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Introduction 9
1.4 Modes of Operation
Refer to 2.0 - Theory of Operation and 3.0 - Operation for more details.
1.4.1 Normal (Load on UPS)
The utility AC source provides power to the rectifier/charger in each UPS module. Each rectifier/charger con-
verts the utility AC power to DC and supplies DC power to the UPS module inverter while simultaneously float
charging the battery plant. Each UPS module inverter converts DC to AC and furnishes AC power to the critical
bus. The System Control Cabinet (SCC) is the tie point for the paralleled modules and monitors and controls the
critical bus performance.
1.4.2 Input Power Failure
If the utility source power fails or is outside the acceptable range, the battery plant becomes the primary supplier
of DC power to the inverter.
1.4.3 Recharge
After the utility source power is restored, or an alternate power source becomes available, each rectifier/charger
slowly walks-in to once again power the inverters and recharge the battery plant.
1.4.4 Overload
Overloads in critical systems may be caused by inrush currents during connected equipment start-up or by faults
in the critical load. The Liebert Series 600T UPS system can maintain full output voltage regulation while sus-
taining the following overloads:
• Up to 150% for 30 seconds
• Up to 125% for 10 minutes
• Up to 104% for an indefinite period of time
Also, for clearing momentary faults above 155% of rated SCC current, the static switch turns on for 40 millisec-
onds to supply power from the bypass source. Up to 1000% of the rated current can be supplied for less than one
cycle, while up to 500% of rated load can be sustained for the full 40 milliseconds of pulsed-parallel operation.
The critical load remains on the UPS modules for the above conditions. If the UPS system overload capacity is
exceeded, an automatic transfer to bypass is initiated, which closes the system bypass circuit breaker (SBB) and
opens the UPS output circuit breaker.
Whenever you have an overload condition, you should determine the cause of the overload.
1.4.5 Redundant
If the multi-module UPS system includes one more module than the number required to supply the critical load,
the UPS system can operate in the redundant mode. This means that the load will remain on the UPS system if
one of the modules is disconnected because of a n overload, an internal fault, or for maintenance. If the additional
module is not included in the system design, or if the additional module is disconnected from the critical bus, the
UPS system operates in the non-redundant mode.
1.4.6 Bypass (Internal)
The SCC control logic initiates an automatic transfer to the bypass source if the overload-current-versus-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 maintenance.
The SCC internal maintenance bypass will allow most key components and operating modes to be checked with-
out disturbing the critical bus. However, certain key power-carrying components, such as the output and bypass
circuit breakers, will require complete system shutdown or isolation through an external maintenance bypass
cabinet for 100% service.
1.4.7 Maintenance Bypass
The installation of a Maintenance Bypass Cabinet, Panelboard or Switchboard is recommended to allow you to
totally isolate the UPS from all power sources. Use of the Maintenance Bypass is described in 3.0 - Operation.
1.4.8 Off-Battery
The battery plant can be disconnected from the rectifier/charger by using an external Module Battery Disconnect
(MBD circuit breaker). The UPS module continues to function normally, though it does not have power outage
back-up capability until the battery plant is reconnected.
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