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.


The "flow" of electricity. Much like water, a current will follow the path of least resistance. As a result, electric current always finds the shortest path to ground. 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.


A voltage variation of short duration but long enough to be noticeable to the human eye as a light flicker.


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.


Per the National Electrical Code, the physical connection from an electrical circuit connecting equipment or wiring to the earth.

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.

Isolated ground:

An insulated equipment grounding conductor that is run in the same conduit as the supply conductors. This conductor is insulated from the metallic raceway and all ground points throughout its length.

Isolation transformer:

A device that electrically separates and protects sensitive electronic equipment by buffering electrical noise and re-establishing the neutral-to-ground bond.

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:

A brief interruption in power commonly lasting between 1/30 (2 cycles) of a second 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.


Any short-term (less than 1 minute) decrease in voltage.


See "Transient".

Standby generator:

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


See "Transient".

Surge protective device (spd):

A device that is designed to limit instantaneous high voltages. Also known as a surge suppressor, surge arrestor and transient voltage surge suppressor.


Any short-term (less than one minute) increase in voltage.


A sudden dramatic increase in voltage that typically lasts less than 1/120 of a second.


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.


The electrical "pressure" that creates the flow of current.

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.