There are two different name, L (Live) and N (Neutral) on the AC input terminal. Please refer to Fig. 1 for the wiring of the AC input. The terminal L must be connected to the unearthed conductive part of AC main supply and the terminal N must be connected to the earthed conductive part of AC main supply, to apply the safety agency approval. This purpose is protection by a blowout fuse from electric shock by earth fault.
Also, the terminal FG needs to be connected to the earth to protect from electric shock and noise interference. Even if the terminal L and the terminal N is connected to another side or the terminal FG is not connected to the earth, then the power supply might not get broken and might not change its characteristic.
Even if terminal L and N are connected oppositely, power supply operates normally. However, please note that the conductive part would be kept under the earthing fault condition. Please note that conducted emission and ripple noise characteristics would be changed when FG is not connected to ground.
Most of AC-DC power supplies are classified as CLASS Ⅰ equipment in safety approval requirement, therefore FG terminal should be connected to the safety ground of the equipment.