Basic event symbol

Sequence number Basic events Symbols Weight... 

General event symbol Basic event symbol... 

Figure 18-1 Event Symbols. The General Event symbol, Basic Event symbol, and Undeveloped Terminal event symbol are the same as commonly used for other fault trees. The use of the stretched circle for a Satisfactory Event and the scroll for a Normally Expected Event are unique to the MORT chart.

The fault tree is a graphic representation of the relationships between basic events and the selected top event. Table 4.24 presents the standard symbols used in fault tree construction to show these relationships. 

The flat tire example is pictured using a fault tree logic diagram, shown in Figure 11-12. The circles denote basic events and the rectangles denote intermediate events. The fishlike symbol represents the OR logic function. It means that either of the input events will cause the output state to occur. As shown in Figure 11 -12, the flat tire is caused by either debris on the road or tire failure. Similarly, the tire failure is caused by either a defective tire or a worn tire. 

The results of this procedure are shown in Figure 11-14. The symbol P represents the probability and R represents the reliability. The failure probabilities for the basic events were obtained from Example 11-2. I... 

The above layout would be cumbersome to construct for each fault tree generated, so many fault tree analysis programs allow CCF groups to be created. Basic events are added to the group and a Beta factor between 0 and 1 specified. This is normally represented visually on the fault tree by displaying a p symbol next to the included events, as shown in Fig. 4A-4A 4 below. 

Trees are called trees because, basically, they have a structure that resembles a tree, that is, narrow at the top with a single event symbol and then branching out as the tree is developed. 

The project evaluation tree is an analytical tree to be used primarily as a graphic check in basically the same manner as the management oversight and risk tree (see Chapter 18). The PET chart, however, contains fewer than 200 event symbols and no transfers the MORT chart contains approximately 1,500 events symbols, multiple transfers, transfers within transfers, and drafting breaks. The PET chart is divided into three branches procedures, personnel, and plant and hardware. 

The symbols used on the MORT chart are basically those used for other analytical trees (Chapter 10) and fault tree analysis (Chapter 15). They include the rectangle as the general event symbol, the circle as the base event symbol, the diamond as an undeveloped terminal event, the and gate, the or gate, and the ellipse as a constraint symbol (Rgs. 18-1 and 18-2). 

The primary symbols used for most analytical trees have been used in the MORT event tree as well. These include the rectangle (primary or top event, and secondary, contributory or main events), the diamond (undeveloped event), the circle (basic event), the and gate, the or gate, the oval (conditional or constraint symbol), and the triangle (transfer gate or symbol). In addition to these symbols, the MORT... 

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. 

Fault tree analysis (FTA) provides a logical representation of many events and component failures that may combine to cause one critical event (e.g., pipeline explosion). It uses logic gates to show how basic events may combine to cause the critical top event. The top event would normally be a major hazard such as "pipeline SCC" as in the example shown in Fig. 12.10. The most commonly used tree symbols and gates used in the construction of fault trees are illustrated in Fig. 12.11 and briefly described here [12] ... 

FTA is a graphical safety analysis technique that develops an FT diagram that utilizes special symbols. Basic event FT symbols and their descriptions are shown in Figure 2.26. 

Figure 2.26 FT symbols for basic events, conditions, and transfers.

An example of an undeveloped event may be the failure of the instrument air supply. An undeveloped event symbol and a single failure rate can be used to model the instrument air supply rather than model all of the components. FTA treats undeveloped events in the same way as basic events. 

FTA is a deductive method that uses Boolean logic symbols (i.e., AND gates, OR gates) to break down the causes of the top event into basic equipment failures and human errors. The analysts begin with the top event and identify the causes and the logical relationships between the causes and the top event. Each of the causes, called intermediate events, is examined in the same manner until the basic causes for every intermediate event have been identified. 

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