Automatic transfer switch or in short ATS is an electronic circuit which is attached with generator. It operates generator automatically when there is outage of power from the utility. In this circuit there are comparators which look at the line voltage and when the line voltage of utility goes down they switch on the generator and when voltage of generator get stable, they switch on the contactor of main supply to load and thus there is no need for generator operator for all time. Battery is main power source for this panel for operation or self start of generator. The other work it does, is to maintain the voltage level and if the level goes down it cut off the line and starts the generator. This is how Automatic transfer switch work.