Complaints may be made by the Commission on its own motion or by any person having an interest, either direct or as a representative of any persons having a direct interest in the subject matter of such complaint by petition or complaint in writing setting forth any act or thing done or omitted to be done by any public utility, including any rule, regulation or rate heretofore established or fixed by or for any public utility in violation of any provision of law or of any order or rule of the Commission, or that any rate, service, classification, rule, regulation or practice is unjust and unreasonable. Upon good cause shown and in compliance with the rules of the Commission, the Commission shall also allow any such person authorized to file a complaint, to intervene in any pending proceeding. The Commission, by rule, may prescribe the form of complaints filed under this section, and may in its discretion order two or more complaints dealing with the same subject matter to be joined in one hearing. Unless the Commission shall determine, upon consideration of the complaint or otherwise, and after notice to the complainant and opportunity to be heard, that no reasonable ground exists for an investigation of such complaint, the Commission shall fix a time and place for hearing, after reasonable notice to the complainant and the utility complained of, which notice shall be not less than 10 days before the time set for such hearing. (1949, c. 989, s. 1; 1963, c. 1165, s. 1.)