power to critical load 720 while effectively disconnected from utility grid 710 in such situations the grid typically recovers after a period of time and resumes making power available at this point site controller 765 is operating in grid isolated mode 240 converting the output of genset 780 into ac to power critical load 720 52u contactor 705 is open so normal loads 722 are powered by the grid while critical load 720 is powered by inverter subsystem 706 when utility grid 710 resumes providing power inverter 750 detects the voltage through sensor 741 and synchronizes its output voltage frequency and phase to the supply from utility grid 710 when inverter 750 and the site controller 765 detects that synchronization has been achieved 52u contactor 705 is commanded to close then when inverter 750 senses that 52u contactor 705 has closed and that its synchronization with utility grid 710 has been maintained it switches from grid isolated mode 240 to grid parallel mode 250 likewise when site controller 765 senses that 52u contactor 705 and m 1 contactor 725 are closed it also switches from grid isolated mode 240 to grid parallel mode 250 because these modes of operation are substantially the same except for the.