Unless approved by majority of the electors voting thereon at any general or special election, the Commission shall not have the power to authorize any issue of bonds except special assessment bonds, bonds for the city portion of local improvements, not to exceed forty percent of the cost of such improvement, refunding bonds, bonds for relief from fire, flood or calamity, for payment of judgments, revenue bonds and other bonds excluded by statute from the requirement for such vote. Only those electors having the Constitutional qualifications for voting on the approval of bond issues shall be entitled to vote on such question.