Since 2019, the North American Elevator Safety Code (ANSI/ASME A17.1 / CSA B44) has defined requirements for detecting persons and objects between the doors (2D) or approaching the elevator entrance (3D).
Slack cable switches are used to detect the suspension tension on vertically moving doors. These stop the motorized drive as soon as the door movement is obstructed, e.g. if the suspension cable slackens.