An Uninterruptible Power Supply refers to a power system that provides emergency power to a load when the input power source or mains power fails, regarded as near-instantaneous protection from input power interruptions. The three general categories of modern UPS systems are online, line-interactive, and offline.

In an off-line ("standby") UPS system, the load is powered directly by the input power, and the backup power circuitry will only be invoked when the utility power fails. Some high frequency noise filtering and surge suppression may be included in this path. The UPS switches on its inverter as soon as mains supply failure is detected or below the normal load and simultaneously switches the output relay to the inverter side to supply battery sourced power to the load. This transition involves a delay on account of the time to reliably detect mains failure and switch over a relay, and the output is broken for this period (usually for 2 to 12msec).

• Finite transfer time from mains power to inverter when the mains power supply fails.

The true LINE INTERACTIVE design is in fact a combination of OFFLINE ONLINE, in which the inverter plays a dual role of charging the battery when mains supply is present as well as regulating the output voltage and working as a normal inverter in absence of mains supply, so to the user it appears like an ONLINE System. Line Interactive UPS offers enhanced power protection over the basic Offline designs by providing additional line conditioning.

• Lower operating temperatures.

• Require frequent battery use in areas of extreme voltage distortion.

In an online (aka double-conversion) UPS, the input AC is charging the backup battery source which provides power to the output inverter, so the failure of the input AC won't cause activation of the transfer switch. When the mains fail, the source of DC power for the inverter section shifts to the battery without any break whatsoever in the output to the Inverter. It also bears all the vagaries and noise borne out of the mains supply and insulates the secure bus supply from it. No transfer time during the failure. When power is restored, the rectifier will resume carrying most of the load and begin charging the batteries

Benefits of Online-UPS:

• No fluctuations in the voltage, indicating stable voltage quality.