Alpha Technologies CFR 600, CFR 600XT, CFR 1000 - Manual (Page 29)

4. OPERATION
4.2 Using the Standard Interface Device
23
Line Synchronization
When both the “LINE POWER” and “LINE FAILURE” LEDs are ON, the
UPS is synchronizing its output frequency to the utility input prior to resuming
AC line operation. Synchronization takes approximately 15 seconds. If the
line frequency (generator frequency) is out of tolerance, the UPS can not
synchronize to it, and stays in the inverter mode.
Line Present Operation
The green “LINE PRESENT” LED indicates that the UPS is running on
AC line (utility / mains) power.
Line Failure (AC Input Out of Tolerance)
Whenever AC line voltage becomes unacceptably high or low (+10 % /
-25%), or the line frequency exceeds + 3%, the "LINE PRESENT" LED
flashes and the "LINE FAILURE" LED lights indicating the UPS is running on
backup power. NOTE: High generator THD can also cause this condition.
Line Failure Operation
Whenever a utility power outage occurs, the UPS initiates LINE
FAILURE operation without interruption of output power. The amber “LINE
FAILURE” LED lights to indicate that the UPS is running on backup power.