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What is a UPS?

An uninterruptible power supply (UPS) is a device providing emergency power to a load when the main power fails. A UPS offers near-instantaneous protection by using attached batteries and associated electronic circuitry for lower power users. For high power users, a UPS may use diesel generators.

Photo of a UPSUPS runtime is relatively short, lasting minutes, but typically long enough to allow time for an auxiliary power source to bring a stable source of power online or to allow the protected equipment ample time to properly shut down.

UPS units range in size from units designed to protect a single computer without a video monitor (around 200 VA rating) to large units powering entire data centers, buildings, or even cities.

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:

  • Power failure: defined as a total loss of input voltage. Surge: defined as a momentary or sustained increase in the main voltage.
  • Sag: defined as a momentary or sustained reduction in input voltage.
  • Spikes: defined as a brief high voltage excursion.
  • Noise: defined as a high frequency transient or oscillation, usually injected into the line by nearby equipment.
  • Frequency instability: defined as temporary changes in the mains frequency.
  • Harmonic distortion: defined as a departure from the ideal sinusoidal waveform expected on the line.