The case of the covered conductor

Unless required elsewhere in the Code, equipment grounding conductors are permitted to be bare, covered or insulated according to Section 250.119, Identification of Equipment Grounding Conductors. Individually covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes except as permitted in this section. Really nothing is wrong here, except that a covered conductor is encased within material of composition or thickness that is not recognized by the Code as electrical insulation. The symbol for the grounding connection is also known as the “reverse Christmas tree” according to the UL Directory (White Book).