When the door/access (to a cell, for instance) is closed, the upper keys stay trapped while the lower cylinder is empty. As the lower key is inserted and turned, the door/access can be opened. As long as it is open, this key will stay trapped. In order to continue with the interlocking sequence, the upper keys need to be released. The lower key won’t be able to turn and lock the door until the upper ones are inserted and trapped.

 Specifications

Design