UNDERSTANDING UPS OPERATIONEATON Eaton 9395 550/275 UPS (225–550 kVA) Installation and Operation Manual 164201716 Rev 2 www.eaton.com/powerquality6-16Main Power FlowUPS 1UPS 2UPS 3UPS 4Output toCritical LoadBatteryUPS Input Tie CabinetUPS 1 OutputBypass InputUPS 2 OutputUPS 3 OutputUPS 4 OutputBatteryBatteryBatteryClosedOpenBreakersFigure 6‐11. Path of Current through the UPSs in Bypass Mode – Distributed BypassThe distributed bypass system can be transferred from Online mode to Bypass modemanually. However, the distributed bypass system automatically switches to Bypassmode whenever the UPSs can no longer supply the critical load. If the distributedbypass system transfers to Bypass mode from Online mode due to an output voltagedeviation, the distributed bypass system automatically attempts to return to Onlinemode (up to three times within a 10‐minute period). After three transfer attempts oran overload, the system locks the critical load to the bypass source and requiresoperator intervention to transfer.Bypass mode is a normal operating mode, not an alarm condition. However, if thedistributed bypass system is unable to return to Online mode following an automatictransfer to Bypass mode, an alarm condition is recorded.Bypass may also be used when the UPSs or UPMs in the system must be shut downto perform routine maintenance or repairs.In the Parallel Redundant (N+1) arrangement, the bypass circuitry in each UPSoperates to support the applied loads on bypass. If the UPSs are online and one UPStrips offline, the remaining UPSs do not go to bypass as long as they have thecapacity to support the load.In the Parallel Capacity (N+0) arrangement, if one UPS trips offline and goes tobypass, the remaining UPSs also go to bypass.
