Liebert Series 600T - Manual (Page 28)

22 Theory of Operation
5. Manual Retransfers
Manual retransfers may be initiated at any time provided no retransfer inhibition conditions are present.
6. Retransfer Inhibited
A retransfer from the bypass source to the UPS system shall be inhibited if any of the following conditions
exist:
a. Manual (and Automatic) Retransfer Inhibitions:
1. Bypass frequency exceeds ±0.5 Hz of the nominal.
2. UPS system-to-bypass voltage difference ( V) exceeds a predetermined percentage (normally
5%).
3. System circuit breaker (UPS output or SBB) is inoperative.
4. OK to Transfer signal from the control logic is not present.
5. Not enough UPS modules are ON LINE to supply the connected critical load.
b. Automatic Retransfer Inhibitions (in addition to those above):
1. The load transfer to bypass was not caused by a system overload.
2. Excessive cyclical overloads within a one-hour period.
3. Retransfer conditions are not satisfied within 5 minutes of the initial transfer.
2.2.6 Redundant Mode
The typical multi-module UPS system is configured with a back-up (redundant) UPS module, connected in par-
allel to share the critical load with the other module(s). This parallel redundant system includes one more module
than is required to supply the full critical load.
A parallel redundant system will always be in the Redundant Mode if all modules are ON LINE. In addition, if
the critical load is reduced so that not all ON LINE modules are required, the UPS system can also operate in the
Redundant Mode with one or more modules OFF LINE. The critical load will remain on the UPS system—even
if one or more of the modules is taken OFF LINE for maintenance or because of an internal fault—as long as the
remaining ON LINE modules can carry the full connected system load.
The system control logic monitors the connected critical load and the number of UPS modules ON LINE. If the
UPS system is operating in the Redundant Mode, the REDUNDANT status message will be displayed at the
System Control Cabinet (SCC). If the UPS system is operating in the Non-Redundant Mode (redundant module
is OFF LINE or was not included in the system configuration), the NON-REDUNDANT status message is dis-
played at the SCC. In the Non-Redundant Mode, the critical load will be transferred to bypass if any UPS module
goes OFF LINE and the remaining ON LINE module(s) cannot carry the connected load without being over-
loaded.
The system control logic, located in the SCC, automatically adjusts the allowable overload limits based on the
connected load and the number of modules ON LINE.