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UCA (Unsafe Control Action) table is where you analyze the control actions you defined in the Control Structure Diagram and identify the unsafe ones in a particular context which leads to a hazard.
In order to open an UCA table, a Control Structure Diagram needs to be created first.
If there is a Control Structure Diagram in the project where the Control Actions are defined, click [Identify UCA (Unsafe Control Action) in the [STPA Procedure] tab.
Or go to [Diagram] – [STAMP/STPA] – [UCA Table] to open the UCA table.
Now an UCA table opens with a list of Control Actions that are depicted in a Control Structure Diagram.
Editing the Control Action names and source and target names in this table will apply to ones in the Control Structure Diagram.
There are four ways a control action can be unsafe and these are included in the columns as “guide words” by default:
1. Not Providing – Not providing the control action leads to a hazard.
2. Providing causes hazard – Providing the control action leads to a hazard.
3. Too early / Too late – Providing a potentially safe control action but too early or too late.
4. Stop too soon / Applying too long – The control action is applied too long or is stopped too soon.
By filling out all the 4 columns for each control action, you are testing if the control action meets these requirements: A correct control action is provided at the correct time with correct duration. And if any fails to meet the requirements, you can identify them as Unsafe Control Actions.
In the UCA table, you should specify which hazard that the UCA leads to, and the context in which the control action is unsafe.
We suggest you fill out all the fields in UCA table. So do not leave any fields blank or put N/A even when the Control Action is identified as Non-UCA. Instead, specify the reason why this Control Action is safe.