Eaton Power Xpert 9395 550/275 Installation And Operation Manual page 125

Understanding UPS Operation
sophisticated detection and switching logic inside the UPSs ensures that operating mode changes are
automatic and transparent to the critical load, while internal monitoring systems indicate the current mode of
operation. The UPSs switch operating modes in response to these system events:
• A command is an intervention that is externally initiated by an operator or by some site action. A command
causes the UPSs to switch operating modes; it usually does not require any further action.
• A notice is a minor system event that may or may not require attention.
• An alarm is a system event that requires immediate intervention.
The following paragraphs describe the differences in the parallel system operating modes, using block
diagrams to show the power flow during each mode of operation.
6 6 . . 4 4 . . 2 2 O O n n l l i i n n e e M M o o d d e e – – D D i i s s t t r r i i b b u u t t e e d d B B y y p p a a s s s s
In Online mode, utility AC power is supplied to the UPSs. Each UPS then conditions the incoming AC power
and provides clean, regulated AC power to the tie cabinet or distribution panel for parallel systems up to four
modules. The applied load is shared equally among the available UPMs in the system.
Figure 57 shows the path of electrical power through the parallel system when operating in Online mode.
If the utility AC power is interrupted or is out of specification, the UPSs automatically switch to Battery mode to
support the critical load without interruption. When utility power returns, the UPSs return to Online mode.
If the UPSs become overloaded or unavailable, the distributed bypass system switches to Bypass mode. The
distributed bypass system automatically returns to Online mode when the overload condition is cleared and
system operation is restored within specified limits.
104 Eaton Power Xpert 9395 UPS Installation and Operation Manual 164201716—Rev 14