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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 |
