Patent ID: 7282813

Claim:
In a system for selectively supplying AC power to a load from first or second AC sources, the system having input terminals comprising line and neutral terminals for receiving AC power from the first AC source and having output terminals comprising output line and neutral terminals for receiving AC power from the second source and for supplying AC power to the load, wherein the input terminals and the output terminals are selectively connectable by a transfer switch configurable to connect the input terminals to the output terminals or to isolate the input terminals from the output terminals, a process for detecting an unsafe condition comprising: receiving a first voltage signal representing a first voltage on the input line terminal relative to the output neutral terminal; receiving a second voltage signal representing an output voltage produced by the second AC source and supplied to the output terminals of the system; receiving an input current signal representing input current in the input line terminal; and when the transfer switch is configured to isolate the output terminals from the input terminals, causing the second AC source to phase shift said second voltage when said first and second voltages are in-phase; and after phase shifting said second voltage, causing the second AC source to cease supplying power to the output terminals when said first and second voltages are still in phase.