electrical
I pulled off a light switch and was going to replace it, but the wiring seems non-standard. The switch is evidently a three way, but has no ground. It has a brass common screw and two traveler screws. See the picture in the box…the black wires are not even being used?!
Wiring diagram for Leviton PR180 3-Way Wall Mount Occupancy Sensor.
Because you are replacing the switch, not installing a new switch, you can get away without a ground. You should install a nonconducting, noncombustible faceplate though.
NEC 2008
404.9 (B) Exception. Where no means exists within the snapswitch enclosure for connecting to the equipment grounding conductor or where the wiring method does not include or provide an equipment grounding conductor, a snap switch without a connection to an equipment grounding conductor shall be permitted for replacement purposes only. A snap switch wired under the provisions of this exception and located within reach of earth, grade, conducting floors, or other conducting surfaces shall be provided with a faceplate of nonconducting, noncombustible material or shall be protected by a ground-fault circuit interrupter.
If you were installing a new switch, you would be required to provide an equipment grounding conductor at the outlet. And the switch would have to be properly grounded, in accordance with the National Electrical Code (NEC).
There was a time when an equipment grounding conductor was not required at each outlet, so it's fairly common to come across this situation (especially when working in older homes). You'll often see exceptions like this written into codes, so as not to require a full rewire just to replace a switch.
If you are replacing a switch a ground is not required, as per the above exception. However, if you're installing a switch; replacement or otherwise, into a metal box that is grounded. The switch will be ground via the devices yoke and mounting screws. So if the metal box is grounded, the switch is also grounded.
If the box is nonmetallic, and there are other grounded devices within the same enclosure. You can ground the new switch using a jumper between the switches grounding screw, and the other devices grounding screw. Just keep in mind, that you can't terminate two conductors under a single screw terminal. So if you do this, you'll have to use pigtails to make the connection between the devices.
Best Answer
The metal box appears to be grounded, so the switch can acquire ground via the mounting screws. (this doesn't work for receptacles).
Here is the crux of how a 3-way circuit works.
Note that your light is probably shifted to the left, i.e. in between the power supply and the first switch.
3-way switches have 2 travelers (on brass screws) and 1 common wire (on black screw). 3-way cables contain 2 travelers and 1 other wire; the function and color of the other wire varies and can be anything.
When it comes to 3-way circuits, identify the travelers (the ones on the brass screws; also in the same cable) and wrap them (both) with yellow tape to re-mark them. This will save you a great deal of time and trouble.
The two yellow-marked travelers will be on the brass screws.
As to why the white is being used as a hot, this appears to be a "switch loop" which is 3-way. The /2 cable carries always-hot (on the white by law) and the switched hot. The /3 cable carries 2 travelers and the switched hot.