Offline UPS (or VFD - Voltage and Frequency Dependent) offer the most basic level of power protection.

When mains supply is present, the UPS output is supplied via a built-in EMI/RFI filter which provides the load with protection from spikes and transients by clamping peak voltage to pre-defined levels.

When the mains supply fails or fluctuates outside of the UPS's operating window, a relay connects the load to the inverter output (resulting in a 4-8ms transfer time). In normal operation, with mains supply present, both output voltage and frequency will track the input voltage and frequency respectively.

As the inverter is switched off when the UPS is operating normally, the term 'Offline' is given to any UPS of this design. The inverter output on Offline UPS is typically a square-wave.

 

Offline UPS Line Diagram

 

Offline UPS are basic models, designed for use in small, non-critical applications that require protection against momentary loss of power.