Unfortunatly, reducing washers are suitable for grounding per UL. (not that I agree they should be)
From the White Book:
Quote
[Outlet Boxes and Fittings Outlet Bushings and Fittings
Guide Information
GROUNDING Metal reducing washers are considered suitable for grounding for use in circuits over and under 250 V and where installed in accordance with ANSI/NFPA 70, "National Electrical Code."
