30 CFR § 77.501
Electric distribution circuits and equipment; repair.
No electrical work shall be performed on electric 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.