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If you have a grounded conduit going in to a metal box (no ground wires), do you need to attach a grounding pigtail to the metal box and then to the outlet ground screw? Or is the metal/metal/outlet screws connection enough to provide grounding?
I ask because I have seen both a grounding pigtail from the outlet to the box and no grounding pigtail from the outlet to the box.
For receptacles, the screws are not enough. That is because you plug in loads, and that load could have a rather significant current draw. That is too much for screws. (and this explains why the screws are enough on switches).
However, there is one thing that is enough. Notice how any common receptacle has a metal "yoke". The ends of the yoke may also have removable drywall ears. Notice also the metal box has metal where the screw goes in. If all these things are true:
Then the hard-flush mating of those bare metal surfaces is an adequate grounding path for the receptacle, and you don't need a ground jumper.
Remember most recpetacles have one hole tapped #10-32 for a ground screw. If you need to wire a ground, you can use that hole, use a grounding clip, or drill and tap your own. If you do, it must be -32 thread of finer; random sheet-metal screws are Right Out. 10-32 is the quasi-standard and cute green 10-32 screws are readily available at any building supply, with or without preattached green pigtails. I'm not a fan of the "with" because they are solid wire, which makes it the only solid wire in my work.
Under current/recent NEC rules I believe the grounding pigtail is required, so that the outlet will still be grounded even if it's not screwed to the box [or because the ground pigtail is regarded as a better connection to the box than the mounting screws are, I'm less sure of the intent than that current rules require the pigtail.]
Consider that if they considered the mounting screws adequate (which they did, for decades, there was no other ground connection on the outlet) you could have a box connected by cable, and tie the ground wire in to the box, yet (I believe we are all pretty sure that) a grounding pigtail to the outlet is required in that case.
