“Safe” doesn’t mean the object is harmless. It just means that it’s been contained and it is controllable or thought to be controllable, or the conditions in which it does its thing are understood enough to be preventable and it won’t act on its own. I.e. it will not fuck us over doing whatever it does unless someone actively messes with it or its containment system.

Your lever is perfectly safe provided no one touches it.

Unfortunately as Terry Pratchett correctly observed, if you installed a button in the deepest darkest cave and hung a sign on it that reads, “End of the world button, do not touch,” the paint on the sign wouldn’t even have time to dry.