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Glossary Of Terms


AC-Alternating Current

Electrical current that continually reverses direction, with this change in direction being expressed in Hertz, or cycles per second.


Quantitative unit of measurement of electrical current. Abbreviated as Amp or A.

ANSI C62.41-1991

A technical Standard that characterizes the electrical power line surge environment. Originally published by the Institute of Electrical and Electronic Engineers as Standard IEEE 587 -1980, it was updated in 1991 and now recognized as an American National Standard.


An electrical component which stores electrical charges. It consists of two metallic plates separated by a dielectric (non-conducting) material. When used in conjunction with other components it can provide a high-frequency filtering capability.

Clamping Level

This generally is used to describe the voltage level which causes the surge diversion device to start to divert surge energy. A related, but more important parameter is the Suppressed Voltage.

Response Time

The time it takes a surge protection device to switch from its "off" condition to an "on", diverting mode. This occurs when a incoming surge voltage exceeds the clamping threshold level of the MOV or other suppression component. Organizations such as IEEE, NEMA and Underwriters laboratories consider response time to be a non-issue since MOV’s, Avalanche diodes etc (and the surge protectors that use them) have response times that are 100 to 1000 times faster than any transient that they are likely to encounter.

Category C, B and A

Classes described by the ANSI Standard C62.41 that define the surge waveforms that would be representative at various locations within a building. Category C products are intended for use at the main service panel or distribution panels that require large surge current handling capability. Category B defines mid-building, branch panel situations while Category A defines the local service panel or specific equipment.

Combination Pulse

A high energy test pulse specified by ANSI C62.41-1991. Also called a "unipolar pulse".

Common Mode Voltage

A voltage, that appears on the phase and neutral wires of the power system when compared with the system ground wire.


Current, expressed in units of amperes, or simply amps, is the flow of electrons through a conductor. AC, or alternating current, is a current in which the flow of electrons reverses periodically. In the United States the current reversal occurs 60 times a second.

EMI - Electro-Magnetic Interference

Electrically induced noise or transients.


Maximum allowable energy (in joules) for a single impulse on a 10/1000 s current waveform. Indicative of the maximum amount of energy that the suppressor can dissipate. This energy  is dependent upon three (3) variables:   Voltage, Current, and Time. Any variation of the three will effect this figure.


Consists of a combination of components that allows only certain frequencies, or a band of frequencies, to pass.


The frequency of alternating voltage is the number of times per second that it changes polarity from positive to negative. In the United States, the power line frequency is 60 Hertz, 60 cycles per second.


For safety reasons, electrical systems in the USA have a wire connected to earth ground at the service entrance. This "ground" wire is run along with the two current carrying wires.


Headroom is the voltage difference between the peak of the 50/60 Hz power line sine wave voltage and the ‘Threshold voltage" of the MOV (or other) suppression elements. A minimum spacing of 15% above the sine wave peak is considered essential.


The unit of frequency, one cycle per second of alternating current.

IEEE 587-1980, ANSI C62.41-1991

A technical Standard originally published by the Institute of Electrical and Electronic Engineers (IEEE) in 1980, updated in 1991, and now recognized as an American National Standard.


Similar to electrical resistance, since it is a measure of the opposition to the flow of electrical current. Impedance is meaningful only for a changing current and changes value as the frequency of the applied waveform changes.


The property of an electrical component , which opposes the flow of electric current. An inductor has the property of impedance, the opposition to the flow of electric current.


A joule Is a measure of the energy contained in an impulse or conversely it is a measure of the absorption capability of a surge protection device.1 joule = 1 watt x 1 second.

Let-Through Voltage

The residual transient voltage that would appear across equipment after an upstream surge protection device has operated. It is important to remember that the "let-through" voltage is the sum of the voltage drop across the surge protector itself plus the voltage drops that appear across the wiring that connects the protector to the power lines. The protector clamping voltage is only one part of the let-through voltage and frequently is of secondary importance to the wiring drop.

Maximum Operating Voltage:

Maximum allowable continuous sinusoidal voltage (RMS) at 50-60hz. If suppressor is exposed to a continuous voltage higher than RMS voltage stated in specification, the suppressor may suffer damage.

Measured Limiting Voltage:

The maximum magnitude of voltage that is measured across the terminals of the SPD during the application of impulses of  specified wave shape and amplitude.

MOV-Metal Oxide Varistor

In many respects a nearly ideal suppression component. In standby mode, the MOV presents a very high resistance in shunt with the power line – drawing negligible current. When an incoming transient exceeds a critical voltage threshold, the MOV switches rapidly to a near "short-circuit" diverting mode -handling many thousands of transient amperes. When the transient surge expires, the MOV components reset instantly to the reset mode-ready to respond to future transients.

Modes of Protection

Refers to the presence of MOV (or other) components connected between phases to neutral, phases to ground, neutral to ground and between phases.


One of the wires used in the USA to distribute power within a building. The neutral wire is generally bonded to earth ground at a building service entrance, but unlike the ground wire, the neutral wire also carries load current.


A signal frequency(s) that may be riding on top of the power line sine wave. A number of systems use the power lines to carry signals and data to other locations. Attempts to filter out the "so called noise" may disrupt the current or future operation of these systems. It is wiser to provide L/C filtering immediately in front of sensitive equipment, if it is ever needed. It seldom is!

Normal Mode Voltage

Voltage appearing between the phase wires and neutral of the power system wiring.


Nationally Recognized Testing Laboratory, one example of which is Underwriters Laboratories, Inc. (UL).

Peak Current – I peak

A common measure, used by marketing people, to express the relative peak current handling capability (8/20us)of a surge suppression device. Frequently, it merely represents the peak current rating of an MOV multiplied by the number of MOV’s in parallel. The "true" peak current rating of a protector requires a careful assessment of the fuse characteristics, the number of parallel protection circuits, are all of them monitored etc. Bellcore specifications to protect their Central Offices, a highly computerized facility, have found and specified surge current values of 20kA, (8/20 us) waveform, to be satisfactory.

Phase Angle

The point on the sine wave at which a transient occurs. IEEE states that transients can occur at any phase angle. It is important to be able to see suppression device response to transients at varying phase angles.


Power, in watts, is the product of voltage (in volts) and current (in amps). Energy in joules is equal to power (in watts) multiplied by time (in seconds).


A property of electrical conductors or electrical insulators which characterizes their ability to conduct or resist the flow of electricity.


A low-energy test waveform specified by ANSI C-62.41-1991.


Surge Protection Device. Also referred to as TVSS (Transient Voltage Surge Suppressor).

Suppressed Voltage Rating SVR

Suppressed Voltage Rating is a rating based on the measured limiting voltage determined during the transient-surge suppression test. UL 1449 2nd edition designates the rating of a surge suppressor range from 330 volts up to 6 kV. SVR ratings are not in themselves indicative of superior performance, since installation and cabling play a critical role in overall performance.

Series Type Surge Protector

A form of surge protector which handles the continuous AC power line current but opposes surge current flow toward the load. Series type surge protectors must be rated to handle the continuous 50/60 Hertz current, hence they are seldom employed at building entry or mid-building locations. See Shunt Type Surge Protectors.

Service Life

The number of surges of given magnitude that can be suppressed by the suppressor, a measure of reliability.

Shunt Mode

Shunt type surge protector which divert large surge current directly to ground, are not constrained by the continuous power line currents and thus are employed effectively on power systems with capabilities exceeding 5000 Amps (rms).

Sine Wave

The waveform that appears on the AC power lines. The 50/60 Hertz sine wave is a periodic voltage waveform that oscillates above and below a zero axis. When displayed on an oscilloscope it appears as an undulating wave with voltage appearing on the "y" axis and time on the "x" axis.

Single Phase

The portion of a power source that represents only a single phase of the three phases that are often available.


See surge


A brief transient wave of voltage, current or power in an electrical circuit, lasting for less than 1% of the power wave cycle duration.


A momentary voltage increase of the power line voltage, lasting up to several seconds. A swell is not considered to be a transient over voltage, but the TVSS device must operate at a level in excess of the peak voltage of the swell voltage. Otherwise, the surge protector will be attempting to clip the power line 50/60 Hz waveform and will sustain major damage. This necessary spacing is called headroom.


Transient Voltage Surge Suppressor. Also called Surge protection Device (SPD).


An abnormal over voltage of microsecond duration. Also called a surge or spike.

          Underwriters Laboratories, Inc.

UL 1449 2nd Edition

A safety testing specification for power line surge suppressors based in large part on ANSI C62.41 and C62.45 Waveform and Testing Standards. UL 1449 2nd Edition addresses the issue of "SLOW BURN" in which end-of-life failure of suppression components could result in significant damage to the TVSS product.

Voltage Drop

The change in potential between two points in a circuit caused by a current flow through components within a circuit.

Voltage Reference

A voltage point from which a measurement is taken.

Voltage Threshold

The voltage level at which the connected circuit changes its response.


The unit of measure of actual power. Watts are the product of volts times current.


The graphic depiction of an electrical voltage, current or power, typically versus time.

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Last Updated 09/27/10