30 CFR § 57.12016
Work on electrically-powered equipment.
Electrically powered equipment shall be deenergized before mechanical work is
done on such equipment. Power switches shall be locked out or other measures
taken which shall prevent the equipment from being energized without the
knowledge of the individuals working on it. Suitable warning notices shall be
posted at the power switch and signed by the individuals who are to do the
work. Such locks or preventive devices shall be removed only by the persons
who installed them or by authorized personnel.