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Power Quality Vocabulary


This is a listing of the power quality terms used in this website and their meanings.


alternating current (AC) - a type of current which changes (or alternates) its direction at regular intervals. Since the current flows in one direction for the same amount of time that it flows in the other direction, the average value of the current flow is zero.
AC utility - commercial power. The electric power furnished by an electric power utility company.
ampere (AMP or A) - the unit of measure for current. it shows the amount of electricity per second that flows through a conductor such as a wire.
auto transfer switch (ATS) - self acting equipment for transferring one or more load conductor connections from one power source to another.
battery backup - uninterruptible power supply used for computers and other electronic systems to project the data from loss during a power outage. Note: Most battery back-up units do not protect the hardware or prevent "screen freeze".
battery charger - a devise or a system which provides the electrical power needed to keep the battery backup fully charges.
bi-directional converter - a device which changes (or converts) alternating current power to direct current power and vice versa.
blackouts - total loss of commercial electrical power
blink - a short duration voltage sag or outage.
branch circuit - an individually protected electrical circuit originating at the service panel and ending at the electrical outlets
brownouts - a long duration voltage sag (low voltage) sometimes caused by the utility to reduce the load on the electrical system.
building transformer - an electrical device for changing high voltage. usually utility voltage on the incoming side and user voltage on the customer side.
bump - an undefined term used to describe a short duration rise in voltage, may or may not be detrimental to equipment operations.
bypass - a circuit used to change the path of the electrical power so that it goes around (or bypasses) its normal path. In the UPS, it is used to bypass the major electronics in the UPS so they can be safely serviced.
circuit breaker (CB) - a device used to manually open (break) or close a circuit to interrupt or apply electric power to an electrical apparatus. A circuit breaker can also open a circuit automatically when it senses an overload.
clean ground - an undefined term used to describe an earth connection that does not contribute to misoperation of electrical equipment. see computer ground, or isolated ground.
clean power - an undefined, imprecise term sometimes used to describe the electrical power requirement so it will not cause problems for electronic equipment operation.
common mode noise - abnormal signals appearing between a current between current carrying line and associated ground. can be between neutral or phase and ground.
converter - a device which changes electrical energy from one form to another, such as from alternating current to direct current.
crest factor - the ratio of peak to RMS value of the voltage or current waveform. a sinewave has a crest factor of 1.414 to 1. a switchmode power supply current waveform usually has a crest factor of 3 or 4 to 1.
critical load - circuit or feeder that has an important or costly operation associated with it.
current - amount of electricity that flows through a conductor, such as a wire.
delta connection - a method of connecting a three phase source or load in series for a closed circuit (3-wire, plus ground)
direct current (DC) - a type of current which never reverses its direction. Since the current flows in only one direction, the average value of the current cannot be zero unless the current has stopped flowing.
dirty power  - an undefined, imprecise term sometimes used to describe the electrical power when it causes problems for electronic equipment operation.
distortion - abnormal waveshape of voltage or current.
drop-out - total loss of voltage for a short period of time
efficiency - the ratio of the output power from the UPS to the input power from the utility. This shows the percentage of the input that is available as useful output power. For example, a UPS that is 95% efficient delivers 95% of the utility power it receives to the load. The remaining power takes the form of dissipated heat (which often must be removed via air conditioning to maintain acceptable operating temperatures). A less efficient system delivers less power to the load and dissipates correspondingly more heat.
electrical disturbance - a change in the electrical power that causes misoperations of appliances connected to the electrical system. Can be caused by other electrical loads or by events outside the building.
electromagnetic compatibility - a term used to describe the tolerance level of electrical appliances to normal electrical disturbances and the ability of the utility to maintain a constant level of power quality.
emergency shutdown - a device or system used to instantly or quickly shut down all the electrical power available to the UPS and the load. An emergency shutdown device is usually used during a crisis to prevent damage to the UPS and the load. Some computer-room installations require a Remote Emergency Power Off (RE
enclosure - protection for components used in the electrical system
fault - a term used to describe a problem occurring in the electrical system, usually a short to ground that causes a breaker to operate.
filtering - a method of removing noise from the output power by providing a path for the noise to flow to ground
flicker - the electrical power quality parameter that describes the variation light intensity of a fluorescent light caused by very slow variations in voltage. The voltage varies at 1/2 or less of the fundamental frequency.
frequency - the number of times in a specific period (how frequently) alternating current reverses its direction, measured in Hertz (Hz). Each reversal from one direction to another and back again is called a cycle. In North America the utility power completes 60 cycles every second, so the frequency of the utility power is 60 cycles (60 Hertz).
full load - the greatest load that a circuit is designed to carry under specific conditions; any additional load is considered an overload.
glitch - an undefined, imprecise power quality term used to describe a voltage variation (usually very short duration) that causes electronic equipment to misoperate.
ground - generally referring to the earth connection of the electrical system
ground loop - unexpected current in a non current carrying conductor. tow or more points of an electrical system connected to the earth at different points.
ground noise - an undefined, imprecise term used to describe unwanted electrical signals appearing between the earth conductor and any other conductor.
hard-wired - equipment that is connected to its power source using wiring (generally customer or contractor-supplied) attached directly to terminal blocks or distribution panels rather than via an input line cord and output receptacle.
harmonic - a signal at a frequency that is a multiple of the fundamental frequency.
harmonic distortion - waveform changes caused by the power supplies of certain electrical or electronic appliances.
harmonic resonance - the power quality term used to describe the condition that sometimes occurs in electrical system. this condition causes high currents to flow through capacitors and damage them or clear fuses in related circuits.
impulse (notch) - a disturbance of the voltage waveform that is less than about one millisecond
input line cord - the covered bundle of wiring connected to the input terminals of the UPS. The end of the cord not connected to the UPS is connected, via an input plug, to an AC utility outlet to supply power to the UPS.
input plug - the plug intended to be connected to the end of the input line cord. Once it has been connected to the input line cord, it is plugged into a receptacle connected to an AC utility outlet.
interharmonics - this power quality term refers to the distortion of the waveform that is at the frequency which is not an even multiple of the fundamental frequency. For example, a harmonic would be 2, 3, 4 times the fundamental frequency. an interharmonic would be 1.5 or 3.4 or 7.2 multiple of the fundamental.
interruptions - (see also power outage or momentary) - a complete stop in the flow of electricity, lasting from a fraction of a second to hours.
isolation - electrical isolation between input and output. such as an isolation transformer or optical coupler.
isolation transformer - a multiple winding transformer with physically separate primary and secondary windings. Although the primary and secondary windings are physically disconnected, the magnetic field in the windings of the primary creates (induces) electrical power in the secondary winding. In this way the electrical power available at the input can be transferred to the output, but some of the unwanted electrical effects in the input power can be discarded.
inverter - a machine, device, or system that changes direct-current power into alternating-current power.
load - a device that receives power, such as equipment powered by a UPS. The load is also the power or apparent power delivered to such a device.
manual bypass switch (MBS) - a manually operated transfer switch used to bypass the major electronics in the UPS, so the UPS can be safely serviced.
momentary - an undefined, imprecise term used to describe a short duration PQ event, such as a voltage sag or surge.
National Electrical Code (NEC) - a national code of standards and practices for the electrical and electronics industry.
noise - non-damaging distortion of electricity - which interferes primarily with communications appliances-caused by other appliances and electronic lighting.
nominal value - a disignated value which has been accepted for the sake of convenience. For instance, nominal voltages are values assigned to circuits so that the voltages of the circuits can be conveniently discussed as 120VAC nominal units, or 240VAC nominal units, etc.
non-linear load - an electrical load that uses power in pulses or other waveform that does not track the sinewave. almost all electronic power supplies are non-linear loads.
notching - a negative or positive change in the waveshape that repeats cycle to cycle. Most often caused by high peak currents of variable speed drives or other phase loads.
off-line - as in a standby power system which feeds power to the load directly from the utility and then transfers to battery power via an inverter after utility drops below a specified voltage. The delay between utility power loss and inverter startup can be long enough to disrupt the operation of many sensitive loads.
on-line - as in an uninterruptible power system which supplies conditioned power to the load through an inverter or converter that constantly controls to AC output of the UPS regardless of the utility line input. In the event of a utility power failure, there is no delay or transfer time to backup power.
oscillatory transient - a power quality term used to describe a voltage transient that rises sharply to some level and then degrades over time with an waveform that decreases in frequency and amplitude.
over voltage - an increase in the normal voltage level lasting for seconds or minutes.
outage - long term loss of voltage Long term usually means several minutes.
outlet - any point on a wiring system where current is taken to supply electrical power for a load.
overload - a condition in which the load wants more from the power source (such as a UPS) than the power source has been designated to supply.
power factor (PF) - the ratio of total watts (the real power) to the total root-mean-square (RMS) volt-amperes (apparent power); W/VA.
power quality problem - the difference between the quality of electricity at an electrical outlet and the quality of the electricity required to reliably operate an appliance, resulting is misoperation-operation or damage.
power surge - an undefined, imprecise term sometimes used to describe a transient that damages equipment.
power warning failure (PWF) - an option of the UPS that supplies a warning signal to some computer systems that the UPS may shut down. Some computers can take advantage of this signal to automatically back up and shut down before the UPS shuts down.
radiated noise - sometimes referred to as EMF or RFI noise by engineers, is emitted through the air instead of the electrical system and is received by televisions, hearing aids, computer monitors, and other communication appliances
raw power - an undefined, imprecise term sometimes used to describe power as it comes from the utility without filtering.
receptacle - a contact device installed at an outlet designed to accept a single plug. Commonly known as a wall socket.
redundancy - duplication of elements in a system or installation to enhance the reliability or operation of the system.
regulation - a method of limiting voltage to a narrow range.
remote dry contacts - the remote dry contacts on a UPS provide a means to monitor status or perform some procedures on the UPS from a remote locations through dry contacts. (A dry contact is one through which no direct current flows.)
RS-232 - a connector on the UPS which provides a signal hookup for a computer so that information can be exchanged between the computer and the UPS.
sag (also called dip or voltage sag) - a decrease of the normal voltage level lasting less from a few cycles to a few hours.
screen freeze (lock-up) - a term used to describe the situation when a computer or computerized equipment stops its operation and non of the controls function. any automated equipment stopping unexpectedly.
service panel (enclosure) - an electrical cabinet that houses circuit breakers or fuses for building or a portion of a building.
sinewave - the sinusoidal form exhibited by alternating current.
single-phase power - power that is provided by a single source with one output. If there is more than one output, the voltages and currents of the outputs are all in phase.
spikes (voltage) - an imprecise, undefined term used to describe the very short duration voltage transient that caused damage to electronic equipment.
swell - an increase in the normal voltage level lasting from a few cycles to a few hours.
surge - a sudden increase of electric current or voltage
surge arresters - electrical devices used to limit sudden changes in voltage or current. They can be either connected in series to limit current or in parallel to limit voltage. They are used to protect other electrical equipment and electrical systems.
surge protectors - see surge arresters
surge suppressers - see surge arrests
transients - short duration, fast rise time voltage changes, that are caused by lightning, large motors starting, utility switching operations, and other appliances switching.
triplens - a term referring to the harmonics that are a multiple of three times the fundamental frequency. For example, 3rd, 9th, 15th and so on.
THD total harmonic distortion - a waveshape change caused by the presence of multiples of the fundamental frequency of the ac power.
TVSS - transient volt surge suppresser, see surge suppressers
under voltage - a decrease in the normal voltage level lasting for seconds or minutes.
UPS - uninterruptible power supply, containing batteries that store energy which can be used during power interruptions (See battery backup)
voltage - electrical pressure, or electromotive force (emf)
voltage dip - see sag
voltage imbalance - a power quality term used to describe the variation in voltage between phases in a three phase system. Calculate by measuring each phase, take the average of the three phases and calculate the percentage difference to phase with the greatest difference.
voltage regulation - a term used to describe the voltage variation. usually described as a percent of nominal.
voltage sags - see sags
voltage variations - changes in voltage value
waveform - a term used to describe the shape of an electrical signal, obtained by plotting a graph with voltage vs time.
wye - a three phase 4 or 5 wire connection with a single common neutral, a single ground and three phase conductors

For additional Electrical terms go to www.eia.doe.gov/cneaf/electricity/page/glossary.html


If you have any questions, comments or would like more detailed descriptions about this listing of terms, please contact us.