30 CFR § 57.12017
Work on power circuits.
Power circuits shall be deenergized before work is done on such circuits
unless hot-line tools are used. Suitable warning signs shall be posted by the
individuals who are to do the work. Switches shall be locked out or other
measures taken which shall prevent the power circuits from being energized
without the knowledge of the individuals working on them. Such locks, signs,
or preventive devices shall be removed only by the person who installed them
or by authorized personnel.