UNDERSTANDING UPS OPERATIONPower Xpert 9395 UPS (300 kVA) Installation and Operation Manual 164201728 Rev 2 www.eaton.com/powerquality6-6The 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 toisolates the inverter. The backfeed protection contactor is normally always closed,ready to support the static switch unless the bypass input source becomesunavailable.6.2.6 Battery ModeThe UPS automatically transfers to Battery mode if a utility power outage occurs, or ifthe utility power does not conform to specified parameters. In Battery mode, thebattery provides emergency DC power that the inverter converts to AC power.Figure 6‐4 shows the path of electrical power through the UPS system whenoperating in Battery mode.StaticSwitch K5Rectifier InverterK1 K3BatteryConverterBatteryBatteryBreakerMain Power FlowTrickle CurrentEnergizedDe-EnergizedClosedOpenBreakers ContactorsK2Bypass InputRectifier Input OutputFigure 6‐4. Path of Current Through the UPS in Battery ModeDuring a utility power failure, the rectifier no longer has an AC utility source fromwhich to supply the DC output current required to support the inverter. The inputcontactor K1 opens and the battery instantaneously supplies energy to the batteryconverter. The converter either bucks or boosts the voltage so that the inverter cansupport the customer's load without interruption. If bypass is common with therectifier input, the backfeed protection contactor K5 also opens. The opening ofcontactors K1 and K5 prevent system voltages from bleeding backwards through thestatic switch and rectifier snubber components and re-entering the input source.