Table 4.21 Time Estimates for Using the Failure Mode and Effects Analysis Method

DOE-HDBK-1100-2004

Table 4.21 Time Estimates for Using the Failure Mode and Effects Analysis Method

SCOPE

PREPARATION

EVALUATION

DOCUMENTATION

Simple/Small

System

2 to 6 hours

1 to 3 days

Complex/Large

Process

1 to 3 days

1 to 3 weeks

2 to 4 weeks

Source: CCPS 1992

Physical system boundaries encompass the equipment, the interfaces with other

processes, and the utility/support systems to be analyzed. Along with the physical system

boundaries, analysts should specify the levels of resolution for fault tree events reflecting

failures of both equipment and support systems (i.e., major component level,

subcomponent level, system level, and subsystem level). For example, analysts may set

the level of resolution at the subsystem level (electrical bus, cooling loop) for support

systems.

Other boundary conditions are the initial equipment configuration or the initial operating

conditions. Initial conditions reflect the initial state of all components and support

systems that are included in the FTA. This boundary condition describes the system in its

normal, unfailed state.

Unallowed events are those that are considered to be incredible or that, for some other

reason, are not to be considered in the analysis. For example, wiring failures might be

excluded from the analysis of an instrument system. Existing conditions are, for the

purposes of the FTA, events or conditions considered certain to occur. The unallowed

and existing conditions do not appear in the fault tree, but their effects must be

considered in developing other fault events as the fault tree is constructed.

Because a broadly scoped or poorly defined top event can lead to an inefficient analysis,

the top event should be precisely defined to show the "what," "when," and "where" of the

accident. Accordingly, analysts may specify other assumptions, as necessary, to define

the system or process to be analyzed. For example, analysts may assume that the process

is operating at 100 percent of normal capacity.

CONSTRUCTING THE FAULT TREE.

Fault tree construction begins at the top event and

proceeds, level by level, until all fault events have been traced to their basic contributing