Glossary
A
AC – alternating current, or current that reverses direction at
regular rate. When graphed, alternating current can appear as a
series of curves, squares, or triangles. The shape of the graph is
referred to as a wave or waveform.
This term also identifies the voltage that drives such current (AC
voltage) and the circuits through which alternating current flows
(AC circuits) and other characteristics that pertain to alternating
current. See cycle, frequency, hertz, and DC.
AC Waveforms
accuracy – the degree to which an instrument correctly displays
the measured value.
ambient temperature – the temperature of the surrounding air.
BICSI (Building Industry Standard Consulting Service
International) – an industry association concerned with the
design and installation of communication wiring systems.
burden voltage – in a current-measuring instrument, this is the
voltage drop across the input terminals caused by internal shunt
resistance; this characteristic is a potential source of error. An
instrument with lower burden voltage has a lower potential for
error than a similar instrument with higher burden voltage.
bus – a bundle or group of conductors; examples include ribbon
cable or a grouping of copper traces on a circuit board.
bus bar – a large conductor, usually a length of solid copper, used
to carry a large amount of current to various devices.
C
ampere – a measurement of current, often abbreviated amp.
Canadian Standards Association (CSA) – a Canadian
organization, similar to UL in the United States, that tests products
to ensure compliance with national standards.
amplification – the process of increasing the magnitude of a
signal.
capacitance – the ability of a component or circuit to hold an
electrical charge; measured in farads.
amplitude – the magnitude, or size, of a signal.
capacitor – a component which consists of two conductors
separated by an insulator, allowing it to temporarily hold an
electrical charge.
analog – data or information presented in a continuous form;
changes in value over time. When graphed, may appear as a
straight line or as a continuously changing curve. See digital.
anode – the positive pole of a device. See cathode.
cathode – the negative pole of a device. See anode.
ANSI (American National Standards Institute) – a national
organization that identifies and responds to industrial and public
standards needs.
Certificate of Calibration – a detailed report indicating values
obtained against performance specifications. The calibration is
performed using instruments traceable to NIST. Meters with
Certificate of Calibration may be ordered through the Greenlee
catalog. See NIST.
attenuation – a loss of signal strength, or power, due to resistance
or impedance; measured in dB.
Certificate of Conformance – a printed certificate stating that the
device conforms to listed specifications.
autoranging – the capability of a meter to automatically select
the proper range for the selected measurement; a feature of most
modern digital multimeters. See manual ranging.
circuit breaker – a mechanical device that opens, or breaks, a
circuit when an overload occurs.
AWG (American Wire Gauge) – a standardized system for sizing
wires and cables; measurements are based on conductor diameter.
clamp-on – a meter or accessory that closes around a conductor
and makes noncontact measurements by sensing the magnetic
field that surrounds the conductor.
B
common mode voltages – voltage that appears on both leads of
a measuring instrument, which can cause digit rattle or offset.
bandwidth – a measure of the amount of information which can
pass through a device per unit of time; measured in hertz.
CMRR (Common Mode Rejection Ratio) – a measure of the
ability to filter out common mode voltages; CMRR is expressed in
dB.
bar graph – a display that conveys analog information; this type of
display is the digital equivalent of the needle on an analog meter.
coil – a winding of wire; examples include the wire wound around
the armature of a motor and the wire that surrounds the core of a
transformer.
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Glossary
conductance – a measure of conductivity.
conductivity – the ability to allow current to flow; the inverse of
resistance.
contact thermometer – a temperature-sensing device that
requires physical contact to take a measurement. See noncontact
thermometer.
diode – an electrical component that allows current to travel in
one direction only; usually used to control the flow of current.
See LED.
duty cycle – the ratio that compares the amount of time an
intermittent device is switched ’on‘ to the amount of time it is
switched ’off.‘ This term also describes a continuous waveform –
the ratio that compares the amount of time the signal is ’high‘ to
the amount of time it is ‘low.’ It is usually expressed as a percentage.
continuity – refers to an uninterrupted path for current flow, or a
closed circuit.
copper loss – loss of signal strength due to copper’s resistance to
current flow.
E
crest factor – an AC term that refers to the ratio of a peak value
to its corresponding RMS value; it is represented by the Greek
letter χ. See RMS.
electromagnetic interference (EMI) – unwanted signals that
emanate from electric, electronic, and magnetic devices.
current – the flow of electrons in a circuit; measured in amperes,
or amps.
Square wave with a 50% duty cycle
F
farad – the unit of measure for capacitance.
crosstalk – a signal generated by one channel or circuit that
interferes with the signal on another channel or circuit. Crosstalk is
typically measured at the close end of the circuit (near-end
crosstalk) or at the distant end (far-end crosstalk).
far-end crosstalk (FEXT) – crosstalk measured at the distant end
of a telecom circuit. See crosstalk and near-end crosstalk.
fixed resistor – a resistor with a fixed value. See resistor.
cycle – concerning electricity, this is one iteration of a regular
repeated signal, such as AC voltage. See AC, frequency, and hertz.
One cycle each of a sine wave, square wave, and sawtooth wave
D
dB (decibels) – the ratio between two quantities of power; this is
a common measurement for gain or loss in telecom and audio
applications. The dB scale is not linear, but logarithmic – an
increase in three dB represents a doubling of power.
full scale – the maximum measurement to which an accuracy
specification applies.
fuse – a device that opens a circuit and stops the flow of current
when an overload condition occurs.
frequency – an AC term which measures the number of cycles per
second, or hertz. Line voltage in the United States is 60 cycles per
second, or 60 hertz. See AC.
frequency response – the frequency range within which an
instrument can carry out measurement functions at the specified
accuracy.
dBm – a decibel measurement with a reference in milliwatts.
DC – direct current, or current that travels in one direction only.
When graphed, DC current appears as a straight line.
This term also identifies the voltage that drives such current (DC
voltage) and the circuits through which direct current flows (DC
circuits) and other characteristics that pertain to direct current.
See AC.
digital – data or information presented in discrete segments;
changes in value over time. When graphed, appears as a jagged
curve (a series of steps). See analog.
digital multimeter (DMM) – a test instrument with a digital
readout that measures and displays a variety of characteristics or
properties (such as voltage, current, frequency, temperature, etc.).
G
GFCI tester – a device that intentionally disturbs the current flow
through a GFCI. See ground fault circuit interrupter.
ground – a return path for current flowing in a circuit; also known
as common, since a single conductor can provide the return path
for many components or devices. In a power distribution network,
this path is literally the earth.
ground conductor – an extra conductor intended to provide a
low-resistance current path in case of a malfunction.
ground fault – an inadvertent connection to ground.
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Glossary
ground fault circuit interrupter (GFCI) – a receptacle with an
extremely fast circuit breaker built in; it monitors the difference
between line current and neutral current. When line current
exceeds the neutral current by a predetermined margin, the GFCI
opens the circuit to stop the flow of current. Also known as a
ground fault interrupter (GFI).
mechanical relay – a device that uses a varying electrical input to
open and close a set of mechanical contacts; an electromagnetic
switch. See solid-state relay.
H
multimedia – communication of information by multiple methods
or formats.
Hall effect – a technology that allows accurate measurement of
DC current with a clamp-on ammeter. See clamp-on.
multimeter – a test instrument that measures and displays a
variety of characteristics or properties.
harmonic – a signal with a frequency that is some multiple of the
fundamental frequency. Harmonics can damage or degrade the
performance of many electrical and electronic devices.
multimode optical fiber – optical fiber cable in which light travels
in many bound modes.
harmonic distortion – an increasingly common form of power
corruption caused by nonlinear load devices, such as variable
speed motor drives, computers, and lighting ballasts.
headroom – the extra capacity, or margin, by which a telecom
system exceeds the minimum requirements; expressed in dB.
hertz – the unit of measure of frequency; the technical term for
cycles per second. See AC.
megohmmeter – a test instrument that evaluates the integrity of
insulation by measuring the insulation’s resistance.
multiplexing – a process of combining several communication
channels so that the information can be transmitted over a single
circuit; such methods include time division and frequency division.
N
near-end crosstalk (NEXT) – crosstalk measured at the near end
of a telecom circuit. See crosstalk and far-end crosstalk.
network – the interconnected pathways that allow
communication among many computers and peripheral devices.
I
impedance – total opposition to current flow in an AC circuit;
includes resistance, capacitance, and reactance.
interference – unwanted signals that disrupt or degrade normal
signal transmission.
neutral conductor – a conductor that provides the normal return
path for current in a power distribution system.
NIST (National Institute of Standards and Technology) – a
federal agency that works with industry to develop measurements
and standards.
noise – jargon for interference.
L
noise rejection – the ability to reject unwanted signals (noise).
LED (light-emitting diode) – a semiconductor device that
illuminates when current is flowing through it. Unlike a standard
diode, an LED is usually used to indicate the flow of current. See
diode.
line filter – a device that filters out electrical interference; prevents
signals from motors and other devices from spreading throughout
a circuit and disturbing other devices.
noncontact thermometer – a temperature-sensing device that
does not require physical contact to take a measurement; it senses
infrared radiation to determine surface temperature. A noncontact
thermometer can be a self-contained unit or an accessory that
connects to a digital multimeter. See contact thermometer.
nonsinusoidal wave – an electrical signal that is not a pure sine
wave; such signals include square wave and sawtooth wave.
liquid crystal display (LCD) – an efficient, high-contrast readout;
commonly used in DMMs and other test instruments.
load – any device that consumes power in a circuit.
Square Wave
Sawtooth Wave
M
normal mode voltages – AC signals that can cause inaccurate DC
measurements.
manual ranging – a characteristic of some test instruments by
which the user selects the measurement range. See autoranging.
NMRR (Normal Mode Rejection Ratio) – a measure of the ability
to filter out normal mode voltages; NMRR is expressed in dB.
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Glossary
O
R
ohm – a unit of electrical resistance.
range – the domain of values, with specific upper and lower limits,
over which a test instrument can take reliable measurements with
specified accuracy.
Ohm’s law – the simple mathematic formula that ties together
voltage, resistance, and current; E = I x R.
E stands for electromotive force (voltage), I stands for current, and
R stands for resistance. For example, one volt will push one ampere
of current through a resistance of one ohm.
optical fiber – a thin strand of glass or plastic used for carrying
information encoded in light.
P
pair – two color-coded wires in network cabling, usually twisted
around each other to reduce interference.
peak – the point of maximum amplitude of a signal; can be either
positive or negative.
peak-to-peak value – the difference measured between a
positive peak and a negative peak.
phase – an AC term that describes the time relationship between
voltage and current. If the circuit is purely resistive, voltage and
current will peak at the same time; and they will be in phase. A
capacitive or inductive circuit will cause the current to peak before
or after the voltage, and they will be out of phase.
Also refers to the time relationship between two or more voltages
in a three-phase system.
Three-Phase Voltage
phase shift – an AC term that describes the amount by which
voltage and current are out of phase with each other. Typically
measured at a peak or zero point of the wave, phase shift is
measured in degrees.
Voltage and Current Out of Phase
polarity – refers to the proper electrical connection of a device or
component. For example, the positive and negative terminals of a
battery are connected to the positive and negative terminals of a
circuit, respectively.
Power Factor – the ratio of True Power (expressed in watts) to
Apparent Power (expressed in volt-amps) in an AC circuit.
power sum – a test method for four-pair cable that compares the
sum of the pair-to-pair crosstalk from three pairs to the fourth pair.
rectifier – a device which converts AC current to DC current by
allowing the current to flow in one direction only.
Registered Communications Distribution Designer (RCDD) –
professional certification from BICSI.
relative measurement – the difference between a reference
measurement and measurements that follow. For example, the
relative measurement function of a DMM allows the user to easily
subtract the resistance of the test leads from the resistance
measurement of a device or circuit.
resistance – opposition to current flow, measured in ohms.
See ohm and Ohm’s law.
resistor – a device that limits the flow of current in a circuit.
See ohm and Ohm’s law.
resolution – the minimum incremental value a test instrument
can measure and display.
return loss – a measurement of the noise or interference cause by
the impedance of a cable; rendered in dB.
RMS (root mean squared) – refers to some of the methods for
displaying AC measurements. Two AC measurement methods
common to DMMs are average responding RMS calibrated and
true RMS reading.
The average-responding RMS calibrated method takes the average
value of the input signal, multiplies it by 1.11, and displays the
result. This method is accurate if the input signal is a pure sine
wave.
The true RMS-reading method uses internal circuitry to read the
true RMS value. This method is accurate, within the specified crest
factor limitations, whether the input signal is a pure sine wave, a
square wave, sawtooth wave, half wave, or signal with harmonics.
The ability to read true RMS provides much more measurement
versatility.
The Waveforms and Crest Factors table shows some typical AC
signals and their RMS values.
Waveforms and Crest Factors
Waveform
RMS
Value
100
100
100
Average
Value
90
100
87
1.414
1
1.73
Crest
Factor *
(χ)
* The crest factor is the ratio of the peak value to the RMS
value; it is represented by the Greek letter χ.
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Glossary
S
transient voltage – any temporary, unwanted voltage level.
sampling – the process of making or displaying measurements at
regular intervals.
sampling rate – the time interval at which sampling occurs;
expressed as the number of samples per second.
sine wave – the shape of a perfect AC signal. When graphed, an
undulating, regular wave. Because one revolution of an electrical
generator produces one cycle of power, a sine wave is usually
measured in degrees; one cycle equals 360 degrees. See AC, cycle
and hertz.
single-mode fiber – optical fiber which permits only one mode
to propagate. Generally used for long-distance telephone
transmission.
solid-state relay – a relay with no moving parts; it has electronic
switching devices instead of mechanical contacts. See mechanical
relay.
T
temperature coefficient – a factor or formula used to calculate
the ambient temperature’s effect on a test instrument’s accuracy.
thermocouple – a device which converts heat into DC voltage;
used to measure temperature.
topology – the physical configuration of a network.
transformer – a device that uses electromagnetism to increase or
decrease an electrical property.
twisted pair – see pair.
U
unshielded twisted pair (UTP) – a twisted pair of copper
conductors with no metallic braid shielding; used for networks
which carry high-speed voice and data transmission.
V
volt – the measurement unit of voltage. See Ohm’s law.
voltage – also known as electromotive force, this is the electrical
’pressure‘ that causes current to flow in a circuit; measured in volts.
voltage dip – a temporary decrease in voltage.
voltage spike – a temporary increase in voltage, usually of very
short duration.
voltage surge – a temporary increase in voltage; usually of longer
duration.
volt tick – a small, pen-like noncontact voltage detector.
W
watt – a unit of measure for electrical power; in DC circuits or
purely resistive AC circuits, power (in watts) is the product of volts
multiplied by amps.
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