Fault tree symbols

Numerous symbols are used in the construction of a basic fault tree. These symbols, sometimes referred to as fundamental logic symbols, provide the analyst with a pictorial representation of the event and how it interacts with other events on the tree. 

The Rectangle Used to identify the top or primary event, as well as secondary or contributing events (sometimes called main events). The rectangle shape as used on the fault tree indicates an event, or system state, which must be further analyzed on lower levels within the tree. This is the reason that the primary undesirable event is represented by a rectangle. Everything that appears under it is an attempt to further analyze its occurrence. 

The Circle Used to depict a basic event in the FTA process. It can be a primary fault event (i.e., the first in the process to have occurred) and, therefore will require no further development. Use of the circle symbol offers the analyst some flexibility. A causal chain could conceivably become quite extensive. Many times, the analyst will obtain sufficient casual information from analysis of higher level events in the chain. Therefore, in order not to waste valuable time and resources analyzing a single event to its lowest possible level, the analyst can label a particular event as basic, using the circle symbol indicating that no further development is required. For this reason, the symbols of the fault tree places the circle at the base of the tree (i.e., a basic event). The basic event is also often referred to as a root event or root cause, for obvious reasons. 

The House The house is used to identify a normal event whieh occurs during system operation. It is an event that either occurs or does not occur, such as turning a switch on or off. It should be noted that, if either the on state or the off state are possible during normal system operation, then the possible effect of both on the top event should be considered. 

The Diamond An event in the fault tree that is considered undeveloped is represented by a diamond. Use of this symbol identifies an event that the analyst has chosen not to develop further either because of the complexity of the event or because insufficient data are available to further analyze the event. Typically, it may also indicate an area of concern where further development should be considered at some point in the future. 

---

Figure 3.4.4-1 summarizes conventional fault tree symbols. The many symbols are daunting, but remember that the computer only performs AND (Boolean multiplication) and OR (Boolean addition) operations. All else are combinations of these. 

The fault tree symbols and their definitions are presented in Table 16.6.1. The construction of tlie fault tree for a tmik overflow c.xaniple is demonstrated in Figure 16.6.1. 

The fault tree symbols and dieir definitions are presented in Table... 

The fault trees, unlike the event trees, proceed backward from the final event (i.e. the fault of the component or system) to the various causes which may have originated it, with the corresponding probabilities. Figure 11-4 shows a fault tree for the simple system shown in Figure 11-5 and for the fault insufficient flow from V3 . (Some fault tree symbols are shown in Fig. 11-6.)... 

Common Fault Tree Symbols and Fault Tree Analysis Steps... 

Basic fault tree symbols (a) OR gate, (b) AND gate, (c) circle, (d) rectangle. 

Step 3 Determine all types of possible potential causes that can result in the occurrence of the top fault event by using fault tree symbols, such as shown in Figure 4.4, and the logic tree format. 

With the aid of fault tree symbols given in Figure 4.4, develop a fault tree for the top event "patient given incorrect medication or wrong amount."... 

Fault tree symbols are divided into four categories primary event, intermediate event, gate, and transfer. Figure 7.1 defines each of the symbols used in fault tree generation. 

FIGU RE 7.1 Fault tree symbols. (From Roberts, N.H. et al., Fault Tree Handbook, NUREG-0492, U.S. Nuclear Regulatory Commission, Washington, DC, 1981, p. TV-3.)... 

Figure 12.11 Fault tree symbols for gates, transfers, and events.

See Fault Tree Symbols for additional related information. 

See Fault Tree Symbols for additional related information. PROBABILISTIC RISK ASSESSMENT (PRA)... 

In order to demonstrate the application of fault-tree analysis in the area of rail safety, using the fault-tree symbols defined in Chapter 4, a simple fault tree for fhe fop evenf—release of liquefied chlorine from a rail tank shell— is shown in Figure 7.1 [25]. The capital letters in the circles and rectangles of the fault-tree diagram in Figure 7.1 denote, respectively, basic and intermediate fault events associated with the rail tank shell. Each of these capital letters is defined below [25]. 

---