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)

Theutility AC source provides power to the rectifier/charger in eachUPS module.Each rectifier/charger con-
vertsthe utility AC power to DC and supplies DC power to the UPS module inverter whiles imultaneouslyfloat
chargingthe battery plant. Each UPS module inverterc onvertsDC to AC and furnishes AC power to the critical
bus.The System Control Cabinet (SCC) is the tie point for the paralleled modules and monitorsa nd controlsthe
criticalbus performance.

1.4.2 Input Power Failure

Ifthe utility source power fails or is outside the acceptablerange, the battery plant becomes the primary supplier
ofDC power tothe inverter.

1.4.3 Recharge

Afterthe utility source power is restored, or an alternate power source becomes available, eachrectifier/charger
slowlywalks-in to once again power the inverters and recharge the batteryplant.

1.4.4 Overload

Overloadsin critical systems may be caused by inrushcurrents during connected equipment start-up or by faults
inthe critical load. The Liebert Series 600T UPS system can maintain fulloutput voltage regulation while sus-
tainingthe 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 switchturns on for 40 millisec-
ondsto 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 millisecondsof pulsed-parallel operation.
Thecritical 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 systembypass circuit breaker (SBB) and
opensthe UPS output circuit breaker.
Wheneveryou have an overload condition, you should determine thecause of the overload.

1.4.5 Redundant

Ifthe multi-module UPS system includes one more module than the numberrequired to supply the critical load,
theUPS system can operate in the redundant mode. This means that the load will remain on the UPS system if
oneof the modules is disconnected because of an overload, an internal fault,or for maintenance. If the additional
moduleis not included in the system design, or if the additional moduleis disconnected from the critical bus, the
UPS systemoperates in the non-redundant mode.

1.4.6 Bypass (Internal)

TheSCC control logic initiates an automatic transfer tothe bypass source if the overload-current-versus-time
curveis exceeded or if specified UPS system faults occur. Youcan also manually transfer the load to the bypass
(withoutinterruption) if you must take the UPS module out of service for maintenance.
TheSCC internal maintenance bypass will allow most key componentsand operating modes to be checked with-
outdisturbing the critical bus. However, certainkey power-carrying components, such as the output and bypass
circuitbreakers, will require complete system shutdown or isolationthrough an external maintenance bypass
cabinetfor 100% service.

1.4.7 Maintenance Bypass

Theinstallation of a Maintenance Bypass Cabinet, Panelboard or Switchboardis recommended to allow you to
totallyisolate the UPS from all power sources. Use of theMaintenance Bypass is described in 3.0 - Operation.

1.4.8 Off-Battery

Thebattery plant can be disconnected from the rectifier/chargerby using an external Module Battery Disconnect
(MBDcircuit breaker). The UPS module continues to function normally,though it does not have power outage
back-upcapability until the battery plantis reconnected.
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