STL files for Hardware items

What I find with STL libraries is most of them are based on popular need. You find files that are based on a need by the general public. Such as parts for well known printers, parts for Ikea furniture, parts for things that commonly break.

There would be no need for a 3D printed circuit breaker as it would fail under natural heating and due to the way the fibers lay would make for poor electrical isolation. Now if you just needed a model of one then you could print one but again due to the fact that one would not work you will be hard pressed to find one. The other issue with STL libraries is actual print-ability. Many of these sites like Thingiverse began as just a 3D library with no intention of models being printed. So even still much of the stuff submitted there is actually unable to be printed due to complicated geometry or the artist just not knowing what is needed to make a file/model printable.

I know of no repository dedicated to hardware or even "practical " Home gadgets.
There are a few museums and government agency with repositories ie the metropolitan museum of art and NASA.

You may want to try searching youmagine it is a similar to thiniverve bur sponsored bu ultimaker
alterantively try doing  google search for practical 3d printed items there are lits from 5 to 25 items.

I am not sure if I would pt things like home safety in the realms of 3d printing not sure we are there yet. but i have seen lock for garden shed gates soap dishes etc.
Try  seaching youmagine for what you are looking for.

I would never print anything like a breaker I was just using that as an example.