//Home/Methods/Example/Fault Tree

 

Example of methods for safety, reliability and availability analysis - Fault Tree

We are interested in analysing the 415V bus in the simplified power system.

The thumbnail on the right (click to open) depicts a fault tree, modeling the undesired event: "No power on the 415V bus". This event is called the "top event" and is represented by the *AND-gate "415V BUS". In this example, the event: "415V BUS" will be true if both the external grid and the diesel generator fail to deliver power. These two events are represented by the *OR-gates "EXTERNAL GRID" and "DIESEL". The inputs to these gates are basic events representing the failure of the external grid, the isolator switch, the connection switch and the diesel generator.

The isolator switch can cause the top event in two ways. Either by inadvertently disconnecting the external grid or by failing to isolate the power system from the external grid, when it has failed and the diesel generator has started. The latter is modeled under the AND-gate "ISO-A, Isolator switch fail to disconnect the grid". Inputs to the gate are the basic events "ISO-A" and "ISO-D". The basic event "ISO-D" is negated to avoid modeling events with both basic events "ISO-A" and "ISO-D" in them. This would not be a valid combination of failures.

Follow this link to learn what quantitative measures you can calculate in this example.

Back to top
 

Described above are the very basics of fault tree modeling. The fault tree method also include possibilities for modeling boundary conditions using *House events and more advanced NOT-Logic using *NAND, *NOR and *XOR.
House events are "logical switches" that can be either True or False. This makes it possible to "switch" on or off branches in the fault tree and thereby getting different versions of the same fault tree without changing it explicitly.

NOT-Logic is useful for taking care of unwanted combinations of events. For example, redundant standby systems are usually not allowed to be under maintenance at the same time. Using NOT-logic you have the possibility to take this into account in your model.

 

* Basic Event The "root cause" events in a fault tree, for which no further development of the tree logic are made   House Event A house event operates as a "logical switch" that can have only one of the Boolean values TRUE or FALSE (ON/OFF)   Diamond Event Diamond is used as a symbol for basic events representing undeveloped fault tree branches   AND gate All input events TRUE   OR gate At least one input event TRUE   NAND (NOT AND) gate Not all input events TRUE (At least one input event FALSE)   NOR (NOT OR) gate None of the input events TRUE   XOR ( exclusive OR) gate Exactly one input event TRUE   K/N At least K of the N input events TRUE

Back to top