Except as otherwise limited in this Chapter, in all proceedings instituted by the Commission for the purpose of investigating any rate, service, classification, rule, regulation or practice, the burden of proof shall be upon the public utility whose rate, service, classification, rule, regulation or practice is under investigation to show that the same is just and reasonable. In all other proceedings the burden of proof shall be upon the complainant. (1949, c. 989, s. 1; 1963, c. 1165, s. 1; 1985, c. 676, s. 8.)