Ind. Code § 5-1-5-10

Sec. 10. When funds and investments and the known earned income therefrom in amounts sufficient to pay the principal of and interest and any premium on bonds to be refunded as they become due at their respective maturities or at the date fixed for redemption have been irrevocably pledged to the bonds to be refunded, such bonds shall not constitute an indebtedness of the issuing body within the meaning of any constitutional or statutory debt limitation.