UNDERSTANDING TVSS:

1. The term "Joule Ratings" and "Response Times" are currently not recognized by ANSI, NEMA, IEEE, or IEC as being relevant to AC surge protection devices. Numerous companies that purchase surge protectors use a joules rating as an indicator of the capability and survivability of a product. When this is the only value of assessment, the customer runs the risk of procuring poor performance products.

    

   Joules, by definition, is the result of a formula that is Watts/seconds. This in turn is current (A) times voltage (V), creating Watts. In many cases, the current is the maximum current the unit can survive and the voltage is the Let-thru-voltage (LTV) at the current.

    

   Knowing this, it is easy to see that a product with a high LTV will calculate out to have a higher joule rating compared to another product with the same current rating but a lower LTV. In such a case, the worst performance product would have the higher joule rating and mistakenly be considered the better product. Mission Criticalgard’s specifications are in every brochure we print, so you can determine the best product to purchase for your facility.

    

2. Manufacturers of main panel Surge Protection Devices (SPD’s) employ either a number of 20mm MOV’s in parallel or larger block MOV’s. The 20mm devices have one advantage: they are cheap, cents compared to dollars for block devices. Problems can arise, however, in ensuring that parallel devices share the surge current equally. In spite of attempts to match components, the non-linear nature of MOV’s suggest that these small devices will never share the surge current equally. If the surge current is of a longer duration that the standardized 8/20us (say 200 to 1000us) then the 20mm MOV that carries the lion’s share of current will degrade prematurely. Mission Criticalgard’s three terminal large block MOV’s offer additional advantages of lower parasitic impedance and therefore, better transient control and higher surge capacity.

3. Three causes of surges:

   1. Lightning

   2. Utility companies

   3. Internally generated

4. The National Lightning Detection Network, (NLDN) detected 53.4 million lightning flashes within 230 miles of one of the 130 sensors in the USA network and the "peak current" was measured. The database was analyzed by State University of New York in Albany, under contract EPRI TR-103603s and these are some of the conclusions

First stroke in a negative flash:

1. The median peak current was 35KA

2. Less then 1% had in excess of 120KA

 

First stroke in a positive flash

1. The median peak current was 55KA

2. Less the 1% had in excess of 180KA

Q: Do computer manufacturers build transient voltage surge suppression (TVSS) and/or electronic filtering into their products?

A: There are two primary reasons why virtually no U.S. computer manufacturers include transient voltage surge suppression or suppression/filtering in their products: size and cost. With the advent of switch-mode power supplies and increasing market demand, computers and related peripheral equipment are likely to continue becoming smaller and less expensive. However, the quality of the power remains in the hands of the end-user. Almost all computer manufacturers make mention in their operation manuals of the need for quality electronic grade power to assure proper equipment operation.

Q: What surge current is associated with a lightning strike?

A: IEEE Std. 1100-1992 states that lightning generated currents range from a few hundred amperes to more than 500,000 amperes. The return strokes are typically less than the initial strokes, and as many as forty return strokes have been observed in some strikes.

Q: How long do lightning strikes last?

A: IEEE's Emerald Book reports that lightning impulses are relatively fast acting, existing for only 50-100 m sec. Rise time of the return stroke is typically a very short 0.1 - 10 m sec.

Q: How much energy is required to disrupt, destroy or otherwise endanger semiconductor devices?

A: The following table describes the thresholds of failure for selected semiconductors:

Digital integrated circuits 10^-9 10^-6

Analog integrated circuits 10^-8 10^-6

Low noise transistors and diodes 10^-7 10^-6

High speed transistors and integrated circuits 10^-6 10^-5

Low power transistors and signal diodes 10^-5 10^-4

Medium power transistors 10^-4 10^-3

Zeners and rectifiers 10^-3 10^-2

High power transistors 10^-2 10^-1

Power thyristors and power diodes 10^-1 10^-0

The Emerald Book notes that "…single lightning or switching surge often causes physical damage that contributes to latent device failures. Exposure to lower magnitude surges cause either a gradual performance deterioration and/or intermittent operation. In such cases, it is often difficult to differentiate between software- and hardware-induced errors. Latent failures are observed primarily in semiconductor devices and insulating materials."

Q: Why is electrical transient and high frequency noise protection at service entrance a critical first step toward facility-wide protection?

A: More and more often, loads located at or near a building's service entrance are electronically driven. Variable speed drives, security and alarm systems, electronic trip circuit breakers and sophisticated electronic power monitoring systems are frequently found at service entrance. Safeguarding these devices from potentially catastrophic damage requires coordinated service entrance protection that includes both suppression and high frequency noise filtration.

In addition, electrical service entrances are exposed to the highest level of surge current generated by external events such as lightning and utility grid switching (refer to ANSI/IEEE C62.41 - 1991 for representative waveforms at service entrance).

Q: On what conductors does lightning actually enter a facility?

A: Most individuals are surprised to discover that lightning may enter a building coupled Line-to-Ground, Line-to-Neutral or Neutral-to-Ground. According to IEEE, the major mechanisms by which lightning produces surge voltages are:

1. A close-proximity lightning strike to objects on the ground or within cloud layers produces electromagnetic fields that can induce voltage on the primary and secondary circuit conductors (L-L, L-G).

2. Lightning ground-current flow resulting from nearby cloud-to-ground discharges couples onto the grounding network's common ground impedance paths, resulting in voltage differences across the network's length and breadth (L-G, N-G).

3. The rapid drop of voltage that may occur, when a primary gap-type arrester operates to limit the primary voltage, is coupled with transformer capacitance and produces surge voltages in addition to those coupled into the secondary circuit by normal transformer action (L-N).

4. A direct lightning strike injects high currents into the primary circuits, producing voltages by either flowing through ground resistance and causing a ground potential change or flowing through the surge impedance of the primary conductors. Some of this voltage couples from the primary to the secondary of the service transformers, by capacitance or transformer action or both, thus appearing in low-voltage AC power circuits (N-G, L-N).

5. Lightning directly strikes the secondary circuits. Very high currents and resulting voltages can be involved, exceeding the withstand capability of equipment and conventional surge protective devices rated for secondary circuit use (L-G, L-N).

Please review STATE OF THE ART TECHNOLOGY AND UL1449 2nd Edition.