I've replaced a few duplex outlets in my house and it's got me wondering about whether they are properly grounded. They are in metal boxes and the ground wires are tied together and screwed under a grounding screw in the box. The ground is not screwed to the grounding screw on the outlet itself. The cables are Romex NM. The wiring is not in conduit.

My outlet tester indicates that the outlets are grounded, but I want to understand why. I understand that the boxes are grounded (so if the hot wire somehow contacted the box it would trip the breaker instead of making the box live). Are the outlets grounded because they are screwed to the box? Is the grounding screw only necessary for outlets in plastic boxes?

Thanks,
Matt

 

Your receptacles are grounded because the casing is connected to the box with screw. I would still pigtail the ground wire and attaching it to the receptacle itself (with the power off of course)

 

You must use the grounding screw.

The 6-32 screw holding the recp is not an approved grounding method.