UNDERSTANDING UPS OPERATIONEaton 9395 UPS (450–825 kVA) Installation and Operation Manual 164201725 Rev 4 www.eaton.com/powerquality6-6StaticSwitch K5Rectifier InverterK1 K3BatteryConverterBatteryBatteryBreakerMain Power FlowTrickle CurrentEnergizedDe-EnergizedClosedOpenBreakers ContactorsK2Bypass InputRectifierInput OutputBypassBreaker(CB4)InputBreaker(CB1NOTE On a UPS configured as an IOM, the bypass input, static switch, bypass breaker (CB4), and K5 are not present.NOTE On a UPS configured with a Continuous Static Switch (CSS), bypass breaker (CB4) is not present.Figure 6‐3. Path of Current Through the UPS in Bypass ModeIf the UPS transfers to Bypass mode from Online mode due to any reason other thanoperator intervention, the UPS automatically attempts to transfer back to Online mode(up to three times within a ten minute period). The fourth transfer locks the criticalload to the bypass source and requires operator intervention to transfer.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.During a utility power failure, the rectifier no longer has an AC utility source fromwhich to supply the DC output voltage required to support the inverter. The inputcontactor K1 opens and the battery instantaneously supplies energy to the batteryconverter. The converter boosts the voltage so that the inverter can support thecustomer's load without interruption. If bypass is common with the rectifier input, thebackfeed protection contactor K5 also opens. The opening of contactors K1 and K5prevent system voltages from bleeding backwards through the static switch andrectifier snubber components to the utility source.
