Power Quality Definitions


Please note that the definitions are updated regularly (message date: February 4, 2013)


Active Filter. Any of a number of sophisticated power electronic devices for eliminating harmonic distortion.


CBEMA Curve. (see ITIC Curve)

Common Mode Voltage. The noise voltage that appears equally from current-carrying conductor (Line of Neutral) to ground.

Coupling. Circuit element or elements, or network, that may be considered common to the input mesh and the output mesh and through which energy may be transferred from one to another.

Crest Factor. A value reported by many power quality monitoring instruments representing the ratio of the crest value of the measured waveform to the rms of the fundamental. For example, the crest factor of a sinusoidal wave
is 1.414.

Critical Load. Devices and equipment whose failure to operate satisfactorily jeopardizes the health or safety of personnel, and/or results in loss of function, financial loss, or damage to property deemed critical by the user.

Current Distortion. Distortion in the ac line current. See Distortion.


Differential Mode Voltage. The voltage between any two of a specified set of active conductors.

Dip. See Sag.

Distortion. Any deviation from the normal sine wave for an ac quantity.

Dropout. A loss of equipment operation (discrete data signals) due to noise, sag, or interruption.

Dropout Voltage. The voltage at which a device will release to its de-energized position (for this document, the voltage at which a device fails to operate).


Electromagnetic Compatibility. The ability of a device, equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment.

Equipment Grounding Conductor. The conductor used to connect the non-current carrying parts of conduits, raceways, and equipment enclosures to the grounded conductor (neutral) and the grounding electrode at the service equipment (main panel) or secondary of a separately derived system (e.g., isolation transformer). See
NFPA 70-1990, Section 100.

Event (power Quality) Power supply quality parameters are necessary in order to investigate and analyze power supply issues. These parameters include disturbances such as transients, dips, swells, interruptions, flicker, and frequency fluctuations. As a rule, the term “event” refers to the state detected based on thresholds for which abnormal values and abnormal waveforms for these parameters have been set. Events also include timer and repeat event settings, which are unrelated to power supply quality parameters.


Failure Mode. The effect by which failure is observed.

Fast Tripping. Refers to the common utility protective relaying practice in which the circuit breaker or line recloser operates faster than a fuse can blow. Also called fuse saving. Effective for clearing transient faults without a sustained interruption, but is somewhat controversial because industrial loads are subjected to a momentary or temporary interruption.

Fault. Generally refers to a short circuit on the power system.

Fault, Transient. A short circuit on the power system usually induced by lightning, tree branches, or animals which can be cleared by momentarily interrupting the current.

Flicker. Flicker differs from other power distortions because it deals with the effect of the power quality on humans rather than on equipment. Since most buildings use electrical lighting, power fluctuations cause lighting fluctuations.  The result of this can be simply annoying, producing headaches and eye fatigue. The discovery that cyclical light variations with a frequency close to 9 Hz can trigger epileptic seizures in susceptible individuals precipitated an attempt to quantify and specify allowable flicker limits. However, the effect on fluorescent lights (with electronic ballasts) is minimal making the issue less important. Flicker is a fundamental-frequency effect. It is the impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time. The variations which cause flicker can be expressed as sidebands (modulation) of the “carrier” or fundamental signal. The frequencies of these sidebands are in the vicinity of the carrier, typically 0.5 to 20 Hz removed, unlike harmonics, which may be removed by a factor of 50. This means that the line impedance can be more reliably used to determine the direction of flicker, or more precisely, whether the utility is imposing flicker voltages on the customer or the customer is imposing flicker currents on the utility. The IEC standards define flicker in standards IEC 868 and the newer IEC 61000-4-15.

Frequency Deviation. An increase or decrease in the power frequency. The duration of a frequency deviation can be from several cycles to several hours.

Frequency Response. In power quality usage, generally refers to the variation of impedance of the system, or a metering transducer, as a function of frequency.

Fundamental (Component). The component of order 1 (50 to 60 Hz) of the Fourier series of a periodic quantity.


Ground. A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body of relatively large extent that serves in place of the earth.
Note: It is used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it, and for conducting ground currents to and from earth (or the conducting body).

Ground Electrode. A conductor or group of conductors in intimate contact with the earth for the purpose of providing a connection with the ground.

Ground Grid. A system of interconnected bare conductors arranged in a pattern over a specified area and on or buried below the surface of the earth. The primary purpose of the ground grid is to provide safety for workmen by limiting potential differences within its perimeter to safe levels in case of high currents which could flow if the circuit being worked became energized for any reason or if an adjacent energized circuit faulted. Metallic surface mats and gratings are sometimes utilized for the same purpose . This is not necessarily the same as a Signal Reference Grid.

Ground Loop. A potentially detrimental loop formed when two or more points in an electrical system that are nominally at ground potential are connected by a conducting path such that either or both points are not at the same ground potential.

Ground Window. The area, through which, all grounding conductors, including metallic raceways enter a specific area. It is often used in communications systems through which the building grounding system is connected to an area that would otherwise have no grounding connection.


Harmonic (component). A component of order greater than one of the Fourier series of a periodic quantity.

Harmonic Content. The quantity obtained by subtracting the fundamental component from an alternating quantity.

Harmonic Distortion. Periodic distortion of the sine wave. See Distortion and Total Harmonic Distortion (THD).

Harmonic Filter. On power systems, a device for filtering one or more harmonics from the power system. Most are passive combinations of inductance, capacitance, and resistance. Newer technologies include active filters that can also address reactive power needs.

Harmonics phase angle and phase difference: The harmonic voltage phase angle and harmonic current phase angle are expressed in terms of the synchronized source’s fundamental component phase. The difference between each order’s harmonic component phase and the fundamental component phase is expressed as an angle (°), and its sign indicates either a lagging phase (negative) or leading phase (positive). The sign is the reverse of the power factor sign. The harmonic voltage-current phase angle expresses the difference between each order’s harmonic voltage component phase angle and harmonic current component phase angle for each channel as an angle (°). When using the sum display, the sum of each order’s harmonic power factor (calculated from the sums of harmonic power and harmonic reactive power) is converted to an angle (°). When the harmonic voltage-current phase angle is between -90° and +90°, that order’s harmonics are flowing toward the load (inflow). When the harmonic voltage-current phase angle is between +90° and +180° or between -90° and -180°, that order’s harmonics are flowing from the load (outflow).

Inflow and Outflow of Harmonics 7

Harmonic Number. The integral number given by the ratio of the frequency of a harmonic to the fundamental frequency.

Harmonic Resonance. A condition in which the power system is resonating near one of the major harmonics being produced by nonlinear elements in the system, thus exacerbating the harmonic distortion.


Impulse. A pulse that, for a given application, approximates a unit pulse or a Dirac function. When used in relation to the monitoring of power quality, it is preferred to use the term impulsive transient in place of impulse.

Impulsive transient. A sudden non-power frequency change in the steady state condition of voltage or current that is unidirectional in polarity (primarily either positive or negative).

Inrush current:  A large current that flows temporarily, for example when an electric device is turned on. A inrush current can be equal to or greater than 10 times the current that flows when the device is in the normal operating state. Inrush current measurement can be a useful diagnostic when setting circuit breaker capacity.

Inter-harmonics: All frequencies that are not a whole-number multiple of the fundamental frequency. Inter-harmonics include intermediate frequencies and inter-order harmonics, and the term refers to RMS values for the spectral components of electrical signals with frequencies between two contiguous harmonic frequencies. (Inter-harmonics of the order 3.5 assume a drive of 90 Hz or similar rather than a frequency synchronized to the fundamental wave of an inverter or other device. However, inter-harmonics do not generally occur in high-voltage circuits under present-day conditions. Most inter-harmonics are currently thought to be caused by the circuit load.)

Instantaneous. When used to quantify the duration of a short duration variation as a modifier, refers to a time range from one-half cycle to 30 cycles of the power frequency.

Instantaneous Reclosing. A term commonly applied to reclosing of a utility breaker as quickly as possible after interrupting fault current. Typical times are 18-30 cycles.

Interharmonic (component). A frequency component of a periodic quantity that is not an integer multiple of the frequency at which the supply system is designed to operate (e.g. 50 Hz or 60 Hz).

Interruption, Momentary (electric power systems). An interruption of duration limited to the period required to restore service by automatic or supervisory-controlled switching operations or by manual switching at locations where an operator is immediately available. Note: Such switching operations must be completed in a specified time not to exceed 5 minutes.

Interruption, Momentary (power quality monitoring). A type of short duration variation. The complete loss of voltage (<0.1 pu) on one or more phase conductors for a time period between 30 cycles and 3 seconds.

Interruption, Temporary. A type of short duration variation. The complete loss of voltage (<0.1 pu) on one or more phase conductors for a time period between 3 seconds and 1 minute.

Interruption, Sustained (electric power systems). Any interruption not classified as a momentary interruption.

Interruption, Sustained (power quality): A type of long duration variation. The complete loss of voltage (<0.1 pu) on one of more phase conductors for a time greater than 1 minute.

Isolated Ground:  An insulated equipment grounding conductor run in the same conduit or raceway as the supply conductors. This conductor is insulated from the metallic raceway and all ground points throughout its length. It originates at an isolated ground-type receptacle or equipment input terminal block and terminates at the point where neutral and ground are bonded at the power source. See NFPA 70-1990, Section 250-74, Exception #4 and Section 250-75, Exception.

Isolation. Separation of one section of a system from undesired influences of other sections.

ITIC Curve is a modified version of the CBEMA power acceptability curve, but the concept remains the same. It was developed by a working group of the CBEMA, which later changed its name to the Information Technology Industry Council (ITI) in 1994. In addition, the ITIC curve was created in collaboration with EPRI’s Power Electronics Application Center (PEAC). The intent was to derive a curve that can better reflect the performance of typical single-phase, 120 V, 60 Hz computers and their peripherals, and other information technology items like fax machines, copiers and point-of-sales terminals. The ITIC curve has been applied to general power quality evaluation, even though it was primarily developed for 120 V computer equipment just like the CBEMA curve. Also, it is used as a reference to define the withstand capability of various loads and devices for protection from power quality problems. This is because the curve is generally applicable to other equipment containing solid-state devices aside from being specifically applicable to computer-type equipment. However, one should be careful and should keep in mind that the ITIC curve is not intended to reflect the performance of ALL electronic-based equipment. There are too many variables – power loading, nominal operating voltage level, and process complexity, to try to apply a one-size-fits-all ITIC curve.

ITIC curve


K factor: Shows the power loss caused by the harmonic current in transformers. Also referred to as the “multiplication factor.” The K factor (KF) is formulated as shown below:

k: Order of harmonics
Ik: Ratio of the harmonic current to the fundamental wave current [%] Higher-order harmonic currents have a greater influence on the K factor than lower-order harmonic currents.

To measure the K factor in a transformer when subjected to maximum load. If the measured K factor is larger than the multiplication factor of the transformer used, the transformer must be replaced with one with a larger K factor, or the load on the transformer must be reduced. The replacement transformer should have a K factor one rank higher than the measured K factor for the transformer being replaced.


Linear Load. An electrical load device which, in steady state operation, presents an essentially constant load impedance to the power source throughout the cycle of applied voltage.

Long Duration Variation. A variation of the rms value of the voltage from nominal voltage for a time greater than one minute. Usually further described using a modifier indicating the magnitude of a voltage variation (e.g., Undervoltage, Overvoltage, or Voltage Interruption).

Low-Side Surges. A term coined by distribution transformer designers to describe the current surge that appears to be injected into the transformer secondary terminals upon a lightning strike to grounded conductors in the vicinity.


Momentary. When used to quantify the duration of a short duration variation as a modifier, refers to a time range at the power frequency from 30 cycles to 3 seconds.


Noise.Unwanted electrical signals which produce undesirable effects in the circuits of the control systems in which they occur. (For this document, “control systems” is intended to include sensitive electronic equipment in total or in part.)

Nominal Voltage. (Vn). A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class (as 208/120, 480/277, 600).

Nonlinear Load. Electrical load which draws current discontinuously or whose impedance varies throughout the cycle of the input ac voltage waveform.

Normal Mode Voltage. A voltage that appears between or among active circuit conductors.

Notch. A switching (or other) disturbance of the normal power voltage waveform, lasting less than a half-cycle; which is initially of opposite polarity than the waveform, and is thus subtracted from the normal waveform in terms of the peak value of the disturbance voltage. This includes complete loss of voltage for up to a half cycle.


Oscillatory Transient. A sudden, non-power frequency change in the steady state condition of voltage or current that includes both positive or negative polarity value.

Overvoltage. When used to describe a specific type of long duration variation, refers to a voltage having a value of at least 10% above the nominal voltage for a period of time greater than 1 minute.


Passive Filter. A combination of inductors, capacitors, and resistors designed to eliminate one or more harmonics. The most common variety is simply an inductor in series with a shunt capacitor, which short-circuits the major distorting harmonic component from the system.

Phase Shift. The displacement in time of one voltage-waveform relative to other voltage-waveform(s).

Power Factor, Displacement. The power factor of the fundamental frequency components of the voltage and current wave forms.

Power Factor (True). The ratio of active power (watts) to apparent power (voltamperes).

Pulse. An abrupt variation of short duration of a physical quantity followed by a rapid return to the initial value.


Reclosing. The common utility practice on overhead lines of closing the breaker within a short time after clearing a fault taking advantage of the fact that most faults are transient, or temporary.

Recovery Time. Time interval needed for the output voltage or current to return to a value within the regulation specification after a step load or line change. Also may indicate the time interval required to bring a system back to its operating condition after an interruption or dropout.

Recovery Voltage. The voltage that occurs across the terminals of a pole of a circuit interrupting device upon interruption of the current.

RMS value: The root mean square of instantaneous values for a quantity obtained over a particular time interval or bandwidth.


Safety Ground. See: Equipment Grounding Conductor.

Sag. A decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency for durations of 0.5 cycles to one minute.

Shield. As normally applied to instrumentation cables, refers to a conductive sheath (usually metallic) applied, over the insulation of a conductor or conductors, for the purpose of providing means to reduce coupling between the conductors so shielded and other conductors which may be susceptible to, or which may be generating
unwanted electrostatic or electromagnetic fields (noise).

Shielding. Shielding is the use of a conducting and/or ferromagnetic barrier between a potentially disturbing noise source and sensitive circuitry. Shields are used to protect cables (data and power) and electronic circuits. They may be in the form of metal barriers, enclosures, or wrappings around source circuits and receiving circuits.

Shielding (of utility lines). The construction of a grounded conductor or tower over the lines to intercept lightning strokes in an attempt to keep the lightning currents out of the power system.

Short Duration Variation. A variation of the rms value of the voltage from nominal voltage for a time greater than one-half cycle of the power frequency but less than or equal to one minute. Usually further described using a modifier indicating the magnitude of a voltage variation (e.g. Sag, Swell, or Interruption) and possibly a modifier indicating the duration of the variation (e.g., Instantaneous, Momentary or Temporary).

Signal Reference Grid (or Plane). A system of conductive paths among interconnected equipment, which reduces noise-induced voltages to levels which minimize improper operation. Common configurations include grids and planes.

Sustained. When used to quantify the duration of a voltage interruption, refers to the time frame associated with a long duration variation (i.e., greater than one minute).

Swell. A temporary increase in the rms value of the voltage of more than 10% the nominal voltage, at the power frequency, for durations from 0.5 cycle to one minute.

Synchronous Closing. Generally used in reference to closing all three poles of a capacitor switch in synchronism with the power system to minimize transients.


Temporary. When used to quantify the duration of a short duration variation as a modifier, refers to a time range from 3 seconds to 1 minute.

Total Demand Distortion (TDD). The ratio of the root-mean-square of the harmonic current to the root-mean-square value of the rated or maximum demand fundamental current, expressed as a percent.

Total Disturbance Level.  The level of a given electromagnetic . disturbance caused by the superposition of the emission of all pieces of equipment in a given system.

Total Harmonic Distortion (THD). The ratio of the root-mean-square of the harmonic content to the root-mean-square value of the fundamental quantity, expressed as a percent of the fundamental.

Transient. Pertaining to or designating a phenomenon or a quantity which varies between two consecutive steady states during a time interval that is short compared to the time scale of interest. A transient can be a unidirectional impulse of either polarity or a damped oscillatory wave with the first peak occurring in either polarity.

Triplen Harmonics. A term frequently used to refer to the odd multiples of the third harmonic, which deserve special attention because of their natural tendency to be zero sequence.


Undervoltage. When used to describe a specific type of long duration variation, refers to a measured voltage having a value at least 10% below the nominal voltage for a period of time greater than one minute.


Voltage Change. A variation of the rms or peak value of a voltage between two consecutive levels sustained for definite but unspecified durations.

Voltage Dip. See Sag.

Voltage Distortion. Distortion of the ac line voltage. See Distortion.

Voltage Fluctuation. A series of voltage changes or a cyclical variation of the voltage envelope.

Voltage Imbalance (Unbalance). A condition in which the three phase voltages differ in amplitude or are displaced from their normal 120 degree phase relationship or both. Frequently expressed as the ratio of the negative sequence or zero sequence voltage to the positive sequence voltage, in percent.

Voltage Interruption. Disappearance of the supply voltage on one or more phases. Usually qualified by an additional term indicating the duration of the interruption (e.g., Momentary, Temporary, or Sustained.)

Voltage Regulation. The degree of control or stability of the rms voltage at the load. Often specified in relation to other parameters, such as input-voltage changes, load changes, or temperature changes.

Voltage Magnification. The magnification of capacitor switching oscillatory transient voltage on the primary side by capacitors on the secondary side of a transformer.


Waveform Distortion. A steady state deviation from an ideal sine wave of power frequency principally characterized by the spectral content of the deviation.


1) Rx Monitoring Services http://www.rxms.com/defintions.html

2) http://www.hioki.com/product/guide/PW3198UGE4-19B-00H.pdf