Base Events

Procedural, object-oriented, visual, symbolic, pipeline-based, event-based... 

Accept, transform, normalize, and populate (message based) event-driven or time-sensitive data in near-real time. 

OSHA stresses the importance of a team-based approach to all types of hazards analysis, but such an approach is fundamental for those techniques that fall into the first category, i.e., those that are primarily creative or imaginative. (Other techniques such as fault tree analysis are less suitable for team participation. However, even in such cases, a team is needed to identify the base events, and to discuss the cause and effect relationships that exist within the system being analyzed.)... 

For example, the FTA approach is logical and rational. The persons building the tree assume that the base events, and the manner in which they interact with one another, have been defined before the analysis starts. However, fault tree analysts will often identify new incident scenarios and find new types of hazard. In other words, this logical/rational approach to hazards analysis can also be creative and imaginative. ... 

Steady-state procedures that require routine operator intervention can be either event- or time-based. Event-based procedures are usually performed in response to a system change or condition. For example, with regard to the cooling tower example, the facility may have event-based procedures to handle issues such as adding chemicals to the water, or controlling the temperature of the water. Time-based procedures are those that are to take place at a specified time interval. For example, the operator may have to catch a sample of the cooling water every 4 hours in order to check the chemical concentrations in the water. Many maintenance-related activities, such as checking the lube oil level in pumps, are time based. 

Both the Action Taken event and the Conditions Exist event can be developed further. However, Figure 11.14 shows Action Taken as being a Base Event (a base event is identified... 

Intermediate Events are expanded through or and and Gates until evenmally they become Base (or Basic) Events, at which point development of that particular branch of the tree ceases. A Base Event is a stand-alone event that will not be developed further (at least at this stage of the analysis). Base Events, which are sometimes referred to as Primal Events (Lapp, 2005), must be independent... 

The symbol for a Base Event in a fault tree is a circle, as shown in Figure 15.13. 

The system can fail due to problems with the pumps or with the instruments. Eventually, this part of the Tree leads to the creation of six separate Base Events. 

At the conclusion of the analysis all of the Intermediate Events (except for the All Other Events term) will have been converted into gates or Base Events. 

Repeating the above actions for all events, the full set of noncondensed Cut Sets for the final tree of Figures 15.22 and 15.23 is shown in Table 15.2. AU the gates, i.e., those terms starting with the letter G, have been transformed into base events. 

Using the Standard Example once more, sample quantitative base event data are shown in Table 15.7. 

Repeat the above process for each Base Event in turn. 

Base Event Item Set At Zero Event Value Ranking %... 

One of the biggest difficulties with quantifying risk is that so much of the basic data (the frequency and probability values for base events) is of low quality, for the following reasons ... 

Kaizen blitz events are team-based events that take advantage of the power of teamwork. Teams bring diversity in skills, thought, and knowledge. Inviting individuals to join the team should be based on some underlying reasoning. Some examples include ... 

The notion of a temporal firewall is built on the assumption that the external environment of a system or subsystem cannot induce a failure in the temporal domain. With a state-only interface this is straightforward, although coordinated executions between subsystems requires a global time base. Event ports, however, introduce the possibility that assumptions about the frequency of events may be violated at run-time. If static analysis is not adequate, then the source and/or the destination of events must be monitored and controlled. 

The module provides two types of data retrieval process one is a retrieval of scenario based plant responses and the other is a retrieval of symptom based event sequences. The objective of the former one is to search the time-dependent behaviors of the plant (or code) response parameters for given severe accident sequences (including the base/sensitivity analysis results), phenomenological characteristics of the code parameter behaviors (such as the minimum, peak. 

Some base events figure in more than one category and the quantification needed to be organized such that the integrity of the results was maintained and the interdependencies between categories properly handled. 

A circle is used instead of a rectangle for base events. Base events are... 

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. 

The only requirement this individual has for a keyboard is that the keyboard be compatible with the system. The keyboard was considered a base event. 

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. 

For a qualitative FTA, reliability and failure probability data must be provided for all base events. 

Cut sets and minimum cut sets should then be determined. A cut set is a group of base events, which, if they all occur, will cause the top event to occur. Using the rules of Boolean algebra, redundant base events can be eliminated and combined so that groups of base events containing the minimum number of events that could cause the top event to occur can be determined. They are called minimum cut sets. An examination of the minimum cut sets aids in identifying the base events that contribute most to the undesired top event and in determining the most effective ways to reduce the likelihood of top event occurrence. 

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