Terms and definitions

The purpose of this section is to present widely accepted definitions of many common power quality terms. Terms such as these are used in a variety of ways in a number of different documents and are often subject to change. As a result, the definitions provided here may or may not be consistent with similar terms that have been used in the rules that presently govern We Energies power quality response effort.

Constant voltage transformer

Specialized transformer that maintains a relatively constant output voltage for variations in voltage on the input/primary side in the range of 20% - 40%.

Critical load

Devices and equipment identified as important or essential to the safety of personnel or the economic health of a business.


Measurement of amount of electric charge through conducting medium. Measured in amps (amperes).


See "Sag".


A voltage variation. Often after electrical equipment malfunctions for an unknown reason, the malfunction will be attributed to a voltage disturbance.


Voltage variation resulting in noticeable and/or irritable changes in illumination from lighting equipment to human eye.


A slang term for a voltage transient or voltage variation that causes equipment mis-operation. Glitches may be used to describe a notching, distortion, flicker, noise or any waveshape irregularity.

Harmonic distortion

Periodic deviation from the 60Hz sinusoidal voltage and/or current waveform.


See "Outage".


Device that converts direct current (DC) power into alternating current (AC) power.

Isolation transformer

Transfers electrical power from alternating current (AC) power source to some equipment or device while isolating the powered device from the power source, usually for safety reasons. Isolation transformers provide galvanic isolation and used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits that must not be connected.

Line conditioner

General term for a device connected to the electrical power supply, which provides some form of power quality improvement.

Momentary outage

Drop in voltage below 10% of nominal for a duration of time greater than 0.5 cycles and less than or equal to 3 seconds.


An unwanted high-frequency electrical signal that alters the normal voltage pattern (sine wave).

Normal (nominal) voltage

The normal or contracted voltage assigned to a system for determining voltage class.


Drop in voltage below 10% of nominal for a duration greater than 0.5 cycles.


An increase in voltage outside the normal voltage levels (10% or greater) for more than one minute.

Power quality

Relates to powering and grounding electric equipment in a manner that is suitable to its operation.


A device that converts alternating current (AC) power to direct current (DC) power.


Short-term (less than 1 minute) decrease in voltage between 10 to 90 percent of nominal.


See "Transient".

Standby generator

An alternate power supply usually driven by a gas or diesel engine.


See "Transient".

Surge protective device (spd)

Generic term used to describe a device intended to limit transient overvoltages, divert surge currents or both. Also known as a surge suppressor, surge arrestor and transient voltage surge suppressor.


Short-term (less than 1 minute) increase in voltage >110 percent of nominal.


Subcycle disturbance in AC waveform, evidenced by sharp, brief discontinuity of the waveform.


A decrease in voltage outside the normal voltage levels (10% or greater) for more than one minute.

Uninterruptible Power Supply (UPS)

A system designed to automatically provide power during any period when the normal power supply is interrupted. Some UPS designs are capable of providing line conditioning continuously, as well as, providing no delay/interruption in the switch from normal supply power to back-up when the normal supply voltage is interrupted.


Electromotive force in an electric circuit. Measured in volts.

Voltage regulator

A device that maintains steady-state voltage within a desired range. These devices are designed and intended to maintain steady-state voltage and are limited in their regulation range typically between +/- 10%. As a result, voltage regulators provide little to no mitigation against voltage sags/swells, transients and/or harmonic distortion.