Fault tree analysis circle

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). 

In addition to these, the MORT chart also uses a rounded rectangle, or elongated circle, to represent a satisfactory event (an event that may have contributed to an accident or incident but whose existence is essential for normal system operation). Also, instead of the house symbol common to fault tree analysis to represent those events that are considered normal and expected in a typical system, MORT uses a scroll symbol. Figure 13.2 shows the MORT symbols as described here. 

Basic Event As pertains to fault tree analysis (FTA) and/or the Management Oversight and Risk Tree (MORT), a root fault event or the first in the process to have occurred that requires no further development or analysis. Represented graphically as a circle. 

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]. 

A basic event, represented by a circle, is an event having no further analysis. It is a terminus of a branch in the fault tree. Basic events have probabilities when there is quantitative analysis. 

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. 

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