Off-Line UPS Less expensive, off-line UPS are usually considered less sophisticated than other UPS products. During normal operation the power will flow straight through the unit, and if the input voltage fails or fluctuates outside pre-set tolerances the UPS will then feed battery power via an inverter. When the mains power returns, the inverter is turned off and the output is switched back to mains. Typically there will be a break of between 4 and 10 milliseconds during transfer to and from the battery mode.
Line Interactive UPS Line interactive UPS operate in a very similar fashion to an off-line UPS, except with the advantage of better filtering and power conditioning. Whilst not eliminating interference in the mains power, this technology reduces the impact of spikes, surges and sags.
On-Line UPS On-line UPS are so-called because the inverter (the part which produces the AC output) is always on-line. They are much more reliable since, should the inverter fail, they simply drop back to mains power and sound an alarm. In addition, their double conversion design ensures excellent output power quality even when the incoming mains supply is poor or faulty.
