By using special controls for managing prime, emergency and standby power.
The ASCO 7000 SERIES Three Source Transfer System provides all necessary controls to start both primary and secondary backup power sources upon the loss of the utility source. Critical loads are automatically transferred to the first alternate power source that achieves acceptable voltage and frequency. The second alternate power source then is shutdown automatically after a time delay and cool down period. If the first alternate power source fails, the second will be automatically restarted and the load will be transferred from the first alternate power source to the second alternate power source. When normal power is restored, the controls automatically retransfer the load to the utility source. The Three Source Transfer System can be furnished in its own enclosure, or as a package with a standard transfer switch. The package can include a separate enclosure for each transfer switch, or a single enclosure housing both transfer switches.
One line diagram of a typical three source power design. The three source transfer switch (ATS #2) automatically connects to the power source that achieves acceptable voltage and frequency first.