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The Institute of Electrical and Electronics Engineers seems as good of a place as any to go for a definition of the uninterruptible power supply (commonly known as the UPS). According to IEEE Std 446--the IEEE Orange Book--the UPS is "a system designed to automatically provide power, without delay or transients, during any period when the normal power supply is incapable of performing acceptably."
But there are many types of UPS systems available--and a number of ways to categorize them. First, UPS are either off-line or online. When sensing a normal power outage, an off-line UPS operating in standby mode transfers online to provide backup power.
In most cases, the transfer is accomplished within one cycle and goes virtually unnoticed by general loads. These days, however, sensitive electronic loads often can't tolerate...