Except for purposes permitted by law, the city shall not sell, exchange, lease, or in any other way alien or dispose of the property, easements, income, or other equipment, privileges, or assets belonging to and appertaining to any utility which it may own or acquire, unless and except such proposition shall first have been submitted at an election held for that purpose in the manner provided in this Charter, to the electors of the city and approved by them at a regular city election by a three-fifths majority vote of the electors voting thereon. All contracts, negotiations, leases, grants, or other forms of transfer in violation of this provision, shall be void and of no effect as against the city. The provisions of this section shall not, however, apply to the sale or exchange of any article of equipment that is obsolete, worn out, or useless, or which could, with advantage to the service, be replaced by new and improved machinery or equipment.