“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.

Don’t forget the locked box test it explains the classes pretty good

From the wiki

“The Locked Box Test is an informal guideline used to determine an object’s most appropriate Object Class. It goes like this:

• If you lock it in a box, leave it alone, and nothing bad will happen, then it’s probably Safe.
• If you lock it in a box, leave it alone, and you’re not entirely sure what will happen, then it’s probably Euclid.
• If you lock it in a box, leave it alone, and it easily escapes, then it’s probably Keter.
• If it is the box, then it’s probably Thaumiel.

Extending the analogy to other Object Classes:

• If it used to need a box, but no longer does, it’s Neutralized. If this is because of a deliberate decision by the Foundation, it was probably Decomissioned.
• If it turns out it never needed a box in the first place, it’s probably Explained.
• If you can’t fit it in a box and it’s about to end the world, then it’s probably Apollyon.
• If you could have locked it in a box but chose not to, then it’s probably Archon.
• If you can’t fit it in a box but that’s not a big deal, then it’s probably Ticonderoga.
• If you’re not sure what box to put it in yet, it’s probably Pending.”