Unless approved by a three-fifths majority vote of the electors voting thereon at a regular or special election, the city shall not sell, exchange, lease or in any way dispose of any property, easements, equipment, privilege or asset belonging to and appertaining to any municipally owned public utility which is needed to continue operating such utility. All contracts, negotiations, licenses, grants, leases or other forms of transfer in violation of this section shall be void and of no effect as against the city. The restrictions of this section shall not apply to the sale or exchange of any articles of machinery or equipment of any city owned public utility which are worn out or useless or which have been, or could with advantage to the service be, replaced by new and improved machinery or equipment, to the leasing of property not necessary for the operation of the utility, or to the exchange of property or easements for other needed property or easements.