Control Loop Diagram is a diagram to show a control loop with actions and feedback. By creating a control loop diagram of “Unsafe Control Actions”, it will help you identify the hazard causal factor.

Control Loop Diagram
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How to create a Control Loop Diagram

To create a Control Loop Diagram, open a Control Structure Diagram, right-click on a link or Control Action and click [Generate Control Loop Diagram] from the context menu.
How to create a control loop diagram

Then a new Control Loop diagram will be generated with two Components that the selected Link/Control Action connects. Other related Components will be shown as a small boxes.
Control Loop Diagram Sample

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Open Hint Words

There is a [Hint Word Set] menu on the toolbar of the diagram. Clicking [Hint Word Set] will open up a window with hint words (guide phrases) and the structure. Use this hint words to analyze this loop to find out what makes this control action unsafe and identify the hazard causal factor.
Hint words on control loop diagram

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Switch Hint Word Set

You can switch the Hint Word that fits your analysis the most.
To do so, click [Hint Word Set] on the toolbar to open the Hint Word.
Hint words on STPA Control Loop Diagram

And switch to one you want to use.
Change Hintword set

There are two pre-installed Hint Word sets in Astah System Safety.

You can customize the Hint Words and also create your own and select a default set.
To do so, please refer to Hint Word.