Esthy said:
I don't know why the metal reducing washers are considered suitable for grounding? I don't like that, this involve to scrap the paint from the panel and that will bring corrosion ... yes, I know I can spray some paint over, but why?
If the reducing washers are UL listed to UL514B, then the washers have to pass a resistance test (which is identical to an EMT fitting requirement). The voltage drop across the assembled washers can be no greater than 10 mV at 30A DC. In addition, the washers must be of a certain minimum thickness and made of either aluminum or steel. The steel washer must be protected from corrosion (usually zinc plating or galvanization).
If the washers pass this test, they certainly will pass the appropriate ground fault current test.
Of course, if the panel has paint close to the ID of the knockout/hole, it will need to be cleaned off, to be able to provide proper continuity for the washers. This is just good workmanship. Painting the washer back over is probably not necessary, unless the electrican went nuts with the sandpaper, well beyond the perimeter of the washer OD. Corrosion is only a significant concern if using these in an outdoor location. Paint for this application is probably not a bad idea, but I don't think it required. (I am sure someone will correct me if I am wrong..)
In all due respect, your dislike about these washers not being suitable for grounding has no basis in fact if they are installed correctly. This is similar to the many arguments I have heard of a steel vs. zinc fitting. Some contractors don't 'like' zinc or steel even though they both meet the requirements of UL514B. Some contractors have told me that they 'think' that steel has more strength than zinc. That may be ultimately true, but what does it have to do with the appropriate requirements in UL514B and the NEC?
I tell some folks that if they don't 'think' the requirements are good enough, they should petition UL or the NEC to change them. If you do, be prepared with some convincing substantiation and repeatable testing data. Right now, the specification requirements are what manufacturer's live by. If they change, the product will change accordingly.