This is a listing of the power quality terms
used in this website and their meanings.
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.
utility - commercial power. The electric power furnished by an electric
power utility company.
(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.
transfer switch (ATS) - self acting equipment for transferring one or more
load conductor connections from one power source to another.
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".
charger - a devise or a system which provides the electrical power needed to
keep the battery backup fully charges.
converter - a device which changes (or converts) alternating current power
to direct current power and vice versa.
- total loss of commercial electrical power
- a short duration voltage sag or outage.
circuit - an individually protected electrical circuit originating at the service
panel and ending at the electrical outlets
- a long duration voltage sag (low voltage) sometimes caused by the utility to reduce the
load on the electrical system.
transformer - an electrical device for changing high voltage. usually utility
voltage on the incoming side and user voltage on the customer side.
- an undefined term used to describe a short duration rise in voltage, may or may not
be detrimental to equipment operations.
- 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.
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.
ground - an undefined term used to describe an earth connection that does
not contribute to misoperation of electrical equipment. see computer ground, or isolated
power - an undefined, imprecise term sometimes used to describe the
electrical power requirement so it will not cause problems for electronic equipment
mode noise - abnormal signals appearing between a current
between current carrying line and associated ground. can be between neutral or phase and
- a device which changes electrical energy from one form to another, such as from
alternating current to direct current.
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.
load - circuit or feeder that has an important or costly operation
associated with it.
amount of electricity that flows through a conductor, such as a wire.
connection - a method of connecting a three phase source or load in series
for a closed circuit (3-wire, plus ground)
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.
power - an undefined, imprecise term sometimes used to describe
the electrical power when it causes problems for electronic equipment operation.
- abnormal waveshape of voltage or current.
- total loss of voltage for a short period of time
- 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.
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.
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.
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
- protection for components used in the electrical system
- a term used to describe a problem occurring in the electrical system, usually a short to
ground that causes a breaker to operate.
- a method of removing noise from the output power by providing a path for the noise to
flow to ground
- 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.
- 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).
load - the greatest load that a circuit is designed to carry under specific
conditions; any additional load is considered an overload.
- an undefined, imprecise power quality term used to describe a voltage
variation (usually very short duration) that causes electronic equipment to misoperate.
- generally referring to the earth connection of the electrical system
loop - unexpected current in a non current carrying conductor. tow or more
points of an electrical system connected to the earth at different points.
noise - an undefined, imprecise term used to describe unwanted electrical
signals appearing between the earth conductor and any other conductor.
- 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.
- a signal at a frequency that is a multiple of
the fundamental frequency.
distortion - waveform changes caused by the power supplies of certain electrical
or electronic appliances.
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.
(notch) - a disturbance of the voltage waveform that is less than about one
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.
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.
- 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.
- (see also power outage or momentary) - a complete stop in the flow of electricity,
lasting from a fraction of a second to hours.
- electrical isolation between input and output. such as an isolation
transformer or optical coupler.
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.
- a machine, device, or system that changes direct-current power into alternating-current
- 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.
bypass switch (MBS) - a manually operated transfer switch used to bypass the
major electronics in the UPS, so the UPS can be safely serviced.
- an undefined, imprecise term used to describe a short duration PQ event, such as a
voltage sag or surge.
Electrical Code (NEC) - a national code of standards and practices for the
electrical and electronics industry.
- non-damaging distortion of electricity - which interferes primarily with communications
appliances-caused by other appliances and electronic lighting.
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.
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.
- 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.
- 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.
- 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.
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.
voltage - an increase in the normal voltage level lasting for seconds or minutes.
- long term loss of voltage Long term usually means several minutes.
- any point on a wiring system where current is taken to supply electrical power for a
- 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.
factor (PF) - the ratio of total watts (the real power) to the total
root-mean-square (RMS) volt-amperes (apparent power); W/VA.
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.
surge - an undefined, imprecise term sometimes used to describe a transient
that damages equipment.
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.
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
power - an undefined, imprecise term sometimes used to describe power as it
comes from the utility without filtering.
- a contact device installed at an outlet designed to accept a single plug. Commonly known
as a wall socket.
- duplication of elements in a system or installation to enhance the reliability or
operation of the system.
- a method of limiting voltage to a narrow range.
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.)
- 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.
called dip or voltage sag) - a decrease of the normal voltage level lasting less
from a few cycles to a few hours.
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.
panel (enclosure) - an electrical cabinet that houses circuit breakers or
fuses for building or a portion of a building.
- the sinusoidal form exhibited by alternating current.
- 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.
(voltage) - an imprecise, undefined term used to describe the very short duration voltage
transient that caused damage to electronic equipment.
- an increase in the normal voltage level lasting from a few cycles to a few hours.
- a sudden increase of electric current or voltage
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.
protectors - see surge arresters
- see surge arrests
- short duration, fast rise time voltage changes, that are caused by lightning, large
motors starting, utility switching operations, and other appliances switching.
- a term referring to the harmonics that are a multiple of three times the
fundamental frequency. For example, 3rd, 9th, 15th and so on.
total harmonic distortion - a waveshape change caused by the presence of
multiples of the fundamental frequency of the ac power.
- transient volt surge suppresser, see surge suppressers
voltage - a decrease in the normal voltage level lasting for seconds or minutes.
- uninterruptible power supply, containing batteries that store energy which
can be used during power interruptions (See battery backup)
- electrical pressure, or electromotive force (emf)
dip - see sag
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
regulation - a term used to describe the voltage variation. usually
described as a percent of nominal.
sags - see sags
variations - changes in voltage value
- a term used to describe the shape of an electrical signal, obtained by plotting a graph
with voltage vs time.
- a three phase 4 or 5 wire connection with a single common neutral, a
single ground and three phase conductors