Bonds to fund or refund the indebtedness shall not be issued unless authorized by the electors of the city voting at an election to be called and held for that purpose, in either of the following cases:
(a)
When the indebtedness is evidenced by warrants or by judgment obtained for indebtedness or liability incurred by the city exceeding the income and revenue provided for the year in which the indebtedness or liability was incurred.
(b)
When the indebtedness is that of any department, board, or special fund of the city, and has been incurred without submission of the proposition of incurring the indebtedness to the city electors, and without the assent of two-thirds of the electors voting at an election held for that purpose.