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I'm replacing this simple 2-connector light switch with a 4-wire dimmer. The 2 new wires present on the dimmer are neutral and ground, and only neutral has an obvious home. The box has no visible ground wires. Can I safely ground the dimmer to a box screw or leave the dimmer ground wire disconnected?
That wiring is AC cable, modern AC cable. I can clearly see the bonding strip that makes the sheathing a grounding conductor. So since a grounding means exists you must use it.
With switches, simply screwing them to a grounded metallic box grounds them, as opposed to a receptacle which would need to be a self-grounding type. So technically, just installing the switch would be adequate, but since there is a ground wire from the dimmer I would connect it to the box.
NEC says that you can replace a switch where no grounding conductor exists, though you'll have to use a nonconducting, noncombustible faceplate. For clarity, I'd leave the grounding conductor from the switch disconnected. That way if anybody comes along in the future, they won't be confused and think the enclosure is grounded.
National Electrical Code 2014
Chapter 4 Equipment for General Use
Article 404 Switches
I. Installation
404.9 Provisions for General-Use Snap Switches.
(B) Grounding. Snap switches, including dimmer and similar control switches, shall be connected to an equipment grounding conductor and shall provide a means to connect metal faceplates to the equipment grounding conductor, whether or not a metal faceplate is installed...
Exception to (B): 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 this box does indeed have a grounding conductor attached, as @Speedy Petey suggests. Then you can use a grounding screw, or a grounding clip to attach the grounding conductor from the device to the box.
The clip attaches to the edge of the enclosure, while the screw can be screwed through the threaded hole in the back of the box.
