30 CFR § 75.511
Low-, medium-, or high-voltage distribution circuits and equipment; repair.
No electrical work shall be performed on low-, medium-, or high-voltage
distribution circuits or equipment, except by a qualified person or by a
person trained to perform electrical work and to maintain electrical
equipment under the direct supervision of a qualified person. Disconnecting
devices shall be locked out and suitably tagged by the persons who perform
such work, except that in cases where locking out is not possible, such
devices shall be opened and suitably tagged by such persons. Locks or tags
shall be removed only by the persons who installed them or, if such persons
are unavailable, by persons authorized by the operator or his agent.