30 CFR § 75.907
Design of trailing cables for medium-voltage circuits.
Trailing cables for medium-voltage circuits shall include grounding
conductors, a ground check conductor, and grounded metallic shields around
each power conductor or a ground metallic shield over the assembly, except
that on equipment employing cable reels, cables without shields may be used
if the insulation is rated 2,000 volts or more.
Subpart K--Trolley Wires and Trolley Feeder Wires