SIF.
  • A definition of any individually safe process state which, when occurring concurrently, creates a separate hazard.
  • The assumed sources of demand and demand rate of each SIF.
  • Requirements for proof-test intervals.
  • Response time requirements for the SIF to bring the process to a safe state.
  • The SIL and mode of operation (demand/continuous) for each SIF.
  • A description of process measurements and their trip point.
  • A description of process output actions and the criteria for successful operation (e.g. requirements for tight shut-off valves).
  • The functional relationship between process input and output, including logic, mathematical functions, and any required permissions.
  • Requirements for manual shutdown.
  • Requirements relating to energize or de-energize to trip.
  • Requirements for resetting the SIF after a shutdown.
  • Maximum allowable spurious trip rate.
  • Failure modes and desired response of the SIF
  • Any specific requirement related to the procedures to initiate and restart the SIF.
  • All interfaces between the SIS and any other system, including BPCS and operators.
  • A description of the operation modes of the plant and identification of the SIF required to operate within each mode.
  • Application software safety requirements.
  • Requirements for cancellations / inhibitions / deviations, including how they will be delete.
  • The specification of any action necessary to achieve or maintain a safe state in the event that flaws in the SIF are detected.d.
  • The mean time to repair which is feasible for the SIF.
  • Identification of the dangerous combinations of exit states of the SIS that should be avoided.
  • Identification of the extremes of all environmental conditions that the SIS is likely to encounter.
  • Identification of normal and abnormal modes for the plant as a whole (for example, start-up of the plant) and individual operating procedures of the plant.
  • Definition of the requirements for any safety instrumented function necessary to survive a serious accident event.


Instrumentation Engineer

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