Emerson Liebert NXL 250 kVA, Operation And Maintenance Manual

Page: 35 / 112

Operation
27 Liebert
®
NXL
™
2.14.3 Input Power Failure—Load on DC Source
If the utility AC power source fails or is outside the acceptable range, the DC source becomes the
power source for the UPS module inverters. The UPS continues to supply power to the critical load
and also to the UPS controls.
Use the Battery Time screen at the UPS modules to monitor the present DC source voltage compared
to the shutdown value. The time the DC source can sustain the load depends on the size of the load
and the size and condition of the DC source.
Alarm messages that indicate DC source status are Battery Discharge, Low Battery and Battery
Shutdown. The voltage limits for these alarms are displayed on the UPS module Alarm Limit
Settings screen. These limits were selected for your installation by Liebert Services during initial
startup. The battery block in the UPS module Monitor/Mimic Display indicates Charge or Discharge
and the current in amperes.
Figure 32 Input power fail, load on DC source
2.14.4 Off DC Source
The DC source can be disconnected from the UPS, if required for maintenance, by opening the module
battery disconnect (MBD) circuit breaker. In this situation the UPS module will continue to supply
conditioned power to the critical load, but if input power fails the UPS system cannot supply power to
the load.
Figure 33 Load on UPS, DC source not available
!
CAUTION
When the UPS system is operating with the module battery disconnect (MBD) circuit
breaker(s) open, the critical load is not protected from loss of the utility source power.
Bypass
Input
DC Source
DC Bus
Module
Output
BFB
CB2 CB1
MBD
Bypass
Input
DC Source
DC Bus
Module
Output
BFB
CB2 CB1
MBD