30 CFR §18.45

1. (a)A self-propelled machine, that receives electrical energy through a portable cable and is designed to travel at speeds exceeding 2.5 miles per hour, shall have a mechanically, hydraulically, or electrically driven reel upon which to wind the portable cable.
2. (b)The enclosure for moving contacts or slip rings of a cable reel shall be explosion-proof.
3. (c)Cable-reel bearings shall not constitute an integral part of a circuit for transmitting electrical energy.
4. (d)Cable reels for shuttle cars and locomotives shall maintain positive tension on the portable cable during reeling and unreeling. Such tension shall only be high enough to prevent a machine from running over its own cable(s).
5. (e)Cable reels and spooling devices shall be insulated with flame-resistant material.
6. (f)The maximum speed of travel of a machine when receiving power through a portable (trailing) cable shall not exceed 6 miles per hour.
7. (g)Diameters of cable reel drums and sheaves should be large enough to prevent undue bending strain on cables.