Must be properly grounded. Unused openings must be sealed. Must be permanently mounted on the wall or a work-bench. Knock-out boxes are NOT permitted for use with extension cords or pendant devices. NEC 250-45, which requires grounding of certain cord connected equipment, does not include this type of equipment.
Some clocks come with three wire cord.
These clocks provide an extra measure of safety that, while not required by NEC, can prevent injury to persons who may come in contact with the metal parts of the clock.