The termination of equipment grounding and bonding conductors must be by exothermic welding, listed pressure connectors of the set screw or compression type, listed clamps, or other listed fittings. Sheet-metal screws cannot be used to connect grounding (earthing) or bonding conductors or connection devices to enclosures.
Obervations- As Mr. Holt so well put on this…note that drywall screws or wood screws are not mentioned here as being prohibited in the 2002 NEC.
When the HI is browsing around in the ole’ electrical panel have you ever seen nice shinny sheet metal screws wrapped with a EGC or on in the older panels when they TRY to add EGC’s doing this…
Why it is a problem ( and personally why the wood screws and drywall screws are also wrong in my opinion even though the NEC does not call it out ) is because the threads on these types of screws to not make enough contact with the enclosure to ensure a good bond…pre-threaded holes do with at all points within the thread count…but not with the screws listed above.
In a situation where their is a fault and it travels through that "sheet metal " screw the lack of contact could provide a less efficiant bond…and could actually loose the effective bond.
Just figured you all would like to know…Educational Moments…
Images Courtesy of Mr. Mike Holt