One main rule for grounded electrical systems is that all metal enclosures and raceways for service conductors and equipment must be connected to the grounded system conductor.

This rule applies to all metal enclosures and raceways containing service conductors, including metal pull elbows that are installed in a run of underground nonmetallic service raceway.

The exception allows an isolated metal component to remain ungrounded if it has a minimum cover of 18 in. to all parts of the metal component.

Similar language can be found in 250.86 pertaining to isolated metal components used in underground runs of nonmetallic raceways, other than service raceways.

Below is a preview of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC^® edition of NFPA 70.

2020 Code Language:

250.80 Service Raceways and Enclosures. Metal enclosures and raceways for service conductors and equipment shall be connected to the grounded system conductor if the electrical system is grounded or to the grounding electrode conductor for electrical systems that are not grounded.

Exception: Metal components that are installed in a run of underground nonmetallic raceway(s) and are isolated from possible contact by a minimum cover of 450 mm (18 in.) to all parts of the metal components shall not be required to be connected to the grounded system conductor, supply side bonding jumper, or grounding electrode conductor.