Sigma Magic Help Version 15

Fault Tree Analysis

Overview

Fault Tree Analysis (FTA) is a deductive procedure used to determine failure modes that could cause undesired events at the top level. The deductive analysis begins with a general conclusion and then attempts to determine the specific causes of the conclusion by constructing a logic diagram called the fault tree. The main purpose of the fault tree analysis is to help identify potential causes of system failure before the actual failure occurs. It can also be used to evaluate the probability of the top event using analytical methods. After completing a fault tree analysis, you can focus your efforts on improving the weak links.

The FTA uses some standard symbols to create the diagram which includes the top event, logic gates such as "and" and "or" and different types of events such as a basic event, external event, conditional events, etc. Following are the high-level steps to create the FTA diagram:

• Define the fault condition and write down the top-level failure.
• Use technical information or inputs from your cross-functional team to determine the possible reasons for failure.
• Continue to break down each element with additional gates to lower levels.
• Finalize and review the complete diagram. The chain is terminated at the basic fault: human, hardware, or software.
• If possible, evaluate the probability of occurrence for each of the lowest level elements and calculate the probabilities from the bottom-up.

This tool can be added to your active workbook by clicking on Project and then selecting Failure Analysis > Fault Tree Analysis.

Inputs

Click on the Analysis Setup button on the task menu bar to open the menu options for this tool.

Setup

A sample screenshot of the setup menu is shown below.

1	Num Columns: Specify the number of columns on the worksheet to display. This will format the worksheet for your use. You can always come back and change this setting later. The columns not displayed will be hidden by the software. Do not delete the unneeded columns on the worksheet, only hide them or unhide them.
2	Column Width: Specify the width of the columns on the worksheet. This will resize the cells on the worksheet. The size of the cells will not impact the analysis results but can be used to scale the symbols on the worksheet (if you select the scale checkbox on the task menu bar).
3	View Example: Click on this button to open the example file. You can view the example to get an idea of how to fill out this tool or you can use the example as a starting point and modify it to meet your project needs.
4	Help Button: Click on this button to open the help file on this topic.
5	Cancel Button: Click on this button to exit without saving any changes.
6	OK Button: Click on this button to save changes and update your analysis results.

Worksheet

When the tool is opened, the main menu at the top may change to a task-specific menu as shown below. If this menu does not open, click on the F4 button or the small arrow at the bottom right portion of your menu group to open this menu bar. The first step to perform the fault tree analysis is to create the fault tree diagram. To create this diagram, first, you need to add the top event. Note that each fault tree diagram should have one and exactly one top event. This is the main event for which you are creating the fault tree diagram. Next, add the remaining symbols to the diagram based on your process and then link the different symbols together using the And and Or logic. Use the following steps to create the FTA diagram:

• Click on the symbol you are interested in on the task menu bar and then click on the worksheet to place the symbol on the worksheet. Note that you can use the Center checkbox to ensure that the symbol is centered in the work cell and the Scale checkbox to scale the symbol to fit within the work cell. This icon will then be added to the worksheet.
• Once all the symbols are added, you can join them using the straight or elbow connectors. Note that all the symbols have to be connected otherwise you will get an error on solving the fault tree diagram.
• Once the symbol is added to the worksheet, you can click on the Data button to add the data for each event. A sample screenshot of this dialog box is shown below. You will be able to enter a brief description of the event and enter the probability values for events if you choose to perform probability calculations. If you select the checkbox next to the item, then that item will be displayed on the worksheet.
• Note that you can only add probability data for the leaf events (at the end of the fault tree) such as basic events, other events, external events, conditional events, etc. The probability values for top events and intermediate events are automatically calculated from the remaining values.

6	Event Id: Specify a unique element id for this element.
7	Checkbox: Click on the checkbox if you want this item to be displayed on the worksheet.
8	Description: Specify a brief description for this element.
9	Probability: Specify the probability value for this element.

Charts

If you click on the Charts button, you will see the following dialog box.

1	Title: The system will automatically pick a title for your chart. However, if you would like to override that with your own title you can specify a title for your chart here. Note that this input is optional.
2	Sub Title: The system will automatically pick a subtitle for your chart. However, if you would like to override that with your own subtitle you can specify a subtitle for your chart here. Note that this input is optional.
3	X Label: The system will automatically pick a label for the x-axis. However, if you would like to override that with your own label for the x-axis you can specify a different label here. Note that this input is optional.
4	Y Label: The system will automatically pick a label for the y-axis. However, if you would like to override that with your own label for the y-axis you can specify a different label here. Note that this input is optional.
5	X Axis: The system will automatically pick a scale for the x-axis. However, if you would like to override that with your values for the x-axis, you can specify them here. The format for this input is to specify the minimum, increment, and maximum values separated by a semi-colon. For example, if you specify 10;20 then the minimum x-axis scale is set at 10 and the maximum x-axis scale is set at 20. If you specify 10;2;20, then, in addition to minimum and maximum values, the x-axis increment is set at 2. Note that this input is currently disabled and you will not be able to change this setting.
6	Y Axis: The system will automatically pick a scale for the y-axis. However, if you would like to override that with your values for the y-axis, you can specify them here. The format for this input is to specify the minimum, increment, and maximum values separated by a semi-colon. For example, if you specify 10;20 then the minimum y-axis scale is set at 10 and the maximum y-axis scale is set at 20. If you specify 10;2;20, then, in addition to minimum and maximum values, the y-axis increment is set at 2. Note that this input is optional.
7	Horizontal Lines: If you want to add a few extra horizontal reference lines on top of your chart you can specify the values here. The format for this input is numeric values separated by semi-colon. For example, if you specify 12;15 then two horizontal lines are plotted at Y = 12 and Y = 15 respectively. Note that this input is optional.
8	Vertical Lines: If you want to add a few extra vertical reference lines on top of your chart you can specify the values here. The format for this input is numeric values separated by semi-colon. For example, if you specify 2;5 then two vertical lines are plotted at X = 2 and X = 5 respectively. Note that this input is optional.

Verify

If you click on the Verify button, the software will perform some checks on the data you have entered. A sample screenshot of the dialog box is shown in the figure below. The objective of this analysis as well as any checks that are performed is listed in this dialog box. For example, the software may check if you have correctly specified the input options and entered the required data on the worksheet. The results of the analysis checks are listed on the right. If the checks are passed, then they are shown as a green-colored checkmark. If the verification checks fail, then they are shown as a red-colored cross. If the verification checks result in a warning, they are shown in the orange color exclamation mark and finally, any checks that are required to be performed by the user are shown as blue info icons.

Outputs

Click on Compute Outputs to update the output calculations. A sample screenshot of the worksheet is shown below. If any errors are encountered during the analysis, they are reported at this point. Following are some possible errors that you may encounter:

• If you have not defined a top event or if you have multiple top events. For each FTA diagram, there should be exactly one top event.
• If you have not connected the icons properly. Make sure that the connectors are tied to the icons and not just placed next to the icons.
• If you have incorrectly drawn the diagram, such as having no children for some icons, having too many children, etc.

If you specify the probability values, then the software will create a bar chart of all the probabilities for the events. You can use this chart to determine which events have a high probability of failure and which events have a low probability of failure. If no errors are found in your diagram, then an OK text message is displayed at the top of the worksheet. An example screenshot of analysis with probability analysis is shown in the figure below.

Notes

Here are a few pointers regarding this analysis:

• You can create FTA diagrams to any level, but the probability calculations are done up to 100 levels deep. For all practical purposes, this implies there are no restrictions on the size of the FTA diagram.
• You can manually add these icons to your FTA diagram to build the diagram, however, the probability calculations currently are only limited to OR and AND gates. It is recommended that you create your FTA diagram using the basic gates in Sigma Magic software.
• If you don't enter any probability values for any of the elements, then the overall probability graphs are not displayed.
• Do not use Ctrl-C and Ctrl-V to copy and paste elements onto the FTA map since each element has a unique id and manually copying and pasting items will result in duplicate entries. If you need to add an element, you must click on the element on the top menu bar.

Examples

Following examples can be found in the Examples folder.

• Create a fault tree diagram using the information given in the reference file. (FTA 1.xlsm).
• Create a fault tree diagram using the information given in the reference file. (FTA 2.xlsm).