An uninterruptible power supply (UPS), also known as a continuous power supply (CPS) ,fuel cell backup or battery backup is a device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when utility power is not available. It differs from an auxiliary power supply or standby generator, which does not provide instant protection from a momentary power interruption. A UPS, however, can be used to provide uninterrupted power to equipment for 1–20 minutes until a generator can be turned on or utility power is restored. Integrated systems that have UPS and standby generator components are often referred to as emergency power systems.
There are three distinct UPS types:
1. off-line: remains idle until a power failure occurs, and then switches from utility power to its own power source, almost instantaneously.
2. line-interactive.
3. on-line: continuously powers the protected load from its energy reserves stored in a lead–acid battery or flywheel, while simultaneously replenishing the reserves from the AC power. It also provides protection against all common power problems, and for this reason it is also known as a power conditioner and a line conditioner.
While not limited to safeguarding any particular type of equipment, a UPS is typically used to protect computers, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption or data loss. UPS units come in sizes ranging from units which will back up a single computer without monitor (around 200 VA) to units which will power entire data centers or buildings (several megawatts).
Historically, UPSs were expensive and were most likely to be used on expensive computer systems and in areas where the power supply is interrupted frequently. As prices have fallen, UPS units have become an essential piece of equipment for data centers and business computers, and are also used for personal computers, entertainment systems and more.
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