UNDERSTANDING UPS OPERATIONPower Xpert 9395 UPS (450–550 kVA) Installation and Operation Manual 164201701 Rev 3 www.eaton.com/powerquality6-66.2.5 Bypass ModeThe UPS automatically switches to Bypass mode if it detects an overload, load fault,or internal failure. The bypass source supplies the commercial AC power to the loaddirectly. Figure 6‐4 shows the path of electrical power through the UPS system whenoperating in Bypass mode.C A U T I O NThe critical load is not protected from voltage or frequency fluctuations or power outages while the UPS is inBypass mode.StaticSwitch K5Rectifier InverterK1 K3BatteryConverterBatteryBatteryBreakerMain Power FlowTrickle CurrentEnergizedDe-EnergizedClosedOpenBreakers ContactorsK2Bypass InputRectifierInput OutputNOTE On a UPS configured as an IOM, the bypass input, static switch, and K5 are not present.InputBreaker(CB1Figure 6‐4. Path of Current Through the UPS in Bypass ModeIn Bypass mode, the output of the system is provided with three-phase AC powerdirectly from the system input. While in this mode, the output of the system is notprotected from voltage or frequency fluctuations or power outages from the source.Some power line filtering and spike protection is provided to the load but no activepower conditioning or battery support is available to the output of the system in theBypass mode of operation.The internal bypass is comprised of a solid-state, silicon-controlled rectifier (SCR)static switch (SSW) and a backfeed protection contactor K5. The static switch is ratedas a continuous-duty device that is used anytime the inverter is unable to support theapplied load. The static switch is wired in series with the backfeed protectioncontactor, and together they are wired in parallel with the rectifier and inverter. Thestatic switch, being an electronically-controlled device, can be turned on immediatelyto pick up the load from the inverter while the inverter output contactor K3 opens toisolate the inverter. The backfeed protection contactor is normally always closed,ready to support the static switch unless the bypass input source becomesunavailable.
