(a) The Commission, upon the filing of a petition for a proposed change to any rate, may within 60 days after said filing:
(1) Suspend the operation of such rate change for a period not to exceed 7 months after said filing; provided, however, that if the Commission has not reached its decision within said 7 months after filing, the public utility may place their rate into effect under bond in accordance with subsection (b) of this section;
(2) Determine that a portion of such change shall become effective not later than 60 days after the filing of the petition on a temporary basis pending the final decision of the Commission.
(b) Upon termination of the 7 months as set forth in paragraph (a)(1) of this section the proposed rate change shall automatically become effective if the public utility files with the Commission a bond in a reasonable amount approved by the Commission with sureties approved by the Commission, conditioned upon the refund, in a manner to be prescribed by order of the Commission, to the persons entitled thereto of the amount of the excess, if the rate so put into effect is finally determined to be excessive; or there may be substituted for such bond other arrangements satisfactory to the Commission for the protection of the parties interested. In no event shall a public utility put a rate into effect under bond as authorized in this subsection that would constitute an increase in excess of 15 percent of the public utility's gross intrastate operating revenues.
(c) Notwithstanding subsections (a) and (b) of this section, 60 days after said filing, a public utility may put a rate into effect under bond as authorized in subsection (b) of this section, provided that the increase does not constitute an increase in excess of 15 percent of the public utility's annual gross intrastate operating revenues or $2,500,000 annually, whichever is less. This subsection shall not apply to any proposed rate change sought by a public utility under regulations adopted pursuant to § 304(b) of this title.