Abstract: A solid state, static power supply control mechanism for coordinated transfer of a load from a preferred source of AC power to an alternate source of AC power should the level of power transmitted from the preferred source fall below a set, adjustable level and thereafter, upon re-establishment of the proper level of operating power of the preferred source, re-transfer of the load from the alternate source to the preferred source. A unique method of current interlock prevents the alternate power source from connecting to the load until the current from the preferred source is absolutely interrupted, with the same criteria required for re-transfer from the alternate to the preferred source of power. This mechanism and associated switching technique insures supply of power to the load regardless of the preferred power source failure mode and regardless of the phase relation of the preferred and alternate sources of power.