The Line-interactive SBS offers the same bare bones surge protection and battery back-up as the offline, except it has the added feature of minimal voltage regulation while the SBS is operating from the utility source. This SBS design came about due to the off-line SBSs inability to provide an acceptable output voltage to the connected equipment during “brown-out” conditions. A “brown-out” happens when the utility voltage remains excessively low for a sustained period. Under these conditions the off-line SBS would go to battery operation and if the brown-out was sustained long enough, the SBS battery would become fully discharged, turn the power off to the connected equipment and not be able to be turned back on until the utility voltage returned to normal. To prevent this from happening a voltage regulating transformer was added, hence the term line-interactive was born. This feature really does help as low voltage utility conditions are common. The down side for this design, most of the units available have to switch to battery momentarily when making transformer voltage adjustments and this can be a bit annoying in a quiet home office on a bad power day.
