Undeveloped terminal event

The quantitative evaluation of the fault tree is done by analyzing the structure of levels using the Boolean algebra to determine the minimum combinations of terminal events or minimum sets of faults that lead to the undesired event. Its probability is calculated based on the probabilities of the basic events and undeveloped events to be evaluated from available historical data. 

The other event symbol is the diamond, which represents an undeveloped terminal event. The diamond is used in place of a rectangle for a general event that is not going to be further developed (as part of this particular tree at this time). An event may not be developed because the information or the time needed is not available. Sometimes an event is not developed because it is not... 

Another symbol is the oval, which is used as a constraint symbol. Ovals are the footnotes or caveats in the system. They can be used to modify or to show additional information and can be attached to either a general event, a base event, or an undeveloped terminal event. They can be used in conjunction with logic gates. They generally show a specific condition or constraint or modify the meaning of either the event symbol or the gate. 

Logically, any general event symbol (rectangle) has one gate attached to it and two or more events as input to that gate. Circles (base events) and diamonds (undeveloped terminal events) are bottom-tier or terminal events that have nothing below them. 

As with many positive or operational readiness trees, the first tier addresses the personnel, procedures, and plant and hardware that comprise the system. The only personnel for this system is the individual preparing the tree who will be the sole operator of the system therefore, it is represented by self shown as a base event. Procedures for this system consist primarily of the operating and maintenance instructions provided with the hardware and software. Because several sets of instructions may be involved, instructions cannot appropriately be shown as a base event, yet the instructions are not the focus of the exercise. Therefore, they are shown as an undeveloped terminal event. The real focus of the effort is the plant and hardware, in this case, the home computer. The home computer facilities and equipment consist primarily of the computer hardware and the software to make it operate. Other items are also required, like a room to set up the operation, furniture, and electrical outlets, but again they are not the focus of the effort and are therefore shown simply as other in an undeveloped terminal event symbol. 

In this example, the individual obtaining the home computer system is interested primarily in word processing capability that is compatible with the word processing package at the office. The immediate concern, then, is to obtain system software and word processing software. Other programs that may be added later are not of immediate interest and are again indicated by an undeveloped terminal event. The constraint or requirement for all software is that it is compatible with that used at the individual s workplace. 

Rectangles are used as general event symbols, circles are used to show base events, and diamonds are used to show undeveloped terminal events. And gates are used to indicate that, in order to get an output, all inputs below that gate are required, and or gates are used to indicate that, in order to get an output, any one or any combination of the inputs is required. 

The next step is carefully wording the undesired top event. This step is important because the wording of the top event determines, to a large extent, the scope of the effort. There are two basic approaches to limiting the scope and thereby focusing the effort. One approach is to state all of the conditions and restrictions in narrative form in the top block. The second approach is to use constraint symbols and/or undeveloped terminal events on the first tier. 

Undeveloped terminal events should be used if the events do not represent significant risks (extremely reliable components), information is not available to develop the event, the event is not the responsibility of the analyst, or the event is not important at this time. Ideally, a complete, final analysis of a system does not contain undeveloped terminal events. 

Under what circumstances should undeveloped terminal events be included in an FTA ... 

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). 

Risk transfers are found in two places. They are found next to undeveloped terminal events, and they are found near the top of the chart as assumed risks. The implied logic is that barriers, controls, or evasive actions are not possible or practical in those areas where there are undeveloped terminal events. If activity is continued under these circumstances, then some risk is assumed or accepted. The small assumed risk transfers are used to denote a transfer up to the small ovals under the assumed risk portion of the MORT chart. 

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.

FTA termination Fault ttee is terminated either at basic event, which represents the lowest level events, ot it is terminated at undeveloped event (which is not... 

Continue developing the fault tree until all branches have been terminated by basic, undeveloped, or external events. All intermediate events must be expanded. 

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