Ordinances providing for appropriations for current expenses of the municipality, or for public improvements petitioned for the owners of a majority of the foot frontage of property benefited and to be specially assessed for the cost thereof, or for raising revenue, or ordinances wherein an emergency is declared to exist, shall become effective immediately upon passage or at such later date as may be provided therein, and such ordinances shall not be subject to referendum. All other ordinances shall take effect 30 days after passage. An emergency ordinance as referred to herein is one which must be passed and made effective at once or in less than 30 days to meet an emergency in the operation of the City government, or which is necessary for the immediate preservation of the public peace, health, safety, morals or welfare. Each emergency ordinance must contain therein a separate section setting forth the reason for the emergency. No ordinance granting a franchise or fixing a rate to be charged by a public utility shall be passed as an emergency measure.