Automatic Transfer Switch (ATS) controller uses microprocessor as its core which can precisely monitor voltage (loss of main power). The ATS will start functioning in the following circumstances; loss of power, over voltage, under voltage phase. 
250A 4 pole automatic transfer switch unit which, in the event of a power outage, will automatically start a generator and transfer the load over to the Generator supply. When the Main supply returns to normal, the ATS will transfer the load back to the Main supply and shut down the generator.