Subpenas requiring the attendance and testimony of witnesses, and subpenas requiring the production of any books, papers, schedules of charges, contracts, agreements, and documents relating to any matter under investigation or hearing, may be signed and issued as follows:
(a) Hearings before the Commission en banc, an individual commissioner, or a panel of commissioners: By any commissioner participating in the conduct of the hearing.
(b) Hearings before an administrative law judge: By the administrative law judge or, in his absence, by the Chief Administrative Law Judge.