Sometimes referred to as Voltage Independent (VI), a Line Interactive UPS system operates similarly to an Offline UPS, with the addition of a built-in Automatic Voltage Stabiliser (AVS).
The AVS ensures the output voltage remains within a pre-defined voltage window regardless of any voltage variations on the mains input supply. This enables Line Interactive UPS to provide protection against power sags, surges, and brownouts.
An EMI/RFI filter may also be used to protect the AVS and load from spikes and transients by clamping their peak voltages to more acceptable levels.
When the mains power supply fails or fluctuates outside of the pre-set window, the load is transferred via a relay (introducing a 4-8ms transfer time) to the inverter output.
Inverters within Line Interactive UPS can vary in design and performance, and ultimately output waveform. Most Line Interactive UPS will provide a modified sinewave (more commonly known as a 'step-wave') whereas more premium designs will provide a sinewave output.
Line Interactive UPS are typically used in smaller, less critical applications, such as PCs, telephone systems, non-critical networking equipment and small motor loads.
Typically a line-interactive UPS is only available up to 3kVA - however, larger Online UPS will have various 'ECO' or 'economy' operating modes, which allows it to mirror the operation of a Line Interactive model, where the inverter is powered but on standby and only switches on if there’s a power problem.
What Power Problems Do The Different UPS Topologies Protect Against?
| Power Problem | Online (VFI) | Line Interactive (VI) | Offline (VFD) |
| Sags/Brownouts | Yes | Yes | - |
| Surges | Yes | Yes | - |
| Spikes/Transients | Yes | Yes | Yes |
| Electrical Noise | Yes | Yes | Yes |
| Harmonics | Yes | - | - |
| Frequency Variation | Yes | - | - |
| Mains Failure | Yes | Yes | Yes |