The three types of UPS topology defined in BS EN 62040-3 including Standby/Off-line, Line Interactive and on-line uninterruptible power supplies.
On-line UPS systems provide the ultimate level of power protection. They provide a digitally generated sinewave output to their connected loads and are classed as Voltage and Frequency Independent (VFI) of the incoming mains power supply. On-line UPS also feature a built-in automatic bypass to ensure safe failure-to-mains of the connected load(s) if the UPS inverter is overloaded or develops an internal fault condition.
On-line UPS and some line interactive uninterruptible power supplies have inverters designed for longer duration or continuous running on mains power failure. Line interactive UPS may be limited to 1-2hours and some smaller on-online UPS to 4 hours. Most larger online UPS from 1.5kVA upwards can be installed with battery extension packs up to 7 hours or more in duration.
The switch mode power supply (SMPS) within a server and other electronic and IT devices is designed with built-in EMC/filtering and a wide input voltage window. The lower the input voltage the more the current that has to be drawn to power the internal electronics. Installing a UPS before the power supply reduces the work that the power supply has to do, as it is provide with a more stable, secure and cleaner electrical power supply. This in turn can lead to greater reliability and longer term operation of the SMPS and device whether it is a server, IT peripheral or other device.