Power Quality Glossary

Almost everything you wanted to know about power quality is here. Over two hundred defined terms make this an invaluable reference tool.



Alternating Current
A flow of electrical current which increases to a maximum in one direction, decreases to zero, and then reverses direction and reaches maximum in the other direction and back so zero. The cycle is repeated continuously. The number of such cycles per second is equal to the frequency and is measured in “Hertz”. U.S. commercial power is 60 Hertz (i.e. 60 cycles per second).

Alternating Current Complement.

Above ground installation for power lines or telephone lines or cables are installed on a pole or overhead structure.

Current carrying capacity expressed in amperes.

Electrical test instrument used to measure current in a circuit.

A unit of measurement for electrical current or rate of flow of electrons (coulombs per second). If a group of electrons whose total charge is 1 coulomb passes a point in a conductor in 1 second, the electric current is 1 ampere. Its mathematical symbol is “I” the term is often shortened to “amps”.

American National Standards Institute.

Apparent Power
The product of voltage and current in a circuit.

Sparking that results when undesirable current flows between two points of differing potential. This may be due to leakage through the intermediate insulation or a leakage path due to contamination.

Armature Coil
A winding that develops current output from a generator when its turns cut a magnetic flux.

A nonlinear device to limit the amplitude of voltage on a power line. The term implies that the device stops overvoltage problems (i.e. lightning). In actuality, voltage clamp levels, response times and installation determine how much voltage can be removed by the operation of an arrester.

The reduction of a signal from one point to another. For an electrical surge, attenuation refers to the reduction of an incoming surge by a limiter (attenuator). Wire resistance, arresters, power conditioners attenuate surges to varying degrees.

American Wire Gage. This term refers to the U.S. standard for wire size.

A transformer used to step voltage up or down. The primary and secondary windings share common turns, and it provides no isolation.

Auxiliary Source
A power source dedicated to providing emergency power to a critical load when commercial power is interrupted.

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Balanced Load
An alternating current power system consisting of more than two current carrying conductors in which these conductors all carry the same current.

A collection of cells, grouped together to provide higher voltage and/or higher current than a single cell.

Battery Reservoir
A combination of cells or batteries used to power a UPS’s system inverter when it is in the emergency mode.

Battery Disconnect Switch
Master switch that disconnects a battery reservoir from a UPS. Provides personnel protection when batteries or UPS require service.

A total loss of commercial power.

Deliberate connection of two or more points to reduce any difference of potential (voltage).

Branch Circuit
A division of a load circuit with current limited by a fuse or circuit breaker.

Operational sequence of a switch or relay where the existing connection is opened prior to making the new connection.

A low voltage condition lasting longer than a few cycles. “Brownouts” differ from “sags” only in duration.

British Thermal Unit. Energy required to raise one pound of water one degree Fahrenheit. One pound of water at 32 degrees F requires the transfer of 144 BTUs to freeze into solid ice.

Buck-Boost Transformer
A small, low voltage transformer placed in series with the power line to increase or reduce steady state voltage.

A heavy, rigid conductor used for high voltage feeders.

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Two plates or conductors separated by an insulator. Applying a voltage across the plates causes current to flow and stores a charge. Capacitors resist changes in voltage.

An AC-to-DC converter which powers a UPS inverter and maintains the battery reservoir charge.

Clamp-On CT
A current transformer which clamps around a current-carrying conductor so the conductor does not have to be opened for insertion of the transformer primary. Particularly suited for monitoring where current must be sensed at many points for relatively short periods.

Common Mode (CM)
The term refers to electrical interference which is measurable as a ground referenced signal. In true common mode, a signal is common to both the current carrying conductors.

Common Mode Noise
An undesirable voltage which appears between the power conductors and ground.

A tubular raceway for data or power cables. Metallic conduit is common, although non-metallic forms may also be used. A conduit may also be a path or duct and need to be tubular.

A device which changes alternating current to direct current.

The ferrous center part of a transformer or inductor used to increase the strength of the magnetic field.

Core Saturation
Condition when an inductor or transformer core has reached maximum magnetic strength.

The combined negative electrical charge of 6.24 X 1018 electrons.

Crest Factor
(Usually refers to current) – the mathematical relationship between RMS current and peak current. A normal resistive load will have a crest factor of 1.4142 which is the normal relationship between peak and RMS current. A typical PC will have a crest factor of 3.

Critical Load
Equipment that requires an uninterrupted power input to prevent damage or injury to personnel, facilities, or itself.

The movement of electrons through a conductor. Measured in amperes and its symbol is “I”.

Current Transformer
(or CT) – A transformer used in instrumentation to assist in measuring current. It utilizes the strength of the magnetic field around the conductor to form an induced current that can then be applied across a resistance to form a proportional voltage.

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The standard unit for expressing relative power levels. Decibels indicate the ratio of power output to power input dB = 10 log10 (P1/P2).

A standard three phase connection with the ends of each phase winding connection in series to form a closed loop with each phase 120 electrical degrees from the other.

The connection between a delta source and a delta load.

The connection between a delta source and a wye load.

Differential Amplifier
One that has two input signal connections and zero signal reference lead. The output is the algebraic sum of the instantaneous voltages appearing between the two input signal connections.

Direct Current
Electrical current which flows in one direction only.

A nonvolatile mass memory storage device for computers.

DMM (Digital Multimeter)
An instrument used to measure voltage, current and resistance.

A discrete voltage loss. A voltage sag (complete or partial) for a very short period of time (milliseconds) constitutes a dropout.

The change in voltage per change in time.

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Emitter coupled logic. Extremely high-speed electronic circuitry where changes in binary logic are determined by very fast switching between specific voltage levels, rather than by semiconductor saturation and cutoff.

Earth Ground
A low impedance path to earth for the purpose of discharging lightning, static, and radiated energy, and to maintain the main service entrance at earth potential.

Earthling Electrode
A ground electrode, water, pipe, or building steel, or some combination of these, used for establishing a building’s earth ground.

The percentage of input power available for used by the load. The mathematical formula is: Efficiency = Po/ Pi Where “Po” equals power output, “Pi” equals power input, and power is represented by watts.

Electrical Degrees
One cycle of A.C. power is divided into 360 degrees. This allows mathematical relationships between the various aspects of electricity.

A magnetic field cause by an electric current. Power lines cause electromagnetic fields which can interfere with nearby data cables.

A mechanical device which is controlled by an electric device. Solenoids and shunt trip circuit breakers are examples of electromechanical devices.

A Potential difference (electric charge) measurable between two points which is caused by the distribution if dissimilar static charge along the points. The voltage level is usually in kilovolts.

Electrostatic Shield
A metallic barrier or shield between the primary and secondary windings of a transformer which reduces the capacitive coupling and thereby increases the transformers ability to reduce high frequency noise.

Electromotive force or voltage.

Acronyms for various types of electrical interference: electomagnetic interference, radio frequency interference.

Equipment Event Log
A record that is kept of equipment problems and activity, to compare against power monitor data to correlate equipment problems with power events.

Error Burst
A large number of errors within a given period of time as compared to preceding and following time periods.

Electrostatic Discharge (static electricity). The effects of static discharge can range from simple skin irritation for an individual to degraded or destroyed semiconductor junctions for an electronic device.

Event Summary
A plot of recorded power monitor events over time.

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Unit of measurement for capacitance.

Farady Shield
A grounded metallic barrier which can be used for improved isolation between the windings of a transformer. In this application, the shield basically reduces the leakage capacitance between the primary and secondary.

Transmission lines supplying power to a distribution system.

Resonance resulting when the iron core of an inductive component of an LC circuit is saturated, increasing the inductive reactance with respect to the capacitance reactance.

Ferroresonant Transformer
A voltage regulating transformer which depends on core saturation and output capacitance.

A selective network of resistor, inductors, or capacitors which offers comparatively little opposition to certain frequencies or direct current, while blocking or attenuating other frequencies.

Federal Information Processing Standards Publication (1983, September 21) is an official publication of the National Bureau of Standards (since renamed National Institute for Standards and Technology). The document is a recommended guideline for federal agencies with respect to the electrical environment for automatic data processing (ADP) facilities.

Flashing due to high current flowing between two points of different potential. Usually due to insulation breakdown resulting from arcing.

A surge or sag in voltage amplitude, often caused by load switching or fault clearing.

The lines of force of a magnetic field.

Forward Transfer Impedance
The amount of impedance placed between the source and load with installation of a power conditioner. With no power conditioner, the full utility power is delivered to the load; even a transformer adds some opposition to the transfer of power. On transformer based power conditioners, a high forward transfer impedance limits the amount of inrush current available to the load.

Fine print note, National Electrical Code (NEC) explanatory material.

On AC circuits, designates number of times per second that the current completes a full cycle in positive and negative directions. See also “alternating current”.

Frequency Deviation
A variation from nominal frequency.

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GFI (Ground Fault Interrupter)
A device whose function is to interrupt the electric circuit to the load when a fault current to ground exceeds some predetermined value that is less than that required to operate the overcurrent protective device of the supply circuit.

Connected to earth or to some conducting body that serves in place of the earth.

Ground Fault
Any undesirable current path from a current carrying conductor to ground.

Connection of one side of a circuit to the earth or a body that serves in place of the earth, through low impedance paths. Sometimes confused with bonding. Grounding should always conform to the National Electrical Code.

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A sinusodial component of an AC voltage that is multiple of the fundamental waveform frequency.

Harmonic Distortion
Regularly appearing distortion of the sine wave whose frequency is a multiple of the fundamental frequency. Converts the normal sine wave into a complex waveform.

Harmonic Neutralization
A cancellation process: harmonics at the output of a circuit are inverted and fed back in their opposite phase.

Unit of measurement for inductance.

Hertz (HZ)
Unit of frequency, one hertz (Hz) equals one cycle per second.

High Voltage.

A device which is composite of differing technologies to create a better functionality.

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The expression of power resulting from the flow of current through a resistance: P = I2R.

Institute of Electrical and Electronics Engineers.

Forces which resist current flow in A.C. circuits, i.e. resistance, inductive reactance, capacitive reactance.


The ability of a coil to store energy and oppose changes in current flowing through it. A function of the cross sectional area, number of turns of coil, length of coil and core material.

(Also called “choke”) – A coiled conductor which tends to oppose any change in the flow of current. Usually has coils wrapped around ferrous core.

Inrush Current
The initial surge current demand before the load resistance or impedance increases to its normal operating value.

A device used to change DC into AC power.

Isolation Transformer
A multiple winding transformer with primary and secondary windings physically separated and designed to permit magnetic coupling between isolated circuits while minimizing electrostatic coupling. See also “electrostatic shield”.

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s A watt/second. A measurement of work in time. 1 joule equals 0.0002778 watt/hours. 1 kilowatt hour is equivalent to 3,600,000 joules.

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Kilo (K)
A metric prefix meaning 1000 or 103.

(Kilovolt amperes) (volts times amperes) divided by 1000. 1 KVA=1000 VA. KVA is actual measured power (apparent power) and is used for circuit sizing.

(Kilowatts) watts divided by 1000. KW is real power and is important in sizing UPS, motor generators or other power conditioners. See also “power factor”.


(Kilowatt hours) KW times hours. A measurement of power and time used by utilities for billing purposes.

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Lagging Load
An inductive load with current lagging voltage. Since inductors tend to resist changes in current, the current flow through an inductive circuit will lag behind the voltage. The number of electrical degrees between voltage and current is known as the “phase angle”. The cosine of this angle is equal to the power factor (linear loads only).

LC Circuit
An electrical network containing both inductive and capacitive elements.

Leading Load
A capacitive load with current leading voltage. Since capacitors resist changes in voltage, the current flow in a capacitive circuit will lead the voltage.

Linear Load
A load in which the current relationship to voltage is constant based on a relatively constant load impedance.

Line Imbalance
Unequal loads on the phase lines of a multiphase feeder.

The driven device that uses the power supplied from the source.

Load Balancing
Switching the various loads on a multi-phase feeder to equalize the current in each line.

Load Fault
A malfunction that causes the load to demand abnormally high amounts of current from the source.

Load Regulation
A term used to describe the effects of low forward transfer impedance. A power conditioner with “load regulation” may not have voltage regulation. Removing the power conditioner altogether will improve load regulation.

Load Switching
Transferring the load from one source to another.

Load Unbalance
Unequal loads on the phase lines of a multi phase system.

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Magnetic Synthesizer
A three-phase ferroresonant based system with zigzag output windings to allow the Ferro to handle unbalanced loads.

Main Service Entrance
The enclosure containing connection panels and switchgear, located at the point where the utility power lines enter a building.

Operational sequence of a switch or relay where the new connection is made prior to disconnecting the existing connection.

A metric prefix meaning 1,000,000 or 106.

Metal Oxide Varistor (MOV)
A MOV is a voltage sensitive breakdown device which is commonly used to limit overvoltage conditions (electrical surges) on power and data lines. When the applied voltage exceeds the breakdown point, the resistance of the MOV decreases from a very high level (thousands of ohms) to a very low level (a few ohms). The actual resistance of the device is a function of the rate of applied voltage and current.

A unit of length equal to one-thousandth, 10-3 of an inch.

Micro (U)
A metric prefix meaning one millionth of a unit or 10-6.

A metric term meaning one millionth of a meter.

Milli (M)
A metric prefix meaning one thousandth of a unit or 10-3.

A modem is a contraction of modulator-demodulator. The device is used to connect data equipment to a communication line. Modems are commonly used to connect computer equipment to telephone lines.

(Mean Time Between Failure) the probable length of time that a component taken from a particular batch will survive if operated under the same conditions as a sample from the same batch.

Mean Time To Repair.

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Nano (N)
A metric prefix meaning one billionth of a unit or 10-9.

Negative Resistance
The characteristic of a circuit in which current varies inversely with applied voltage.

National Electrical Manufacturers Association.

National Electrical Code.

The grounded junction point of the legs of a wye circuit. Or, the grounded center point of one coil of a delta transformer secondary. Measuring the phase to neutral voltage of each of the normal three phases will show whether the system is wye or delta. On a wye system, the phase to neutral voltages will be approximately equal and will measure phase to phase voltage divided by 1.73. On a center tapped delta system, one phase to neutral voltage will be significantly higher than the other two. This higher phase is often called the “high leg”.

Neutralizing Winding
An extra winding used to cancel harmonics developed in a saturated secondary winding, resulting in a sinusoidal output waveform from a ferroresonant transformer.

Nominal Voltage
The normal or designed voltage level. For three phase wye systems, nominal voltages are 480/277 (600/346 Canada) and 208/120 where the first number expresses phase to phase ( or line to line) voltages and the second number is the phase to neutral voltage. The nominal voltage for most single phase systems is 240/120.

Nonlinear Load
A load in which the current does not have a linear relationship to the voltage. In a light bulb, the current is directly proportional to voltage at all times. In a nonlinear load such as switched mode power supplies, the current is not directly proportional to voltage.

Normal Mode (NM)
The term refers to electrical interference which is measurable between line and neutral (current carrying conductors). Normal mode interference is readily generated by the operation of lights, switches and motors.

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The unit of measurement for electrical resistance or opposition to current flow.

The relationship between voltage (pressure), current (electron flow), and resistance. The current in an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance. E=IR, or I=E/R, or R=E/I. Where E=voltage, I=current, and R=resistance.

Orderly Shutdown
The sequenced shutdown of units comprising a computer system to prevent damage to the system and subsequent corruption or loss of data.

The variation, usually with time, of the magnitude of quantity with respect to a specified reference when the magnitude is alternately greater and smaller than the reference.

See blackout.

A voltage greater than the rating of a device or component. Normally overvoltage refers to long term events (several AC cycles and longer). The term can also apply to transients and surges.

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A single panel or group of panel units designed for assembly in the form of a single panel; including buses, overcurrent protection devices (with or without switches) for the control of power circuits.

Parallel Operation
The connection of the outputs of two or more power conditioners for use as one unit. Paralleling for capacity means that the units are paralleled for the sum of their individual ratings, i.e. two 125 KVA systems paralleled for use as a single 250 KVA system. Paralleling for redundancy means using one or more additional units to maintain power even when one unit fails.

Parity Error
An unintentional change in the bit structure of a data word due to the presence of a spurious pulse or transient.

Peak Line Current
Maximum instantaneous current during a cycle.

Any device used to process data for entry into or extraction from a computer.

Phase Compensation
Switching capacitors into or out of a power distribution network to compensate for load power factor variations.

Pico (P)
A metric prefix meaning one million millionth or 10-12.

An alternating current supply with two or more hot conductors. Voltage is measurable between the conductors and the voltage waveforms for each conductor are usually displaced 120 degrees. When a neutral is present, the voltage from each hot conductor to neutral is equal.

Electrical energy measured according to voltage and current (normally watts). Power in watts equals volts times amperes for DC circuits. For single phase AC circuits, watts equal volts times amperes times power factor.

Power Factor
Watts divided by voltamps, KW divided by KVA. Power factor: leading and lagging of voltage versus current caused by inductive or capacitive loads, and 2) harmonic power factor: from nonlinear current.

The travel of an electrical waveform along a medium. In other words, a surge passing along a power cord to a system.

A protector is another name for an arrester or diverter.

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Radial Array
A group of earthing electrodes or conductors of equal length and ampacity, connected at a central point and extending outward at equal angles, spoke fashion, to provide a low earth impedance reference.

Real Power

Opposition to the flow of alternating current. Capacitive reactance is the opposition offered by capacitor, and inductive reactance is the opposition offered by a coil or other inductance.

The automatic closing of a circuit-interrupting device following automatic tripping.

An electrical device used to change AC power into DC power. A battery charger is a rectifier.

The inclusion of additional assemblies and circuits (as within a UPS) with provision for automatic switchover from a failing assembly or circuit to its backup counterpart.

The return wave generated when a traveling wave reaches a load, a source, or a junction where there is a change in line impedance.

The statistical probability of trouble-free operation of a given component or assembly. Used principally as a function of MTBF and MTTR.

Radio Frequency Interference.

The ability of a power conditioner to supply output power when input power is lost.

(Root mean square) used for AC voltage and current values. It is the square root of the average of the squares of all the instantaneous amplitudes occurring during one cycle. RMS is called the effective value of AC because it is the value of AC voltage or current that will cause the same amount of head to be produced in a circuit containing only resistance that would be caused by a DC voltage or current of the same value. In a pure sine wave the RMS value is equivalent to .707 times the peak value and the peak value is 1.414 times the RMS value. The normal home wall outlet which supplies 120 volts RMS has a peak voltage of 169.7 volts.

Rotating Field
The electrical field that develops in a multiphase generator. The varying currents of through pairs of stator winding cause the magnetic field to vary as if it was a single rotating field.

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Safety Ground
An alternate path of return current, during a fault condition, for the purpose of tripping a circuit breaker. Also, the means of establishing a load at earth level.

A short duration low voltage condition.

(Semiconductor, or silicon, controlled rectifier) an electronic DC switch which can be triggered into conduction by a pulse to a gate electrode, but can only be cut off by reducing the main current below a predetermined level (usually zero).

A semiconductor is an electronic conductor (ex., silicon, selenium or germanium) with a resistivity between metals and insulators. Current flows through the semiconductor normally via holes or electrons.

Service Factor
(Of a motor) a measurement of the motor’s ability to operate under abnormal conditions. A 1.15 times its rated load continuously when operated at its rated voltage, frequency, temperature, etc. Therefore, a 125 horsepower motor could be operated as a 143.75 h.p. motor under normal conditions.

Imposing a metallic barrier to reduce the coupling of undesirable signals.

Sine Wave
A graph, with the x axis for amplitude and the y axis for time, depicting AC voltage or current. The center line of the x axis is zero and divides polarity (direction).

Single Phase
(With a three phase source) one or tow phase conductors. (Single phase source) A single output which may be center tapped for dual voltage levels.

Single Phase Condition
An unusual condition where one phase of a three-phase system is lost. It is characterized by unusual effects on lighting and other loads.

Sinudoidal Waveform
A waveform that can be expressed mathematically by using the sine function.

Soft-Start Circuit
Circuitry that limits the initial power demand when a UPS has been operating in emergency mode and commercial power is restored. Also, it controls the rate at which UPS output increases to normal.

Steady State
A condition in which circuit values remain essentially constant after all initial fluctuating conditions have settled down.

An external force applied to a component or assembly that tends to damage or destroy it.

Location where high voltage transmission lines connect to switchgear and step-down transformers to produce lower voltages at lower power levels for local distribution networks.

A short duration high voltage condition. A surge lasts for several cycles where a transient lasts less than one half cycle. Often confused with “transient”.

Switch Gear
A group of switches, relays, circuit breakers, etc. Used to control distribution of power to other distribution equipment and large loads.

Maintaining a constant phase relationship between AC signals.

Events that have the same period or which occur at the same time. For instance, a synchronous transfer mechanism for a standby power generator transfers power to or from the utility in phase. In other words, the voltage waveform of the generator and of the utility are in phase and the waveforms occur at the same time and interval during the transfer.

Synchronous Motor
An AC motor whose speed is exactly proportional to the power input frequency.

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A connection point brought out of a transformer winding to permit changing the turns ratio.

TAP Switcher
A voltage regulator which uses power semiconductors, rated at line voltage and current, to switch taps of a transformer thereby changing the turns ratio and adjusting output voltage.

(From telemetering) Measurement with the aid of intermediate means that permit the measurement to be interpreted at a distance from the primary detector. A site telemetry system supplies the intermediate means of communication for all major environmental units at the site. Data from these units can then be interpreted by a computer. Site telemetry differs from central monitoring in that it uses the distributed processing power of monitored equipment from a variety of manufacturers.

Three Phase Power
Three separate outputs from a single source with a phase differential of 120 electrical degrees between any two adjacent voltages or currents. Mathematical calculations with three-phase power must allow for the additional power delivered by the third phase. Remember, both single phase and three phase have the same phase to phase voltages, therefore you must utilize the square root of 3 in your calculations. For example, KVA equals volts times amps for DC and for single phase. For three phase the formula is volts times the square root of three times amps.

Total Harmonic Distortion (THD)
The square root of the sum of the squares of the RMS harmonic voltages or currents divided by the RMS fundamental voltage or current. Can also be calculated in the same way for only even harmonics or odd harmonics.

A device that senses one form of energy and converts it to another, i.e., temperature to voltage (for monitoring).

Transfer Switch
A switch used to transfer a load between a UPS and its bypass source.

A static electrical device which, by electromagnetic induction, regenerates A.C. power from one circuit into another. Transformers are also used to change voltage from one level to another. This is accomplished by the ratio of turns on the primary to turns on the secondary (turns ratio). If the primary windings have twice the number of windings as the secondary, the secondary voltage will be half of the primary voltage.

A high amplitude, short duration pulse superimposed on the normal voltage wave form or ground line.

Transient Response
The ability of a power conditioner to respond to a change. Transient step load response is the ability of a power conditioner to maintain a constant output voltage when sudden load (current) changes are made.

Transmission Line
The conductors used to carry electrical energy from one location to another.

Transverse Mode Noise
(Normal mode)- An undesirable voltage which appears from line to line of a power line.

An electronic device that provides switching action for either polarity of an applied voltage and can be controlled from a single gate. Usually composed of two SCR’s connected back to back.

Transistor-Transistor Logic. Electronic circuitry that defines a binary logic state when components are in saturation or cutoff.

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Negative change in amplitude of a voltage.

Uninterruptible Power Source.

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Volts of alternating current.

Volts of direct current.

Volt (V)
The unit of voltage or potential difference.

Volt ohm-meter.

Electrical pressure, the force which causes current to flow through a conductor. Voltage must be expressed as a difference of potential between two points since it is a relational term. Connecting both voltmeter leads to the same point will show no voltage present although the voltage between that point and ground may be hundred or thousands of volts. This is why most nominal voltages are expressed as “phase to phase” or “phase to neutral”. The unit of measurement is “volts”. The electrical symbol is “e”.

Voltage Regulation
The ability of a power conditioner to maintain a stable output voltage when input voltage fluctuates.

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Watt (W)
The unit of power. Equal to one joule per second.

A wye connection refers to a polyphase electrical supply where the source transformer has the conductors connected to the terminals in a physical arrangement resembling a Y. Each point of the Y represents the connection of a hot conductor. The angular displacement between each point of the Y is 120 degrees. The center point is the common return point for the neutral conductor.

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Zero Signal Reference
A connection point, bus, or conductor used as one side of a signal circuit. It may or may not be designated as ground. Is sometimes referred to as circuit common.