said by Jack_in_VA:What's with the concentric knockouts?
It was an example of how important a good path to ground is and the fact that even though a concentric knockout provides a metallic connection it is insufficient to be trusted to handle fault currents.
Now if a concentric knockout is insufficient, doped up threads are truly insufficient.
250.92 Services.
Bonding jumpers meeting the other requirements of this article shall be used around concentric or eccentric knockouts that are punched or otherwise formed so as to impair the electrical connection to ground. Standard locknuts or bushings shall not be the sole means for the bonding required by this section.
Wayne