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3.6.5 Automatic Operations
The Npower UPS is designed to function while attended or unattended by an Operator. The system
control logic monitors the performance of the UPS, the availability of power sources, and the current
required by the critical load.
Automatic UPS operations are described in the following sections.
Overloads
The UPS system is capable of sustaining full output voltage for overload conditions that remain
within or under the current versus time curve of system overload capacity.
For high current demands of short duration (momentary overloads) the critical load is supplied simul-
taneously by both the UPS system and the bypass line. Whenever the critical load requires more than
200% of the capacity of the module, the bypass line will supply up to 6000 amperes for one-half cycle.
If an overload condition exceeds the UPS system overload capacity or the capacity of the modules
online, the UPS system initiates an automatic load transfer to the bypass line.
Automatic Transfers to Bypass
The UPS system will initiate an automatic load transfer to the bypass line if an overload condition
exceeds the current-versus-time curve of overload capacity, or if specified UPS system faults occur.
The Overload Transfer and Output Under-Voltage alarm conditions will initiate an automatic trans-
fer to bypass and the Load On Bypass message will be displayed. The Status Report screens will
include the Automatic Transfer to Bypass message. Other UPS system faults will initiate an auto-
matic transfer to bypass followed immediately by the shutdown and isolation of the UPS system. The
static bypass switch is closed and the contactor is closed. Bypass power is supplied to the critical load
through the static bypass switch.
Automatic Re-transfers to UPS
Automatic Retransfer to UPS is an option that you can select from the System Configuration screen.
If you do not want the UPS system to initiate any automatic re-transfers, set Max Auto-Rexfer
Attempts to zero (0).
In an automatic retransfer, the output contactor closes and the static bypass switch opens.
The following conditions must be present to initiate an automatic retransfer of the critical load from
the bypass source to the UPS inverter:
• The number of Auto-Rexfer Attempts selected must be greater than zero (0).
• Critical load was initially transferred to the bypass source due to a system overload only. A man-
ual retransfer from bypass is required if the transfer to bypass was caused by any condition other
than output overload.
• The load has since dropped below 100% of the rated load.
• Both the Input contactor and the Battery (MBD) remain closed since the overload transfer.
• OK to Transfer signal received from the control logic for at least 10 seconds, within 5 minutes of
the system overload transfer. A manual retransfer from bypass is required for overloads lasting 5
minutes or more.
• Cyclic overloads, which occur up to five (select range is 0 to 5) times in 60 minutes, are automati-
cally returned to the inverter for each event including the Nth overload. A manually initiated
retransfer from bypass is required for the N + 1 overload.