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EN/IEC 62040-3 defines three generic Static UPS topologies. Each topology is then further classified based on its output waveform and dynamic performance.

  • VFI - Voltage and Frequency Independent: more generally known as 'Online' or 'Double Conversion', where the UPS output is completely independent of any voltage and/or frequency abnormalities on the incoming mains supply. The highest classification for an Online UPS is VFI-SS-111.
  • VI - Voltage Independent: typically referred to as 'Line Interactive', where voltage fluctuations are stabilised by built-in regulation devices. Output frequency tracks the input frequency when the mains supply is present. 
  • VFD - Voltage and Frequency Dependent: more commonly referred to as 'Offline' or 'Standby', where the output of the UPS output tracks the voltage and frequency of the input mains supply.


Each of the three Static UPS topologies provides varying degrees of power protection.

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


Online UPS are the preferred choice for any critical application due to their superior electrical performance - including their 'no-break switchover' on mains power supply failure and an automatic internal bypass for safe failure to mains in the event of a fault.