I have a distributed control system where the system power source originates from 480 VAC powering a 20 amp 24 VDC power supply. The DC common is grounded at the power supply. 24 volt supply circuits are run to various field or machine mounted components. Some of these cables are part of an industrial machine but some of them are part of the building as they are run in cable trays which interconnect various components throughout the building. My question is: Does an equipment grounding conductor need to be a part of the cable for these circuits, or can a two conductor cable supply the power without a third, equipment ground? If this were a 120 volt AC circuit, i would already know the answer, but I am not sure if it being 24 VDC makes any difference.