Subsystem Boundary Determination

DOE-STD-1052-93

7.3.4

A copy of the marked-up system drawings should be included in the final

documentation package.

A comparison and review of major drawings should provide assurance that

all components in the selected systems were included in the process and that

important components were not excluded when system boundaries were

established.

7.4

Subsystem Boundary Determination

This section provides a method to determine subsystem boundaries by partitioning the

selected system. The boundary for a subsystem should be established as a logical

grouping of devices based on the functions of the system being analyzed. All

instruments and components that are necessary for the subsystem to perform its

system function should be included within the subsystem boundary. These new

boundaries partition the system under analysis into subsystems. Some of the guidance

for defining system boundaries as described in Section 7.3 is also applicable to the

subsystem boundary determination process. For example, redundant trains of

components should be included in the same subsystem. The subsystem should be

further partitioned into separate trains for analysis to consider the possibility that one

train may perform a function(s) the other train does not perform. This step of

partitioning into subsystems should be completed for all functionally significant

components within a selected system.

Subsystem boundary interfaces also should be identified. These interfaces include

significant mechanical, electrical, and pneumatic inputs/outputs and/or control signals

such as the following:

Inputs cross the boundaries moving into a component of the subsystem.

Examples of inputs include fluids, gases (e.g., air), electrical power,

instrument signals, and steam. These inputs are necessary for the component

to function properly. In this process, inputs are always assumed to be present

and available when needed.

NOTE:

The assumption that these inputs are present and available

when needed precludes the need for using a fault tree analysis

to analyze multiple failures. For example, if the supply breaker

is included in a subsystem, a fault tree analysis would be

appropriate to analyze failures of the breaker along with

failures of other subsystem components. If a fault tree is not

used, the breaker should be analyzed as a separate subsystem

or when the distribution system is analyzed.

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