Imagine a trigger guard on a gun, but for a light switch. This guard prevents the switch from being accidentally flipped, and also acts to discourage flipping the switch in general.

In my case, I have a switch in my apartment which turns off the outlet that my cable modem and router are plugged into. This guard allows me to flip the switch when I need to (to reset the router/modem when they're misbehaving), while making it much less likely that someone will inadvertently kill my internet connection when trying to find the switch for the lights.

This is designed to be installed on a switch whose face plate is held on by #6 screws (about 0.137 in or 3.5 mm) which are spaced apart by 2.375 inches (60.3 mm). It allows a clearance under its arch for the switch to stick out by 0.7 in (18 mm) when it is halfway between on and off. I believe this is or was some kind of standard in the US.

If there is popular demand, I will create and upload other versions of this guard with different screw sizes, spacings, and switch clearances...when I get around to it! I have included a STEP file for those of you who have access to proper solid modeling software. I also encourage anyone who does the OpenSCAD thing to create a customizable version of this thing, but please be sure to link back to this original one if you do. Thanks!