Understanding UPS Operation7-7EATON Powerware® 9390 UPS (100–160 kVA) Installation and Operation Manual S 164201554 Rev E powerware.comDuring an outage, the UPS prohibits transfers to bypass and provides upstream protection.The backfeed protection contactor is opened, preventing system output voltage frombleeding across the static switch snubber components to the bypass input source.To ensure the load remains energized, the UPS automatically transfers the output to theinternal bypass when one of these abnormal conditions occur: the output of the systemexceeds acceptable voltage and frequency tolerances, the system is overloaded, or theinverter fails. The transfer is initiated by turning on the static switch and opening theoutput contactor K3, also called a Make-Before-Break transfer. The transfer occurs in lessthan 4 mS (quarter cycle) to ensure that the output load is not interrupted. The staticswitch remains on until either the inverter is able to support the system output or the unitis placed into a maintenance position where repairs can be made.If the UPS initiates its own transfer to bypass for any reason other than operatorintervention, the UPS attempts to restart the inverter (if not running already) and attemptsa transfer back online to support the output of the system. Three attempts are madewithin ten minutes to return the inverter online automatically before the UPS locks out anyfurther attempts. After three attempts, the UPS remains in bypass and an alarm conditionis enunciated. The UPS can also be transferred to bypass using the front panel controls.Bypass mode is a normal operating mode, not an alarm condition. However, if the UPS isunable to return to Normal mode following an automatic transfer to Bypass mode, analarm condition is recorded.
