I'm trying to install a new light switch from a plastic one to a metal one.
I don't have a ground wire nor is the back box grounded.
Do I need to connect it to a ground?
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Assuming you are in North America: When boxes were steel and the wiring was run with metal conduit or BX cable (flexible metal armoring wrapped around the wires instead of a plastic or fabric jacket), grounding was done via the metal-to-metal contact of the switch to the box, the box to the conduit and the conduit to the breaker panel, which was itself bonded to ground. If you have removed a steel box and replaced it with a plastic one, but the wiring is still in the BX cable, then you can get a fitting for attaching the BX to the box that allows you to attach a ground wire (grounding locknut). But technically, it would not meet current code, because the old BX cable no longer meets the requirements for continuous grounding. NEWER versions, now called "Type AC" cable (for Armor Clad) have a different design that meets code grounding requirements.
For a while, code allowed for switches to not have any ground connection though, so you would have NM (Non Metallic) cable without a ground wire used on them. In that case if you are replacing like-for-like, you can ignore the ground connection on a new switch, there is a Code exception under article 404.9(B) allowing for that.