N.D. Cent. Code § 49-06-06
If no new rate shall have been agreed upon, as provided in section 49-06-05, then pending the investigation and final order of the commission, if it is of the opinion that public interest so requires, the commission immediately shall make an order that the utility shall file with the commission a bond of a corporate surety company, approved by the commission and authorized to do a surety business within this state. The bond shall be payable to the commission for the use and benefit of the customers and patrons of the utility and shall be conditioned that if the rates fixed, determined, and prescribed by the final order are less than the rates charged, received, and collected by the public utility during the period of the investigation, the public utility as principal shall remit to the several customers, patrons, or users of its service during the period between the date of the order directing a valuation or revaluation and the date of the final order fixing, determining, and prescribing the rates to be charged, received, and collected by the utility company the amount payable under this section. If the utility does not remit to its customers and patrons the differences between the amount paid by them and the new rate prescribed by the final order, the commission may maintain an action on the bond for the benefit of the customers and patrons. The amount to be paid and remitted to each of the several customers, patrons, or users shall be such sum as such customer, patron, or user has paid to the utility for the service over and above the amounts that such customer, patron, or user would have paid during that time had the rates fixed, determined, and prescribed in the final order been in effect during that period. The bond or undertaking shall be filed with the commission within thirty days after the service of the order upon such public utility. Service of the order may be made by personal service upon the public utility or by registered or certified mail, and if by registered or certified mail, service shall be deemed completed when the registered or certified mail is delivered to the public utility, as evidenced by the return receipt for the mail.
N.D.C.C. § 49-06-06