I am not an electrician, this is what I found googling. The bolding was added by me to highlight the comment about the metal box. The metal screw between the device and the metal box is the ground.
Also, I did buy grounding wires that have the correct screw and the metal box has places inside to place the screw. But since these boxes were already EXTREMELY tight, placing more wires into the box was a problem.
New info, Better answer
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.
Well if you notice in the forums here people asking about smart switches that do NOT require neutrals because there are no neutral wires in the boxes. Well, if they do not have neutral wires there is a good chance they do not have ground wires either.
actually I’ve seen plenty of switches without a neutral and every last one has a ground.
A regularly used wiring method was/is to run the power to the device(outlet, light, whatever) along with neutral and ground- but rather than connect the hot (typically black) to the device you run a 2 conductor (which in modern days is really 2 + a ground) from that box to the switch box. You connect one of those conductors (say black) to the hot at the device’s box then to one end of the switch, you connect the white to the other end of the switch (and many will wrap the white with tape to indicate it is NOT a neutral) then run that back to actually power the device.
There was never a need to put the neutral in the switch box until smart switches came along, so it was cheaper to leave it out since 2 conductor (2 plus ground) is cheaper than 3 conductor (3 plus ground). In a subdivision all those pennies add up.
Also If the switch is farther from the power source than the load then you would waste wire running the neutral to the switch and back. Or if they ran a 3 conductor the neutral would just be capped in the box anyway until smart switches came along.
3 way switches runs also can get wired without a neutral depending on the topography of the house.
JD, I am considering a Decora deep switchplate from Kyle, paired with a plastic 3-gang extender. But, the Decora plate is metal. Won’t that restrict my performance? I have the new GE Z-Wave plus all over the house.
It might, it might not. Zwave transmits 360°, so the signal might come out through the plastic, it might come out through the back of the switch, it might go down through the wall and reach an outlet or even a device one floor down that way.
Even if your backbox is all metal, which is somewhat unusual these days, just the plastic extender itself might let the signal out.
Sometimes you just have to try it and see, being aware that the metal may be an issue.