When electrical net is power off, has voltage loss or has phase lack, the control box can start generating set itself(Engine can continually start 3/5 times. If failed in starting,control box will have sound alarm and machine will be locked up). After successful starting, the control box will control the switch of the ATS to switch electric circuits of power supply. It's 8-12S from a trouble happens on electric net to power is supplied from the spare machine. When electric supply is normal and ready, the control box controls switches to change the electric circuit of power supply to get back power from electric net, and at the same time.