0–9
10-minute aggregation
The way IEC 61000-4-30 and EN 50160 condense measurements: values are aggregated into 10-minute intervals, and compliance is judged on the share of intervals in a week that stay within limits (typically 95 %). EN 50160 explained.
A
Active power
The power that does useful work, measured in watts (W). In AC circuits it is the average of instantaneous voltage times current, and it is always less than or equal to the apparent power.
Apparent power
The product of RMS voltage and RMS current, measured in volt-amperes (VA). Cables, transformers and UPS systems are sized on apparent power, not active power.
C
Crest factor
The ratio of the peak value of a waveform to its RMS value. A pure sine wave has a crest factor of 1.414; the pulsed current of rectifier loads such as computers and LED drivers is much higher.
Current clamp
A current sensor that closes around a conductor without breaking the circuit. Rigid clamps suit smaller conductors and give good accuracy at low currents.
D
Displacement power factor
The cosine of the angle between the fundamental voltage and fundamental current. Capacitor banks correct displacement power factor; they do not correct the part of a low power factor caused by harmonics.
E
Earth leakage current
Current that flows to protective earth in normal operation, from filters, cable capacitance or insulation degradation. A rising trend is an early warning of insulation failure.
Earth resistance
The resistance between an earth electrode system and the general mass of earth. A high value limits fault current and lets touch voltages rise. Inspector V4 measures it in the same unit that records the supply. Inspector V4.
EN 50160
The European standard describing the voltage characteristics of public supply networks: what a customer can expect in normal operation for frequency, magnitude, flicker, unbalance and harmonics. EN 50160 explained.
F
Flicker
The visible fluctuation of light caused by voltage fluctuations. It is quantified as short-term severity Pst over 10 minutes and long-term severity Plt over 2 hours; EN 50160 expects Plt ≤ 1 for 95 % of a week.
Frequency deviation
A difference between the actual and nominal supply frequency. EN 50160 expects 50 Hz ±1 % for 99.5 % of a year on interconnected systems.
Fundamental frequency
The nominal frequency of the supply, 50 Hz or 60 Hz. Harmonics are numbered as multiples of it.
H
Harmonic resonance
A condition in which a capacitor bank and the system inductance form a resonant circuit at or near a harmonic frequency, amplifying that harmonic. It is why harmonics should be measured before power factor capacitors are added.
Harmonics
Sinusoidal voltages or currents at whole-number multiples of the fundamental frequency (the 5th harmonic of 50 Hz is 250 Hz). They are produced by non-linear loads such as drives, rectifiers, UPS systems and LED lighting. Harmonics and power factor guide.
I
IEC 61000-4-30
The international standard that defines how power quality parameters are measured and aggregated, so that two instruments give comparable results. It defines Class A (for contractual and compliance measurements where results must be comparable) and Class S (for surveys and statistical assessment). Class A vs Class S.
IEEE 519
The IEEE standard for harmonic control in power systems. It limits voltage distortion that the utility delivers and current distortion that the customer injects, both at the PCC. IEEE 519 explained.
IEEE 1159
The IEEE recommended practice for monitoring electric power quality. Its table of categories — transients, short- and long-duration variations, unbalance, waveform distortion, fluctuations and frequency variations — is the most widely used classification of disturbances.
Inrush current
The high current drawn for a short time when a transformer, motor, capacitor or power supply is switched on. It can cause dips on the same supply and nuisance trips.
Interharmonics
Components at frequencies that are not whole-number multiples of the fundamental. Cycloconverters, arc furnaces and some drives produce them, and they can cause light flicker.
Interruption
A reduction of voltage to almost zero: below 0.1 pu in IEEE 1159, and below the interruption threshold (typically 5 % or 10 %) in IEC 61000-4-30. Short interruptions last up to three minutes in EN 50160.
ITIC curve
The voltage tolerance envelope published by the Information Technology Industry Council (successor to the CBEMA curve). It shows the dip and swell magnitudes and durations that IT equipment is expected to withstand.
K
K-factor
A number that expresses how much extra heating harmonic currents cause in a transformer compared with a pure sinusoidal load. It is used to specify or derate transformers that feed non-linear loads.
M
Measurement category (CAT III, CAT IV)
The IEC 61010 rating of how close to the supply origin an instrument may safely be connected. CAT III covers distribution inside a building; CAT IV covers the origin of the installation, such as the service entrance.
N
Neutral-to-earth voltage
Voltage measured between the neutral and the protective earth at the load. Elevated values point to neutral overloading, triplen harmonics or wiring faults, and upset sensitive electronics.
Notching
A periodic voltage disturbance caused when power electronic converters commutate current from one phase to another, cutting short notches into the voltage waveform.
P
Point of common coupling (PCC)
The point on the network where a customer's installation connects and other customers are, or could be, supplied. IEEE 519 limits are assessed there, not at individual machines.
Power analyzer
An instrument that measures power, energy and efficiency, usually as averages over intervals. It does not usually capture or classify supply disturbances. Power analyzer vs power quality analyzer.
Power factor
True power factor is active power divided by apparent power (P/S). It includes the effect of both phase shift and harmonics, which is why it can be low even when the displacement power factor looks fine.
Power quality analyzer
An instrument that measures voltage, current and power and also detects, classifies and records disturbances such as dips, swells, transients, harmonics and flicker, with the waveform around each event. Inspector power quality analyzers.
Power quality monitoring (permanent)
Continuous measurement with an instrument installed for the long term, so that intermittent events are recorded whenever they happen. Portable vs permanent monitoring.
R
Rapid voltage change (RVC)
A quick transition of RMS voltage between two steady states that does not cross the dip or swell thresholds. IEC 61000-4-30 defines how it is detected; motor starts and transformer energising are common causes.
Reactive power
Power that flows back and forth between source and load without doing work, measured in volt-amperes reactive (var). Motors and transformers draw it; capacitor banks supply it. Power factor correction calculator.
Ride-through
The ability of equipment to keep operating through a voltage dip or short interruption. It is judged against curves such as ITIC and SEMI F47. Voltage sag calculator.
RMS value
Root mean square: the value of an AC voltage or current that delivers the same heating as the same DC value. Power quality instruments compute it over each cycle or half cycle to detect dips and swells.
Rogowski coil
A flexible, air-cored current sensor that wraps around large conductors or busbars. It does not saturate, so it suits high currents and fast-changing waveforms.
S
SEMI F47
The semiconductor industry's voltage sag immunity standard: equipment must ride through dips to 50 % for 200 ms, 70 % for 500 ms and 80 % for 1 s.
Short-circuit ratio (ISC/IL)
The available short-circuit current at the PCC divided by the maximum demand load current. In IEEE 519 it selects which row of the current distortion limits applies: a stiffer supply allows more harmonic current.
T
TDD (total demand distortion)
Harmonic current distortion expressed as a percentage of the maximum demand load current (IL), not of the present current. IEEE 519 sets its current limits in TDD so that a lightly loaded site is not judged on a few amps. IEEE 519 explained.
THD (total harmonic distortion)
The RMS of all harmonic components divided by the fundamental, as a percentage. EN 50160 allows voltage THD up to 8 % for 95 % of a week; IEEE 519 sets 8 % at 1 kV and below. THD calculator.
Transient
A sudden, short-lived disturbance. Impulsive transients are fast one-direction spikes, typically from lightning or switching; oscillatory transients ring, typically from capacitor bank switching. Transient overvoltage guide.
Triplen harmonics
Odd multiples of the third harmonic (3rd, 9th, 15th…). In a three-phase four-wire system they add in the neutral instead of cancelling, which can overload neutral conductors.
U
Undervoltage and overvoltage
Long-duration RMS variations lasting more than one minute (IEEE 1159), usually caused by load changes, tap settings or long feeders rather than faults.
V
Voltage dip (sag)
A temporary reduction of RMS voltage below the dip threshold, typically 90 % of declared voltage, lasting from half a cycle to one minute. "Dip" is the IEC term and "sag" the IEEE term; IEEE 1159 defines it as 0.1–0.9 pu. Dip vs sag vs interruption.
Voltage swell
A temporary increase of RMS voltage above the swell threshold, typically 110 %, for half a cycle to one minute. Single-phase-to-earth faults can cause swells on the healthy phases.
Voltage unbalance
Unequal voltages between the three phases, measured as the ratio of negative-sequence to positive-sequence voltage. EN 50160 expects it to stay at or below 2 % for 95 % of a week (up to 3 % in some areas). It overheats three-phase motors.