Alternating current:  an electrical current that reverses direction periodically, usually many times per second

Blackout:  the total loss of AC utility power expressed in percent.  It is normally assumed that the measurements are taken at nominal input and output voltage levels and full load conditions

Double conversion:  in these systems the incoming line is converted to DC and is provided to the input side of the inverter.  The inverter then converts to AC which is used to power the load 

Ground loops:  return currents or magnetic fields from relatively high powered circuits or components which generate unwanted noisy signals in the common return of the relatively low level signal circuits

Harmonic distortion:  excessive harmonic content that distorts the normal sinusoidal waveform.  This can cause overheating of the neutral conductor and might appear to a device as data-corrupting noise

Impedance:  the total opposition that a circuit offers to the flow of the current at a particular frequency.  It may consist of any combination of resistance, inductive reactance, and capacitive reactance

Line interactive:  in the line interactive UPS, line power is not connected into DC but is fed directly to the critical load through an inductor or transformer.  Regulation and continuous power to the critical load is achieved throughout the use of inverter magnetic components such as inductors, linear transformers, or Ferro resonant transformers

Noise:  the noise voltage that appears equally and in phase from each current carrying conductor to ground

Offline UPS:  a type of uninterruptible power supply that allows a transient break in the output while a load is detected and the inverter is turned on

Online UPS:  a type of uninterruptible power supply that provides a continuously filtered, no-break output to the protected load.  An online UPS has zero transfer time; the output to the protected load never sees an interruption

Power conditioner:  a device that takes the raw AC power from the utility and cleans or conditions it by removing harmful aberrations (such as spikes, noise, etc.), making the power more acceptable to electrical or electronic devices.  They do not contain batteries and are not capable of providing power for extended periods of time in the event of a total power loss

Redundancy:  use of multiple devices or modules to provide continued operation following most failures in a single module device 

Sag:  a reduction of the AC voltage, at the power frequency, for durations from a half cycle to a few seconds

Single-phase:  energized by a single alternating voltage

Single conversion:  single conversion UPS systems are those in which, during normal operation, the incoming line is used to provide power to the critical load either through a transformer or in conjunction with some series impedance

Spike:  a transient or impulse with duration much less than 1/2 cycle that can occur anywhere on the waveform in the common or normal mode

Standby UPS:  an uninterruptible power supply topology that allows a transient break in the output while a load is detected and the inverter is turned on

Swell:  an increase in the voltage or current of the main voltage at the power frequency, for durations from a half cycle to a minute

Three-phase:  a system of alternating current supply comprised of three conductors between successive pairs of which are maintained alternating differences of potential, successively displaced on phase by one-third of a period

Transient:  an excursion in a given parameter, typically associated with input voltage or output loading

Uninterruptible Power System (UPS): a system designed to automatically provide power without delay or transients, during any period when the normal power supply is incapable of performing acceptably