Suppressor Design Consideration:
Effects of "Let-Through" Voltage on Electronic Equipment and Surge Arrester/Suppressor Design

How do Arresters or Suppressors work (a layman's description)?

    The most common type of arrester or suppressor (described hereafter as an "SPD" or "Surge Protective Device) effectively limits the amount of voltage that is seen on the "line" and diverts it to neutral or ground.  First, the  SPD must "see" that the voltage is higher than its design specification (the clamping voltage).  Then, it must "pull" the voltage down by tying it to the neutral or ground.  Because this voltage has a "momentum", it will rise past the clamping voltage for a short period before being "pulled" to the proper level.  Imagine, for example, a bullet sitting on a table.  It can easily be stopped from moving by placing your hand (acting as a clamp) on it.  That same bullet, moving at a few hundred feet per second, will not be controlled as easily, and the same bullet, moving at thousands of feet per second, will not be stopped at all.

All  Design is a Compromise

    If there is such a thing as the "perfect design", we've never seen it.  To illustrate, let us use the example of a bulletproof vest.  It would seem simple to define the important characteristic of this product...that it stop bullets.  However, when considered, the problem becomes much more complex.  One must consider what type bullet must be resisted, how much impact is passed to the wearer, how much coverage be applied, garment weight, and wearer maneuverability.  Finally, it must be offered at the lowest cost possible to allow for the maximum number of wearers protected within an organization's operating budget.

Stedi-Power Design Philosophy

    We identify three characteristics as most important.  These values relate directly to how we view our product serving our customers.  We want to provide the most protection, for the longest time, at the lowest price, with the least confusion.  We believe this philosophy allows customers to cover more equipment more directly.  This is important because the NFPA (National Fire Protection Association), the IEEE (Institute of Electronic and Electrical Engineers), the LPA (Lightning Protection Association), and all major insurance organizations strongly recommend installing surge suppression equipment at ALL panels within your facility.

    Finally, we design our equipment to meet both standards as surge arresters and surge suppressors.  This greatly reduces the problem of specifying a complete installation you no longer have multiple models of the same voltage at varying levels of protection (capacity) and safety ratings.

    We consider the most important factor of design, as do most manufacturers, specifying engineers, and professional organizations, to be "let-through-voltage".  Our philosophy of design, however, is somewhat different in that we don't look at it as a simple number.

What is "let-through-voltage"?

    "Let-through" voltage is the amount of voltage that is allowed to pass through a surge arrester or suppressor.  It represents the peak voltage, not the voltage that most people are familiar with, which is actually "RMS voltage".  For example, the peak voltage of a household service (120/240v) is actually 339.36 Volts (RMS voltage multiplied by 1.414). Underwriters' Laboratories defines this as the "Voltage Suppression Level".  Some manufacturers may also use the term "Voltage Suppression Level".

Understanding Lightning Protection and Surge Protective Device Design

    Many manufacturers or specifiers try to simplify how customers decide which product to choose by making blanket statements such as "the lowest let-through voltage" is best.  This is not accurate.  It is not only the voltage that is allowed through the system that is dangerous, it is the amount of time the voltage is allowed to remain at this dangerous or damaging level that is the most important.

    The Information Technology Industry Council (I.T.I, formerly known as the Computer Business Manufacturers' Association, or CBEMA) provides a guideline that is a realistic, at-the-equipment, maximum allowable voltage that equipment can withstand without damage or upset.  This profile is commonly called the "CBEMA Power Curve" or the "ITI Power Curve"  Simply put, it not only describes what amount of voltage will cause upset or damage, but it also shows how long this voltage must be present to cause effect.

	The ITI Power Curve vividly illustrates that it is a combination of VOLTAGE and TIME that results in equipment malfunction and failure.      The Surge Suppression Level (or Suppression Level Voltage) measured by UL (Underwriters Laboratories) measures the peak voltage during the transient event. It does not measure the resulting voltage of the clamping event, and it does not measure the amount of time the voltage is at the measured (peak) level.      It is for this reason we submit our equipment and design to independent, third-party testing laboratories, in additional to those required for certification to test for this specific characteristic. Stedi-Power Surge Arresters and Suppressors are designed to keep voltage increases to no more than 110% of the line-to-line and line-to-ground voltage of your electrical service in accordance with this curve to maintain your voltage constantly within the "no interruption in function" above the line voltage.
This easily illustrates why a simple claim of "lowest let-through is best" is not accurate or justified.  The lowest let-through voltage certified by UL (Underwriters Laboratories) is 330 volts.  You can easily see that a voltage of 264 volts (240 volts times 110%) or 132 Volts (120 Volts times 110%) will result in damage if it is applied for a period of longer than 1/2 second.	Line Voltage (RMS) Times and PEAK Voltages that will result in damage to electronic components (.0000004 Second) (.0001 second) 8/20 μSecond Up to 100 μSeconds More than 100 μSeconds 480 Volts 3,394 V 2,376 V 880 V 277 Volts 1,958 V 1,371 V 509 V 240 Volts 1,700 V 1,188 V 440 V 120 Volts 850 V 593 V 220 V
The example shown here overlays two actual waveforms captured during Stedi-Power product testing by Underwriters Laboratories Commercial Inspecting and Testing Service (Japan).	The background waveform illustrates a 6,000 volt (peak) decaying impulse (with no protection).  This impulse is chosen because it closely approximates a lightning strike.  Another waveform is overlaid to show the actual voltage waveform that the Stedi-Power product allowed to pass.  The waveform is then "colored" to correspond to the ITI (or CBEMA) Power Curve illustrated above.      This illustrates the design philosophy behind Stedi-Power Surge Arresters and Suppressors.  We design for equipment reliability and maintenance....YOUR equipment reliability and maintenance.      Stedi-Power's design assures that voltage remains in the "no interruption in function" region at all times, and are capable of handling more than twice their rated voltage for days...not seconds. The yellow area indicates the "Damage Zone" and the green shaded area indicates the "No Interruption in Function Zone".  Please note that the "wave" is somewhat exaggerated because the two waveforms were captured at different resolutions.  The peak shown, however, is accurate.

Stedi-Power, Inc
5044 B U Bowman Drive #102
Buford, Georgia 30518
PHONE: (678) 546-6780

Last Updated: 07 Jun 2004
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