Robotics and automation

Two sentences on a safety sign look alike and say different things. Die Schutztür darf nicht geöffnet werden is a rule: the door would open, and somebody has forbidden it. Die Schutztür lässt sich nicht entriegeln is a fact about the hardware: try it, and nothing happens. The second pattern is sich lassen plus an infinitive, and German reaches for it wherever English says that something can or cannot be done to a thing. The thing becomes the subject, sich is compulsory, and the infinitive goes to the end: Die Zuhaltung lässt sich erst entriegeln, wenn alle Achsen stehen. Read a risk assessment with this in mind and the two verbs sort themselves out. Dürfen describes what people are allowed to do; sich lassen describes what the machine will let them do. A cell is only safe where the second is as strict as the first, and a guard that sich überbrücken lässt is a rule and nothing more.