Consultant Corner
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Glossary of Power Terms
A
Active Filter
Any of a number of sophisticated power electronic devices for eliminating harmonic distortion.
Alternating Current
A flow of electrical current that increases to a maximum in one direction, decreases to zero, and then reverses direction and reaches maximum in the other direction. The cycle is repeated continuously. The number of such cycles per second is equal to the frequency, measured in Hertz. U.S. commercial power is 60 Hertz (i.e. 60 cycles per second).
ACC
Alternating Current Complement.
Aerial
Above ground installation for power lines or telephone lines or cables that are installed on a pole or overhead structure.
Ampacity
Current carrying capacity expressed in amperes.
Ammeter
Electrical test instrument used to measure current in a circuit.
Ampere
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."
ANSI
American National Standards Institute.
Apparent Power
The product of voltage and current in a circuit.
ARC
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.
Arrester
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.
Attenuation
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.
AWG
American Wire Gage. This term refers to the U.S. standard for wire size.
Autotransformer
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.
B
Battery Reservoir (Plant)
A combination of cells or batteries used to power an UPS's system inverter when it is in the emergency mode.
Battery Disconnect Switch
Master switch that disconnects a battery reservoir from an UPS. Provides personnel protection when batteries or UPS require service.
Blackout
See "outage".
Bonding
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.
Break-Before-Make
Operational sequence of a switch or relay where the existing connection is opened prior to making the new connection.
Brownout
A low voltage condition lasting longer than a few cycles. "Brownouts" differ from "sags" only in duration.
BTU
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.
Busbar
A heavy, rigid conductor used for high voltage feeders.
C
Capacitor
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.
CBEMA Curve
A set of curves representing the withstand capabilities of computers in terms of the magnitude and duration of the voltage disturbance. Developed by the Computer Business Equipment Manufacturers Association (CBEMA), it has become a de facto standard for measuring the performance of all types of equipment and power systems, and is commonly referred to by this name.
Charger
An AC-to-DC converter that powers an UPS inverter and maintains the battery reservoir charge.
Clamp-On CT
A current transformer that 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 that 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.
Common Mode Voltage
The noise voltage that appears equally from a current-carrying conductor to the ground.
Conduit
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.
Converter
A device that changes alternating current to direct current.
Core
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.
Coulomb
The combined negative electrical charge of 6.24 X 1018 electrons.
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
(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.
Current
The movement of electrons through a conductor. Measured in Amperes and its symbol is "I."
Current Distortion
Distortion in the ac line current. See Distortion.
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.
D
Decibel
The standard unit for expressing relative power levels. Decibels indicate the ratio of power output to power input dB = 10 log10 (P1/P2).
Delta
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.
Delta-Delta
The connection between a delta source and a delta load.
Delta-Wye
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.
Differential Mode Voltage
The voltage between any two of a specified set of active conductors.
Dip
See "sag."
Direct Current
Electrical current that flows only in one direction.
Disk
A nonvolatile mass memory storage device for computers.
Distortion
Any deviation from the normal sine wave for an ac quantity.
DMM (Digital Multimeter)
An instrument used to measure voltage, current and resistance.
Dropout
A discrete voltage loss. A voltage sag (complete or partial) for a very short period of time (milliseconds) constitutes a dropout.
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).
DV/DT
The change in voltage per change in time.
E
ECL
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.
Efficiency
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.
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.
Electromagnetic Field (EMF)
A magnetic field cause by an electric current. Power lines cause electromagnetic fields that can interfere with nearby data cables.
Electromagnetic Field Interference (EMI)
Interference with electronic equipment that is caused by electromagnetic fields. This interference may take many forms, from simple flickering of computer monitors to complete equipment failure.
Electromagnetic Shield
See "Shield".
Electromechanical
A mechanical device which is controlled by an electric device. Solenoids and shunt trip circuit breakers are examples of electromechanical devices.
Electrostatic
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 that reduces the capacitive coupling and thereby increases the transformers ability to reduce high frequency noise.
EMF
Electromagnetic force or voltage.
EMI, RFI
Acronyms for various types of electrical interference: electromagnetic 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.
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.
Error Burst
A large number of errors within a given period of time as compared to preceding and following time periods.
ESD
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.
F
Failure Mode
The effect by which failure is observed.
Farad
Unit of measurement for capacitance.
Farady Shield
A grounded metallic barrier that 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.
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 - that can be cleared by momentarily interrupting the current.
Feeders
Transmission lines supplying power to a distribution system.
Ferroresonance
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 that is dependent on core saturation and output capacitance.
Filter
A selective network of resistor, inductors, or capacitors that offers comparatively little opposition to certain frequencies or direct current, while blocking or attenuating other frequencies.
FIPS PUB 94
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.
Flashover
Flashing due to high current flowing between two points of different potential. Usually due to insulation breakdown resulting from arcing.
Flicker
Impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time.
Fluctuation
A surge or sag in voltage amplitude, often caused by load switching or fault clearing.
Flux
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.
FPN
Fine print note, National Electrical Code (NEC) explanatory material.
Frequency
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
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.
G
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.
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 Fault
Any undesirable current path from a current carrying conductor to 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.
Grounded
Connected to the earth or to some conducting body that serves in place of the earth.
H
Harmonic
A sinusoidal component of an AC voltage that is multiple of the fundamental waveform frequency. Certain harmonic patterns may cause equipment problems.
Harmonic Content
The quantity obtained by subtracting the fundamental component from an alternating quantity.
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 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.
Harmonic Neutralization
A cancellation process: harmonics at the output of a circuit are inverted and fed back in their opposite phase.
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.
Henry
Unit of measurement for inductance.
Hertz (HZ)
Unit of frequency, one hertz (Hz) equals one cycle per second.
HV
High Voltage.
Hybrid
A device that is composite of differing technologies to create a better functionality.
I
I2R
The expression of power resulting from the flow of current through a resistance: P = I2R.
IEEE
Institute of Electrical and Electronics Engineers.
Impedance
Forces which resist current flow in AC circuits, i.e. resistance, inductive reactance, capacitive reactance.
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).
Inductance
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.
Inductor
(Also called "choke") - A coiled conductor that tends to oppose any change in the flow of current. Usually has coils wrapped around ferrous core.
Infrared Analysis
A power-quality diagnostic measure which uses infrared energy to analyze the heat characteristics of electrical equipment and can identify potential trouble areas that are overheating.
Inrush Current
The initial surge current demand before the load resistance or impedance increases to its normal operating value.
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
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.)
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, 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.
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.
Inverter
A device used to change DC into AC power.
Irregularity
See "Voltage Variations".
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.
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".
J
Joule
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.
K
Kilo (K)
A metric prefix meaning 1000 or 103.
KVA
(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.
KW
(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".
kWh
(Kilowatt hours) KW times hours. A measurement of power and time used by utilities for billing purposes.
L
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
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.
Line Imbalance
Unequal loads on the phase lines of a multiphase feeder.
Load
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.
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.
M
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.
Make-Before-Break
Operational sequence of a switch or relay where the new connection is made prior to disconnecting the existing connection, also soft-load-transfer switching
MEGA (M)
A metric prefix meaning 1,000,000 or 106.
Metal Oxide Varistor (MOV)
A MOV is a voltage sensitive breakdown device that 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.
MIL
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.
Micron
A metric term meaning one millionth of a meter.
Milli (M)
A metric prefix meaning one thousandth of a unit or 10-3.
Modem
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.
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.
Monitoring
See "Power Quality Monitoring".
MTBF
(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.
MTTR
Mean Time To Repair.
N
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.
NEMA
National Electrical Manufacturers Association.
NEC
National Electrical Code.
Net Current
The vector sum of all the currents in a circuit or group of wires (primary, secondary and neutral), which should equal zero. Alternative return paths of neutral currents result in a non-zero net current. Magnetic fields from net current condition cannot be economically shielded. They must be fixed for other measures to be effective.
Neutral
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.
Noise
High-frequency waveform interference that can be caused by a number of factors, including arc welding or the operation of some electric motors. See FAQ.
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 ( r 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 that is measurable between line and neutral (current carrying conductors). Normal mode interference is readily generated by the operation of lights, switches and motors.
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.
O
OHM
The unit of measurement for electrical resistance or opposition to current flow.
OHM'S Law
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.
Oscillation
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.
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.
Outage
A total loss of commercial power.
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.
P
Panelboard
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.
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.
Peak Line Current
Maximum instantaneous current during a cycle.
Peripheral
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.
Phase Shift
The displacement in time of one voltage-waveform relative to other voltage-waveform(s).
Pico (P)
A metric prefix meaning one million millionth or 10-12.
Polyphase
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.
Power
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.
Power Factor, Displacement
The power factor of the fundamental frequency components of the voltage and current wave forms.
Power Quality
The parameters of voltage that affect equipment operation.
Power Quality Monitoring
The use of specialized equipment to track power quality parameters diagnose power quality issues over a period of time.
Power Quality Standards
Standards that are relevant for power quality:
Ansi c84.1-1995: Electrical Power Systems and Equipment Voltage Ratings (60 Hz)
Ansi c65.1: Electric Power Systems and Equipment Voltage Ratings (60 Hertz)
IEEE Standard 100-1996: IEEE Standard Dictionary of Electrical and Electronics Terms
Propagation
The travel of an electrical waveform along a medium. In other words, a surge passing along a power cord to a system.
Protector
A protector is another name for an arrester or diverter.
Pulse
An abrupt variation of short duration of a physical quantity followed by a rapid return to the initial value.
S
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.
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.
SCR
(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).
Semiconductor
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 Factors (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.
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, that reduces noise-induced voltages to levels which minimize improper operation. Common configurations include grids and planes.
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 that may be center tapped for dual voltage levels.
Single Phase Condition
An unusual condition in which one phase of a three-phase system is lost. It is characterized by unusual effects on lighting and other loads.
Sinusoidal 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.
Spike
See "Transient".
Steady State
A condition in which circuit values remain essentially constant after all initial fluctuating conditions have settled down.
Steady State Voltage
Used to refer to the voltage of incoming utility-provided electricity operating under normal conditions.
Stress
An external force applied to a component or assembly that tends to damage or destroy it.
Substation
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.
Surge
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". See FAQ.
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.
Switch Gear
A group of switches, relays, circuit breakers, etc. Used to control distribution of power to other distribution equipment and large loads.
Synchronization
Maintaining a constant phase relationship between AC signals.
Synchronous
Events that have the same period or that 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 Closing
Generally used in reference to closing all three poles of a capacitor switch in synchronism with the power system to minimize transients.
Synchronous Motor
An AC motor whose speed is exactly proportional to the power input frequency.
T
TAP
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.
Telemetry
(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.
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.
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 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 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. See FAQ.
Transducer
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.
Transformer
A static electrical device that, 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.
Transient
Pertaining to or designating a phenomenon or a quantity that 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. See FAQ.
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.
Triac
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.
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.
TTL
Transistor-Transistor Logic. Electronic circuitry that defines a binary logic state when components are in saturation or cutoff.
U
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.
UPS - Uninterruptible Power Source
A device that provides continuous, acceptable power to its loads regardless of the input power supplied.
V
VAC
Volts of alternating current.
VDC
Volts of direct current.
Volt (V)
The unit of voltage or potential difference.
Voltage
Electrical pressure, the force that 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 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
See "Interruption."
Voltage Magnification
The magnification of capacitor switching oscillatory transient voltage on the primary side by capacitors on the secondary side of a transformer.
Voltage Regulation
The ability of a power conditioner to maintain a stable output voltage when input voltage fluctuates.
Voltage Sag
See "Sag."
Voltage Spike
See "Transient".
Voltage Surge
See "Swell".
Voltage Variations
Any one of a number of abnormal electric sine wave characteristics that may lead to power-quality problems. See FAQ.
VOM
Volt ohm-meter.
W
Watt (W)
The unit of power. Equal to one joule per second.
Waveform Distortion
A steady state deviation from an ideal sine wave of power frequency principally characterized by the spectral content of the deviation.
WYE
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.
Z
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.
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