Metal cable trays, metal raceways, and metal enclosures for conductors shall be grounded, except that:
Metal enclosures such as sleeves that are used to protect cable assemblies from physical damage need not be grounded; and
Metal enclosures for conductors added to existing installations of open wire, knob-and-tube wiring, and nonmetallic-sheathed cable need not be grounded if all of the following conditions are met:
Runs are less than 25 feet (7.62 m);
Enclosures are free from probable contact with ground, grounded metal, metal laths, or other conductive materials; and
Enclosures are guarded against employee contact.