Editing Uninterruptible power supply (section)

==Common power problems==
The primary role of any UPS is to provide short-term power when the input power source fails. However, most UPS units are also capable in varying degrees of correcting common utility power problems:
# [[Voltage spike]] or sustained [[overvoltage]]
# Momentary or sustained [[Brownout (electricity)|reduction in input voltage]]
# [[Voltage sag]]
# Noise, defined as a [[Line noise|high frequency transient]] or [[Transient (oscillation)|oscillation]], usually injected into the line by nearby equipment
# Instability of the [[mains frequency]]
# [[Harmonic distortion]], defined as a departure from the ideal [[sinusoidal]] [[waveform]] expected on the line

Some manufacturers of UPS units categorize their products in accordance with the number of power-related problems they address.<ref>E-book on choosing a UPS topology based on application type {{cite web|url=http://www.emersonnetworkpower.com/en-US/Brands/Liebert/Documents/White%20Papers/UPS%20Trap%20Doors%20for%20SMB.PDF|title=Avoiding Trap Doors Associated with Purchasing a UPS System|access-date=2018-12-11|archive-url=https://web.archive.org/web/20130326072420/http://www.emersonnetworkpower.com/en-US/Brands/Liebert/Documents/White%20Papers/UPS%20Trap%20Doors%20for%20SMB.PDF|archive-date=2013-03-26|url-status=dead}}</ref>

A UPS unit may also introduce problems with [[electric power quality]]. To prevent this, a UPS should be selected not only by capacity but also by the quality of power that is required by the equipment that is being supplied.