There are two ways how this could be done. Depending if you would like to execute dynamic or static calculation.

For dynamic simulation (RMS) - Right mouse button selection on the element (for example line) -> Define -> Switch Event ... . In the new open window select when this interruption should occur and which phase will be affected by it. Execute the RMS calculation command and plot the results. Have a look at the first attached png file.

For static calculation - define new Switch Event inside of the Short Circuits folder in the active study case. After switching event is being defined open the short circuit calculation command. Select "complete" method and activate the Multiple Faults option. Select the event under Short-circuits option and press execute. Have a look at the second and third png file.