For a grounded system, Section 250.24(B) requires an unspliced main bonding jumper to connect the equipment grounding conductors and the service-disconnect enclosure to the grounded conductor within the enclosure for each service disconnect in accordance with 250.28.

According to NEC^® Section 250.24(A)(5), A grounded conductor cannot be connected to normally non–current-carrying metal parts of equipment, to equipment grounding conductors, or be reconnected to ground on the load side of the service disconnecting means unless permitted elsewhere in Article 250.

Inside the main service disconnect enclosure, grounded conductors are permitted to have continuity with the metal enclosure and equipment grounding conductors.

Downstream of the grounded conductor connection inside the service disconnect, all grounded conductors must be isolated from any contact with metal enclosures and equipment grounding conductors of the electrical system. This ensures that if a phase to neutral fault occurs in a feeder or branch circuit, the greatest amount of fault current will return on the grounded neutral conductor which will facilitate a faster opening of the overcurrent device protecting the circuit.

If there are multiple paths that fault current can take because grounded neutral conductors have been connected to equipment grounding conductors downstream of the service disconnect, then the amount of return fault current on the grounded conductor will be diminished and spread out among the other parallel paths. A lesser amount of returning fault current on the grounded neutral conductor results in a slower opening time of the overcurrent device installed to protect against the fault condition.

For complete code sections, refer to the actual NEC^® text at NFPA.ORG. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.