USP
VI systems (line-interactive)
Inexpensive UPS systems to protect against most network problems
VI systems (offline) offer protection against:
Power failures
Voltage spikes
Voltage sags
Voltage frequency differences
VFI Systems (online)
VFI (online) UPS systems deliver extensive protection against power failure except in the event of lightning.
VFI systems (online) offer added protection against:
Voltage failure
Voltage distortion
Voltage oscillation
Frequency fluctuation