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

Maintains a relatively constant output voltage for variations up to 20% in the input voltage.

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.


Undervoltage due to large motor starts or feeder/transformer drops under load. Sometimes used to describe voltage sags or undervoltages.


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

The alteration of the normal voltage or current pattern (sine wave) due to equipment generating frequencies other than the standard 60 cycles per second.


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

A device that is connected to the electrical power supply which provides voltage regulation for sags and swells. Can also reduce electrical noise.

Momentary outage

Brief interruption in power commonly lasting between 0.5 cycles and 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.


Complete loss of electrical power.


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 without delay or transient, during any period when the normal power supply is interrupted. Certain types of UPSs continuously condition the power to eliminate the effects of most sags, swells and momentary interruptions.


Electromotive force in an electric circuit. Measured in volts.

Voltage regulator

A device that maintains output within a desired limit despite varying input voltage. These devices usually provide little to no protection against voltage transients or noise.

Waveform distortion

Any power quality variation in the waveshape of the voltage or current.