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Introduction
6
1.3.4 Overload
Overloads in critical systems may be caused by inrush currents during connected equipment startup
or by faults in the critical load or distribution network. The Liebert Npower UPS system can maintain
full output voltage regulation while sustaining the following overloads:
• Up to 125% for 10 minutes
• Up to 150% for 1 minute
• Up to 200% for 10 cycles
For momentary faults above 200% of rated current, the static switch turns on for 10 cycles to supply
power from the bypass source. Up to 6,000 amps of current can be supplied for the first half cycle.
The critical load remains on the UPS module for the above conditions. If the UPS system overload
capacity is exceeded, an automatic transfer to bypass is initiated, which keeps the static switch on
and opens the inverter output contactor.
Whenever you have an overload condition, you should determine the cause of the overload.
1.3.5 Bypass (Internal)
The UPS control logic initiates an automatic transfer to the bypass source if the overload-current-ver-
sus-time curve is exceeded or if specified UPS system faults occur. You can also manually transfer the
load to the bypass (without interruption) if you must take the UPS module out of service for mainte-
nance.
With the rotary switch in the Bypass position, most key components and operating modes can be
checked without disturbing the critical bus. However, certain key power-carrying components will
require complete system shutdown or isolation through an external maintenance bypass cabinet for
100% service.
1.3.6 External Maintenance Bypass
The installation of an External Maintenance Bypass Cabinet or panel board is recommended to allow
you to totally isolate the UPS from all power sources. Use of the External Maintenance Bypass is
described in SIB External Maintenance Bypass on page 30.
1.3.7 Off-Battery
The battery plant can be disconnected from the rectifier/charger by using an external Module Battery
Disconnect (MBD) circuit breaker. The UPS continues to function normally, though it does not have
power outage back-up capability until the battery plant is reconnected.
Figure 1 UPS controls and display screen (with example of the monitor/mimic screen)
MIMIC DISPLAY
SYS. STATUS
BATTERY
INV.
LOAD
RECT.
SS
CHG