We also have to think about 250.96
quote:250.96 Bonding Other Enclosures.
(A) General. Metal raceways, cable trays, cable armor, cable sheath, enclosures, frames, fittings, and other metal non–current-carrying parts that are to serve as grounding conductors, with or without the use of supplementary equipment grounding conductors, shall be effectively bonded where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed on them. Any nonconductive paint, enamel, or similar coating shall be removed at threads, contact points, and contact surfaces or be connected by means of fittings designed so as to make such removal unnecessary.
I keep a cheep wire wheel in my cordless case for removing paint when needed.
In Soares - Book on Grounding - Chap. 8 (8th ed.) there is reference to a UL report showing virtually all conduit fittings perform satisfactorily under fault conditions on painted surfaces. There is an exception: die cast fittings using locknuts having flat or ribbed surfaces did not perform as well. Quote (p. 130) "It appeared as though locknuts with serrations consistently penetrated through the enclosure paint and provided better electrical contact between the fitting and the metal of the enclosure than did the locknuts with flat or ribbed surfaces." ...and "All of the conduit fittings that were constructed of steel bodies and steel locknuts completed the test with acceptable results."
This UL test was done in 1992, I don't know for sure, but I haven't seen the flat or ribbed type locknuts in quite a while.
