The OR gate provides a TRUE (failure) output if one or more of its input faults
are present. In contrast, the AND gate provides a TRUE (failure) output if all of its
input faults are present. Symbols for both OR and AND gates are shown in Fig. 12.7.
The fault tree construction proceeds by generation of fault events successively
until the fault events need not be developed further. These fault events are known
as basic fault events and the fault tree itself is the logic structure relating the top
fault event to the basic fault events.
Four basic symbols used in fault tree construction are shown in Fig. 12.7. The
meanings of both OR and AND gate symbols were discussed earlier. Circle and
rectangle symbols denote a basic fault event and the resultant fault event which
occur from the combination of fault events through the input of a gate, respectively.
The development or construction of a fault tree is a top-down process (i.e.,
starting from the top event moving downward). It consists of successively asking
the question, “How could this event occur?” The following basic steps are involved
in performing FTA:21
• Define factors such as system assumptions, and what constitutes a failure.
• Develop system a block diagram showing items such as interfaces, inputs,
and outputs.
• Identify undesirable or top fault event.
• Using fault tree symbols, highlight all causes that can make the top event
occur.
• Construct the fault tree to the lowest level required.
FIGURE 12.7 Basic fault tree symbols: (a) basic fault event, (b) OR gate, (c) AND gate,
(d) resultant fault event.
(a)
Output fault
Output fault
Input faults
Input faults
(b)
(c) (d)
©2002 CRC Press LLC
• Analyze the fault tree as per the requirements.
• Identify necessary corrective measures.
• Document and followup on highlighted corrective measures.
The following example demonstrate the development of a fault tree.