Fault Tree Symbols and Logic

The fault tree uses logic gates to describe graphically how the top event occurs. You read a fault tree from the top event down to the constituent events. The higher gates are the outputs from lower gates in the tree. Therefore, the top event is the output of all the input faults or events that occur. 

Spend time clearly understanding the difference between fault and failme. When 1 review fault trees, that is one of the biggest mistakes that 1 frequently find. 1 have seen many fault trees filled with failures but few faults. 

The system or subsystem fault is an undesirable state of existence of the system or subsystem. That system fault (the top event) is comprised of component (or subsystem) faults. It depends to what level of detail you wish to delve whether the FTA faults are to the component, black box, or subsystem level. The fault tree of a nuclear plant is very large even at the subsystem level. However, it would be advantageous to go to the component fault level to analyze the plant safety subsystems. 

Fault trees are very diverse and can be used in many ways. They are one of my most favorite safety analysis tools. I have used them for such diverse activities as understanding integrity management of an upstream oil pipeline system, employee and management actions taken during a plutonium spill at a laboratory, and the Sydney, Australia, Waterfall rail accident investigation. 

The component fault is the state of existence of that component that contributes to the mechanism that leads to the next-level fault. In understanding what the component fault is, it is important to consider what the component state is in and when it is in that state of existence. Component faults are comprised of primary, secondary, and command faults. However, most primary and secondary faults are comprised of component failures, so they are usually called primary and secondary failures. 

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