220 ILCS 5/8-102
The Commission is authorized to conduct or order a management audit or investigation of any public utility or part thereof. The audit or investigation may examine the reasonableness, prudence, or efficiency of any aspect of the utility's operations, costs, management, decisions or functions that may affect the adequacy, safety, efficiency or reliability of utility service or the reasonableness or prudence of the costs underlying rates or charges for utility service. The Commission may conduct or order a management audit or investigation only when it has reasonable grounds to believe that the audit or investigation is necessary to assure that the utility is providing adequate, efficient, reliable, safe, and least-cost service and charging only just and reasonable rates therefor, or that the audit or investigation is likely to be cost-beneficial in enhancing the quality of service or the reasonableness of rates therefor. The Commission shall, before initiating any such audit or investigation, issue an order describing the grounds for the audit or investigation and the appropriate scope and nature of the audit or investigation. The scope and nature of any such audit or investigation shall be reasonably related to the grounds relied upon by the Commission in its order.
Any audit or investigation authorized pursuant to this Section may be conducted by the Commission, or if the Commission is unable to adequately perform the audit or investigation, the Commission may arrange for it to be conducted by persons independent of the utility and selected by the Commission. The cost of an independent audit shall be borne initially by the utility, but shall be recovered as an expense through normal ratemaking procedures. Any audit or investigation shall be conducted in accordance with generally accepted auditing standards.
220 ILCS 5/8-102