Rockwell Automation Publication ICSTT-RM446N-EN-P - April 2018
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Functional Safety Management
Chapter 2
System Functional and Safety Requirements
A set of system functions and their timing requirements will be specified.
Where possible, the functions should be allocated to defined modes of
operation of the process. For each function, it will be necessary to identify the
process interfaces. Similarly, where the function involves data interchange with
third party equipment, the data and interface should be clearly identified.
Where non-standard field devices, communications interfaces or
communications protocols are required, it is especially important that detailed
requirements for these interfaces are established and documented at this stage.
The client should provide the functional requirements, where this information
is not supplied the System Integrator should define the requirements and agree
them with the client. It is, however, necessary to collate these requirements into
a document, including any clarification of the requirements. It is recommended
that logic diagrams be used to represent the required functionality and highly
recommended that all requirements are reviewed, clarified where required and
approved by the client.
During the system safety requirements stage the functional requirements are
analyzed to determine their safety relevance. Where necessary, additional
safety requirements shall be identified and documented to ensure that the plant
will fail-safe in the case of failures of the plant, safety-related system, external
equipment or communications, or if the safety-related system's environment
exceeds the required operating conditions.
The appropriate safety integrity level and safety-related timing requirements
shall be defined for each safety-related function. For each function the required
safety failure mode shall be determined. The client should supply this
information or it should be defined and agreed with the client as part of this
phase. The System Integrator shall ensure that the client approves the resulting
safety requirements.
System Engineering
The system engineering stage realizes the design of the safety-related system. It
is recommended that the engineering be divided into two distinct stages, the
first defining the overall system architecture, and the second detailing the
engineering of the individual architectural blocks.
The architectural definition shall define the safety requirements class for each
architectural element and identify the safety functions allocated to each
element. Additional safety functions resulting from the chosen system
architecture shall be defined at this stage.
The detailed engineering design shall refine the architectural elements and
culminate in detailed information for system build. The design shall be in a
form that is readily understood and allows for inspection and review of each
stage of the process and final design.
