UNDERSTANDING UPS OPERATIONEaton 9395 550/275 UPS (225–550 kVA) Installation and Operation Manual 164201716 Rev 2 www.eaton.com/powerquality 6-15Main Power FlowUPS 1UPS 2UPS 3UPS 4Output toCritical LoadBatteryUPS Input Tie CabinetUPS 1 OutputBypass InputUPS 2 OutputUPS 3 OutputUPS 4 OutputBatteryBatteryBatteryClosedOpenBreakersFigure 6‐10. Path of Current through the UPSs in Online Mode – Distributed Bypass6.4.3 Bypass Mode – Distributed BypassIn Bypass mode, the output of the system is provided with three-phase AC powerdirectly from the bypass input. While in this mode, the output of the system is notprotected from fluctuations, spikes, or power outages from the source. No batterysupport is available to the output of the system in the Bypass mode of operation.The distributed bypass system automatically switches to Bypass mode if it detects aUPS system overload or load fault.If one UPS becomes unavailable, the distributed bypass system dynamically updatesthe redundancy calculation to determine if the remaining UPSs can support the load. Ifthe load can be supported, the system does not switch to bypass.Figure 6‐11 shows the path of electrical power through the parallel system whenoperating in Bypass mode.In a distributed bypass system, each UPS operates similar to a single UPS, but inparallel with each other. The bypass source for the load is derived from the bypassinput of one, two, three, or four UPSs, depending on the system configuration,through the internal static switches. If a module is taken offline, the other modulesremain online to support the load. If more modules than can support the load must betaken offline, the load must be transferred to maintenance bypass or shut down.
