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7.1.1 Bypass Supplies
The Liebert NX contains an electronically controlled switching circuit that enables the critical load to
be connected to either the inverter output or to a bypass power source via the static bypass line. Dur-
ing normal system operation the load is connected to the inverter and the inverter contactor is closed;
but in the event of a UPS overload or an inverter failure, the load is automatically transferred to the
static bypass line.
To provide a clean (no-break) load transfer between the inverter output and static bypass line, the
bypass static switch activates, connecting the load to bypass. To achieve this, the inverter output and
bypass supply must be fully synchronized during normal operating conditions. This is achieved
through the inverter control electronics, which make the inverter frequency track that of the static
bypass supply, provided that the bypass remains within an acceptable frequency window.
A manually controlled, maintenance bypass supply is incorporated into the UPS design. It enables the
critical load to be powered from the utility (bypass) supply while the UPS is shut down for routine
maintenance.
7.1.2 Operating Modes
The UPS is designed to operate as an on-line, double-conversion, reverse-transfer system in the fol-
lowing modes:
Normal Mode
Operating in normal mode, the Liebert NX’s rectifier derives power from a utility AC source and sup-
plies regulated DC power to the inverter, which regenerates precise AC power to supply the connected
equipment. The rectifier also uses the utility source power to charge the batteries.
Battery Mode
When utility AC power fails, the Liebert NX protects the critical load by instantaneously channeling
battery power to the inverter, which continues supporting the critical load without interruption.
When utility power returns and is within acceptable limits, the Liebert NX automatically shifts back
to Normal mode, with the rectifier powering the critical load.
Bypass Mode
When the Liebert NX is in bypass mode, the load is directly supported by utility power and is without
battery backup protection.
The Liebert NX’s inverter static switch and bypass static switch will shift the load from the inverter
to bypass mode without an interruption in AC power if the inverter is synchronous with the bypass
and any of the following occurs:
• inverter fails
• inverter overload capacity is exceeded
• inverter is manually turned off by the user
NOTE
When the UPS is operating in bypass mode or on maintenance bypass, the connected
equipment is not protected from power failures or surges and sags.
NOTE
If the inverter is asynchronous with the bypass, the static switch will transfer the load from the
inverter to the bypass WITH interruption in AC power to the critical load. This interruption
will be less than 10ms. This interruption time may be altered by modifying the Output transfer
interrupt time setting.