When there is only one power supply is working, the product will automatically switch to that power supply. When the common power supply and the backup power supply are supplied at the same time, the common power supply is used Preferentially.(Attention: in automatic mode, forcibly rotating the power switch handle will cause damage to the transfer switch). Manual mode : Power supply should be transfered manually by rotating the handle. Industrial, residential and various occasions where continuous power failure is not allowed. Auto mode and Manual mode for choise. 250A 4P Dual Power Automatic Transfer Switch. Life Electrical:8000times; Mechanical: 3000times. This item is in the category "Business & Industrial\Light Equipment & Tools\Transfer Switches". The seller is "angeltaylor" and is located in this country: US. This item can be shipped to United States.