WE HAVE TWO UNITS OF LIEBERT UPS SYSTEM FOR O/P CAPACITY 10KVA OUR PROBLEM IS AS UNDER UPS RUNNING ON GRID POWER WHEN GRID POWER FAILS UPS REMAIN ON AND GIVE BACKUP ON BATTERY POWER AND WHILE WE FEED GENERATOR POWER FOR UPS INPUT ALL IS WORKING OK BUT ACTUAL PROBLEM STARTS WHEN WE AGAIN FEED GRID POWER TO UPS SYSTEM IT DOES NOT OUT PUTTING POWER MEANS THE DISPLAY SHOWS AS UNDER O/P LOAD : 0 O/P VOLT : 0 INVERTER LED IS BLINKING DISPALY SHOWS INVERTER FAULT PLZ GIVE ME SOLUTION FOR THAT