07-17-2021, 06:05 AM
The box being plastic is irrelevant. The grounding wires on the switches should be attached to a ground wire coming from the main breaker panel. For all except the oldest houses, this will be either a third wire in the cable that enters the box, or in some older homes the armored cable itself constitutes the ground path. For a light switch the ground is probably less important than a receptacle or wired in device of some kind so if there is no ground wire in the cable entering the box I wouldn't worry about it.