How does a UPS provide protection from sag, surges and brownouts?

A line interactive uninterruptible power supply has a built-in automatic voltage regulator (AVR) that can step the incoming mains power supply voltage up or down using as a series of taps or relays. If the mains power supply voltage or frequency fails outside a preset threshold window the supply is seen to have failed and the inverter section switches into circuit to supply power to the load from the battery set. The inverter ‘inverts’ the DC (direct current) supply from the battery set to AC (alternating current) to support the connected loads.