Sorry if I posted pics that were less than perfect, and showed other issues as well
. Send me another pic, and I'll put that in; I guarantee that I wasn't thinking of this issue when I took the pics, and both are cropped from much larger pictures.
(It is worth noting that the 'two screw' requirement - which says nothing about using two nuts as well - is also a fairly recent addition to the code. That's another discussion).
Another variation of this issue involved the "nuts" used by one manufacturer. Rather than being any type of standard hardware, they are little clips that you slip over the yoke. These gizmos further drive home the point that you can't inspect for this without taking things apart. It's one thing to call for inspection before you fill a ditch; but to call before closing every box is absurd.
There's not a product in existance that doesn't present additional hazards once you start taking it apart. Removing the cover is not something that happens in the normal use of a receptacle. Moreover. its not true if the ground wire terminates at the device, with no direct attachment to the box. As I said before, bonding works in both directions.
Sparkyinak again stresses the problem with the 'listing language.' I'm not sure there are any devices, covers, or boxes specifically listed for use with each other. Rather, all are made to NEMA specs that define dimensions, etc., and that has been good enough until now. If the code panel wanted to say 'must be manufactured and listed as an assembly,' they need to say so.
I disagree that the handbook states that the code is not discussing these covers. The language of 250.146(A) seems specifically written for these covers, and the handbook illustrations show these types of covers. I can't imagine what else the panel had in mind, unless it was complete, manufacturered assemblies - in which case they need to say so.
These covers are very common; if the panel forgot they existed, then maybe they need to get out of the office more often.
Let's remember how this whole thing got started.
At one time - mayne for the 96 edition - there was a fear expressed thet using the mounting screws alone as a means to bond an outlet to the box was not a very good idea. Now, this discussion had been going on since at least the 60's, with all manner of discussions about fiber washers vs. metal clips, etc. Since the code allows an 1/8" gap between the mud ring and the face of the wall, though, the discussion focused on using the #6 mounting screws as the only bonding attachment. At that time, the jumper requirement was added for receptacles, but not switches.
With that change, the jumper was not required if there was solid yoke to box contact. To make that more certain, the next change required the box to be mounted on the surface; that way, there was no doubt that there was solid yoke to box contact.
Then the fear was expressed that, with these 'industrial' covers, we were relying on just the one screw, the one holding the receptacle to the faceplate, to make our bond, and this screw might come loose. That's when we got the requirement for there to be two screws.
Now it appears someone felt that there just wasn't enough contact between some covers and the box, so this 'flat, non-raised' mumdo-jumbo gets added.
In a few short years, we have gone from the jumper being the exception to a view that you need a jumper, no matter what - and it's all been done one step at a time.