22 Theoryof Operation
5. ManualRetransfers
Manualretransfers may be initiated at any time provided no retransfer inhibitionconditions are present.
6. RetransferInhibited
Aretransfer from the bypass source to the UPS system shall be inhibitedif any of the following conditions
exist:
a. Manual (and Automatic)R etransferInhibitions:
1. Bypassfrequency exceeds ±0.5 Hz of the nominal.
2. UPSsystem-to-bypass voltage difference ( V) exceedsa predetermined percentage (normally
5%).
3. Systemcircuit breaker (UPS output or SBB) is inoperative.
4. OKto Transfer signal from the control logic is not present.
5. Notenough UPS modules are O N LINE to supply the connectedcritical load.
b. AutomaticRetransfer Inhibitions (in addition to those above):
1. Theload transfer to bypass was not caused by a system overload.
2. Excessivecyclical overloads within a one-hour period.
3. Retransferconditions are not satisfied within 5 minutesof the initial transfer.
2.2.6 Redundant Mode
Thetypical multi-module UPS system is configured with a back-up (redundant)UP S module,connected in par-
allelto share the critical load with the othermodule(s). This parallel redundant system includesone more module
thanis required to supply the full critical load.
Aparallel redundant system will always be in the Redundant Mode if all modulesare ONLINE. In addition, if
thecritical load is reduced so that not all ON LINE modules are required,the UPS system can also operate in the
RedundantMode with one or more modules OFF LINE. The criticalload will remain on the UPS system—even
ifone or more of the modules is taken OFF LINEfor maintenance or because of an internal fault—aslong as the
remainingON LINE modules can carry the full connected system load.
Thesystem control logic monitors the connected criticalload and the number of UPS modules ON LINE. If the
UPSsystem is operating in the Redundant Mode, the REDUNDANT status messagewill be displayed at the
SystemControl Cabinet (SCC). If the UPS system is operating in the Non-RedundantMode (redundant module
is OFF LINE or was not included in the system configuration), the NON-REDUNDANT status mess age is dis-
playedat the SCC. In the Non-Redundant Mode,the critical load will be transferredt o bypassif any UPS module
goes OFF LINE and the rema ining ON LINE module(s) cannot carry the connected loadwithout being over-
loaded.
Thesystem control logic, located in the SCC, automaticallyadjusts the allowable overloadlimits based on the
connectedload and the number of modules ON LINE.
DISCONTINUED
PROD U C T