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Theory of Operation
19
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. UPS system-to-bypass voltage difference (ΔV) exceeds a predetermined percentage
(normally 5%).
2. System circuit breaker (UPS output or SBB) is inoperative.
3. OK to Transfer signal from the control logic is not present.
4. 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, con-
nected in parallel 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 con-
nected 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-Redun-
dant Mode (redundant module is OFF LINE or was not included in the system configuration), the
NON-REDUNDANT status message is displayed at the SCC. In the Non-Redundant Mode, the criti-
cal 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 overloaded.
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.