What is a UPS?

Electricity suppliers distribute what is known as alternating current (AC), at a voltage of 230 volts and a frequency of 50 Hertz. The only way of storing electricity, in a battery for example, is to convert the alternating current from the mains power supply into what is known as direct current (DC). Simply speaking, a UPS is a buffer-battery located between the power socket and the computer. It converts the AC mains power supply into direct current in order to charge the battery and then reconverts it back (to AC) in order to supply the load, in this case a computer.

A Variety of Technologies

The original UPS technology is what is normally referred as On-Line technology. Using this principle, the current is continuously regenerated in order to ensure that power supply to the load is never interrupted. The first UPSs appeared thirty years ago and Merlin Gerin was one of the precursors in this field. At the time, the hardware requiring protection was so sensitive to the slightest electrical disturbance that it was necessary to integrate a large number of sophisticated technical features in the product, in order to guarantee the quality of the electrical current.

Legacy segmentations

The appearance of the PC at the beginning of the eighties changed the basic principles underlying UPS technology.
The PC introduced a new constraint with respect to the price. With high power UPSs, the very low relative cost of protection, compared with the cost of the hardware protected, meant that it was possible to integrate sophisticated technology. The situation was very different when the price limit was set at 10% of the price of a PC, costing $ 10 000 (ten years ago!). The only way of overcoming this obstacle was to use less expensive technology. This became possible in 1985 with the appearance of switch-mode power supplies for PCs.
The power supplies are fitted with internal capacitors and are thus able to store power for a few milliseconds, which is long enough to switch to an external backup battery without any risk of interrupting the power supply. The was how Off-line UPSs started. Their components were not specified to supply power continuously, but only during the ten minutes of battery backup time. This represented a significant reduction in costs. The first generation of Off-line UPSs was beset by teething problems, in particular with respect to compatibility, which did not enhance their reputation. There is still a deep-rooted idea that On-line means « top of the range » while Off-line is somehow inferior.

Technology or functionalities?

This idea is misguided, as it is based on technological considerations and not on functionalities. In order to cut the cost of their products as much as possible, Off-line UPS manufacturers started by considerably reducing filtering capacity, the quality of the electrical signal or the dynamic characteristics of their products. Then certain functions were re-introduced in order to enhance the product. New functions, such as communication, appeared. At the same time, certain On-line UPS manufacturers, anxious to reduce the price differential with their Off-line competitors, started removing some of the top-of-the-range features from their products. These developments have resulted in a rather confusing market, in which it is not uncommon to find Off-line UPSs delivering better performance than some On-line devices.
To help you to understand, we have prepared a comparative table showing the various UPS technologies (defining the power conversion technique) and the functionalities implemented (which are not related to the technology and may consequently be included with any type of product, or not, as the case may be).

Technologies	Functionalities
Off-Line (or Stand By)	voltage regulation (square wave, sine wave)
Line Interactive (or Active Stand By)	output waveform (threshold-type or continuous)
Reversible (or Triport)	frequency regulation (threshold-type or continuous)
On-Line (or Continuous Operation)	galvanic isolation power factor correction fault-tolerance (by pass, static switch) communication (contacts, RS 232)

MGE UPS SYSTEMS choices

A Typical Mistake

Many people believe that any UPS with a voltage correction device (booster) could be called Line Interactive. Though MGE UPS SYSTEMS provides a booster on its ES 7/10 models, which use an off line topology, and on its ESV 8/11, which use a Line Interactive topology.
In actual fact, the voltage transfer threshold of the ES 7/10 is factory set and cannot be adjusted by the user, while the one of ESV 8/11 can be set to different values according to the users need. This makes the whole difference between an Off Line and a Line Interactive.