Grounded DC Motors

The question is unclear. Do you mean

"Do the metal enclosures of DC motors need to be earthed? If so, why?"

and

"Does the negative wire of a DC motor need to be connected to earth? If so, in what situations?"

"The question is unclear"

You are a master of understatement.

The purpose of ground/earth is personnel protection under fault conditions. If any part of the supply system is referenced to earth, then yes, the casing of the motor may need to be grounded/earthed.

The body reacts differently under shock conditions depending on whether the shock is AC or DC. If the shock is AC, there is a chance that the muscles that are brought into spasm by the shock may release the body from the shock twice per cycle, i.e. when the voltage applied to them goes through the 0V points. The same cannot be said for DC where there is no passage through a 0V point.

The voltage of this DC system has not been shared with the forum.

Make sure that the ingress protection rating for all parts of the DC system exceeds IP2x.

Grounding the equipment and Grounding the system are two different items.

All those parts of an enclosure of an electrical equipment should be kept at ground potential so that a person or animal coming in contact is not getting electrocuted. This is ensured by bonding all those parts of enclosure and effectively connecting to earth grid. Grounding the motor means bonding and earthing enclosure for ensuring safety of personnel and animals. This is Grounding of equipment.

Grounding system refers to Neutral earthing or system earthing. This does not apply to DC system.

I do not know whether I have made clear the point. Request your feedback please.

My comment(No.4) is seen as posted from 'guest',since I forgot to login before posting.