When the City is authorized to acquire, own, or operate any public utility, it may in accordance with the provisions of the State Constitution, issue mortgage bonds for the purpose beyond the general limit of bonded indebtedness prescribed by the general laws of the State of Michigan provided that any mortgage bonds issued beyond the general limit of bonded indebtedness prescribed by law shall not impose any liability upon the City but shall be secured only upon the property and revenues of the public utility, including a franchise, as prescribed by State law.