Sec. 24. (a) The board of a municipally owned utility, as defined by IC 8-1.5-3-2, shall, at least semiannually, set aside from the net earnings a sufficient amount to pay the interest and principal, as they become due, on revenue bonds issued in payment for the utility or for its improvement. This money may not be used for any other purpose.
(b) The commission shall approve the amount set aside for the payment of the interest and principal when the commission approves the rates and charges of the municipality.