Previous NEC editions have recognized that metal elbows in an underground system of rigid nonmetallic conduit are not required to be grounded when isolated from possible contact by a minimum cover of 18 in. With recent changes in the Code allowing for more nonmetallic raceways, this rule now says that metal elbows in an underground system of any nonmetallic raceway need not be grounded. It's the Exception note to 250.80.