The ASCO Three-Source Priority Systems are similar to the ASCO three-source systems except they protect two critical loads rather than one critical load. Upon the loss of the utility power source, the system provides all necessary controls to start both alternative power sources. The most critical load is transferred to the first alternate power source that achieves acceptable voltage and frequency. The second most critical load is then transferred to the second alternate power source when it achieves acceptable voltage and frequency.
If the alternate power source feeding the most critical load should fail, the most critical load would be transferred to the second alternate power source and the second most critical load would be shed. When the utility power source is restored, the system controls automatically retransfer the loads to the utility and both alternate power sources are run for a cooldown period and then shutdown.