What is a transient?
An electrical transient is a temporary excess of voltage and/or current in an electrical circuit which has been disturbed. Transients are short duration events, typically lasting from a few thousandths of a second (milliseconds) to billionths of a second (nanoseconds), and they are found on all types of electrical, data, and communications circuits.

What causes electrical transients?
The simple act of turning on (or off) a light, motor, copy machine or any other electrical device can disturb the electrical circuit and create transients. In general, the larger the load current the greater the disturbance when the load is switched off or on.

The switching of high ampacity loads such as electric welders and electric motors are known to create transients. Studies have shown that a majority of transients (roughly 80%) are generated inside a given facility. Cloud-to-cloud lightning discharges or nearby lightning strikes are capable of creating electric field intensities in the hundreds to thousands of volts per meter. A two meter length of wire (i.e. a power or signal conductor) exposed to an electric field intensity of 300 volts per meter can develop an induced transient voltage of 600 volts (2 meters X 300 volts/meter = 600 volts). If this 600 volt transient appears across an unprotected power, telephone, data, or coaxial line the result can be system destruction. A driver knocking down a utility power pole or even events considered minor, such as a curious squirrel exploring a utility power transformer can be responsible for creating power interruptions and significant transient disturbances. Tree branches and even wet kite strings touching power lines have disrupted energy flow and caused power line transients. Noisy electrical neighbors sharing your electrical distribution system, such as welding shops or manufacturing facilities can also be a major source of transients.

Should I cover the main or the branch panel? Should I cover both?
Cover both and consider that a recommended minimum of two levels of protection is optimum. The main panel or service entrance surge protective device (SPD) provides the first line of defense against large externally generated transients stepping them down to safe levels. These externally generated transients include those caused by lightning, downed power lines and similar distribution problems, as well as noisy electrical neighbors. The main panel device also calms the internally generated transients which reach it from loads directly connected to it and from connected sub or branch panels. The main panel device prevents the redistribution of these internally generated transients. Branch panel SPDs handle the surge remnants which may remain after a massive hit at the main panel has been stepped down. Additionally, branch panel SPDs prevent transient cross contamination between the various circuits and their connected loads. Point-of-use and individual equipment protection (always consider AC and signal protection) isolates the protected system from transient activity on the same circuit.

What is let-through voltage?
Transient suppression voltage or let-through voltage is defined in UL 1449 as “The maximum peak voltage occurring within 100 microseconds after the application of the test wave.” What we are really talking about is the maximum amplitude or height of the voltage after the SPD has done its job.

Why are UL ratings important?
The most important reason to look for the UL 1449 Second Edition Standard for Safety Transient Voltage Surge Suppressors listing is to ensure that the products are listed as complying with this safety standard. The UL 1449 measured limiting voltage ratings (let-through voltages) are determined as a part of the safety testing of the suppressor of UL 1449. This is the only data some manufacturers have because they do not have a UL 1449 certified laboratory or the staff, equipment, experience and certification required to effectively design, test and certify their products with the various ANSI/IEEE C62.41-1991 test wave forms.

How long will this surge protector last?
The owner of an I.T. panel device has the exclusive, longest and strongest warranty protection in the industry, twenty years (Protector® Products). For good reason, I.T. SPDs have been field proven to provide outstanding reliability and longevity. We expect our units to last far longer than their warranty. In fact, over the past twenty years I.T. has manufactured more than a quarter million panel devices, and the majority of products installed are still on the job today.

What safety listings do you carry?
The safety listings for the various I.T. SPDs vary by product design and intended application as required by the marketplace. The current safety listings are provided on the product data sheets. Some units are UL recognized components under UL 1449 and are designed to be included in other original equipment manufacturers’ equipment.

How do you protect large motors?
We protect a large motor just like any other electrical device or system. First, we conduct a survey to determine the electrical system flow and distribution characteristics (e.g., voltage, current, number of phases, number of conductors, Wye or delta, etc.). Second, we determine the points of entry or points creation of transients, such as lightning, and/or large inductive or capacitive loads. Next, we provide an I.T. System Shield® protection solution from the service entrance down to the individual equipment level as required. Generally, we will protect large motors with higher peak surge current models, depending upon the results of the survey and consideration of other factors such as exposure level, criticality to operations, and ampacity.

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Glossary of Industry Terms


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.

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.

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Dynamic Measured Limiting Voltage Test
Test conducted with normal operating voltage applied. (formerly known as “let-through” voltage)

EMI – Electro-Magnetic Interference
Electrically induced noise or transients.

Maximum allowable energy (in joules) for a single impulse on a 10/1000 µs 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.

Filter Frequency Range
The range of frequency in which a filter operates. This is usually dictated by the 3db points on the low and high ends of the frequency scale.

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.

Frequency (Noise) Attenuation
The range of attenuation for a given frequency range. A larger negative number indicates a greater attenuation.

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.

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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.

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.

Input Power Frequency:
Frequency range in which the suppressor operates without causing damage to suppressor or equipment, or interference with the power signal. Applicable to AC circuits.

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.

Lead Length
The length of leads, whether integral to the unit or added to effect field connection, extending from the suppression device enclosure on a hard wire panel unit. This is an important factor in testing as specifications should reflect actual installation application.

ê – Delta Configuration.
Y – Wye Configuration.
W + G – Description of connection for application, i.e. 2W + G means 2 wires + ground.

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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 (Let-Through) Voltage
The maximum magnitude of voltage that is measured across the terminals of the SPD during the application of impulses of specified waveshape 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.

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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).

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Peak Surge Current
Maximum current allowed for a single 8×20 microsecond waveform,µs impulse waveform with continuous voltage applied. The higher the number, the stronger the unit.

Peak Transient Voltage
The peak transient voltage which is applied to unit under test according to ANSI/IEEE C62.41 1991, section 4.1: “. . this 6 kV level, therefore, can be selected as a typical cutoff for the occurrence of surges in indoor power systems.”

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.

Physical Dimensions
The length, width and height or depth of the suppression device. When considering space constraints in any application, this specification affords quick determination of acceptability.

Positive or Negative Polarity
Indicates direction in which the surge occurs.

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).

Protection Modes
Protection mode indicates suppressor’s ability to protect different paths of transient activity. Normal mode = line to ground and/or neutral to ground.

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

Response Time
The time in which a suppression device responds to a transient.

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

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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).

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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

Static Measured Limiting Voltage Test:
Test conducted with no normal operating voltage applied; eg: neutral-to-ground, which has no voltage normally applied. formerly known as “let-through” voltage)

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

Surge or Transient Amperage:
IEEE has developed wave form guidelines for testing. In this guideline, discharge currents (surge currents) are given as 200A, 5OOA, and 3000A. These values, when combined with the appropriate transient waveshape represent possible transient activity according to ANSI/IEEE C62.4 1 – 1991.

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.

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

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

Transient 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.

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.

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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.

Weight of suppression device. Useful in determining application and installation requirements.

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