Abstract: A system for transferring a load from a first to a second AC power source, including a monitor for sensing both power sources and producing an output signal when the differences in voltage, frequency, and phase between the two sources are within prescribed limits. Contacts close, in response to the monitor output signal, to connect the second power source to both the first power source and the load. Other contacts open, to disconnect the first power source from the second power source and load, but only after the second power source has been connected to the first power source and the load. The contact arrangement may include an automatic transfer switch and separate contacts in parallel with one or both sets of transfer switch contacts, or it may involve two independently operable contactors, or it may involve a four position switch.