A UPS (Uninterruptible Power Supply) is a device that is installed between the raw mains power coming into a building or room and the critical equipment to be protected against power fluctuations or failures.
It is a system that provides backup power (usually derived from batteries) when the electrical mains supply fails or drops to an unacceptable voltage level.
Single phase and three phase systems are available.
Small UPS systems (that plug into the nearest power socket) usually provide back-up power for a few minutes from integral batteries; enough to power down a computer in an orderly manner. Longer back-up times are available by using additional battery enclosures.
Larger systems (depending upon the power rating) can have internal batteries and/or additional battery enclosures. Batteries can be housed in matching enclosures or cladded battery stands. Open battery racks are also available.
For very long battery back-up times a generator can provide the input power to the UPS system and support air conditioning or lighting. This may provide a more cost effective alternative to batteries.
UPS systems can have the facility to alert fileservers to shut down in an orderly manner when an outage has occurred or notify users by text/e-mail. Building Management Systems (BMS) can also be notified.