POWER QUALITY FOR SOUND & VIDEO

Will My Surge Suppressor Work?

With the U.S. government's surge suppressor classification system, we finally have a rational basis for selection

Suppressor replacement schedule

Most surge suppressors in use today use components (MOVs) that wear out with use, and replacement schedule for these products should be planned before failure occurs. Quoting from one manufacturer that acknowledges this problem with their MOV products, "All (our) surge suppressors are eroded by every surge they absorb ... All have limited capacity. Once that capacity is reached, the unit is no longer protecting your equipment and should be replaced."

Unfortunately, few suppressors have established meaningful endurance ratings, making it almost impossible to predict when to replace them. If your application is important to you, your company or your customers, it is probably prudent to replace any suppressor that is more than a year old unless it has a demonstrated endurance capability of grade A, B or C. A certified grade A, class 1 product should provide continuing protection in the most severe electrical environment for about 10 years. The MOV endurance ratings in the following section may give you some guidance concerning a replacement schedule.

Technology issues

Although the new classification system does not reference any particular technology, technology plays a key role in being able to provide the desired performance and endurance. The dominant technology in use today is MOV shunt, or surge diversion technology, principally because MOVs are inexpensive and have large one-time surge ratings.

As a consequence of the large one-time surge rating, manufacturers emphasize this rating but may neglect to inform you that it is a one-time rating. In other words, the device can be expected to fail if exposed to a second such surge.

MOVs with endurance ratings of 14 mm and 20 mm are commonly used. Table C shows their endurance ratings (sometimes called pulse ratings or pulse life ratings) as taken from MOV manufacturers' data sheets for the industry-standard 8/20 microsecond current wave.

Because MOVs have limited endurance, manufacturers of surge suppressors using MOVs often provide a failure indicator on their products. A failure indicator tells us the product is prone to failure, but worse yet. it is virtually useless for several reasons, the most significant of which is that the MOV failure will most likely be discovered when equipment fails at the same time. Isn't it better to use components that are not designed to fail in the application? The cost of downtime, rework caused by noise and equipment damage is most certainly greater than the incremental cost of high-performance, verified protection.

The clamping level of an MOV often given is the onset of clamping, not the effective clamping level (suppressed voltage). For instance, for the Harris Semiconductor (formerly GE) V130LA20A, with industry standard 8/20 surges for maximum rms voltage is 130 V. the varistor clamping voltage at 1 mA is 200 V. the maximum clamping voltage at 100 A is 340V and the maximum clamping voltage at 3,000 A is 500 V.

A surge suppressor rated by a suppressor manufacturer for a clamping level of 200 V may actually have an effective clamping level (suppressed voltage) of 500 V, a specification not often acknowledged by the suppressor manufacturer.

When a surge hits, the MOV clamping level tends to drift from the initial value. If the clamping level drifts low, the MOV can start conducting on the peak of the power wave, introducing system noise, and go into thermal run-away, catch fire or explode. If the clamp level drifts higher, the effectiveness is degraded.

To reduce the chance of fire and explosion, some manufacturers are using the higher voltage MOV, such as the Harris V150LA20A, which has a maximum rms voltage of 150 V. a varistor clamping voltage at 1 mA of 240 V, a maximum clamping voltage at 100 A of 395 V and a maximum clamping voltage at 3,000 A of 580 V. As you can see, the effective clamping level (suppressed voltage) at surge currents of 3,000 A (the highest surge current called for in the U.S. government classification system) is almost 600 V, not the 240 volts likely to be claimed.

Some manufacturers use multiple MOVs and simply sum the individual MOV ratings and rate their product accordingly. Before you buy into such a scheme, insist on seeing test data from an independent lab, data that includes clamping level (suppressed voltage) at the desired surge current as well as endurance test results.

From the MOV endurance ratings just presented, it is clear that meeting the highest levels of the new U.S. government classification system standards by suppressing 1,000 large surges using 14 mm or 20 mm MOVs would be challenging.

An alternative technology

In the effort to provide Improved endurance, reduced ground wire surge contamination and improved effective clamping level (suppressed voltage) performance. a technology using filter concepts has emerged. The technology is based on the concept shown in Figure 2.


If the surge can be sensed and a switch opened during the very brief moment of the surge, and if the switch closes immediately after the surge, the surge would simply disappear with no harm done to the protected equipment and no ground wire circuit contamination. The industry-standard 8/20 surge lasts for less than 1% (20 [microJs) of the powerline cycle (16.67 ms). Such a short interruption of power would have no discernible effect on the protected equipment.

This concept is embodied in the design of the Series Mode^® suppressor shown in simplified form in Figure 3. Because no sacrificial components (such as MOVs) are used and filters do not wear out from use, no inherent endurance limitation exists for this technology. Indeed, this is the first technology to pass the new U.S. government classification system to the highest levels of performance and endurance.

Basically, the series inductor (called a surge reactor by some manufacturers) is designed to be "transparent" to the power wave but acts like an open circuit to surge frequencies, keeping them from entering the protected equipment. This is the same principle used by all inductor input L-C filters, which let the desired frequencies pass unabated but stop the undesired frequencies.

The secret here is to use materials for the series inductor, which work effectively even with high voltages and high currents applied. A major problem with the inductor design is that. to reduce the cost of the inductor, a manufacturer might use certain core materials that make the inductor small and inexpensive but will go into magnetic saturation under surge conditions of high surge voltage and high surge current, rendering the inductor ineffective for suppressing surges. Handling 6,000 V and 3.000 A (up to 4.5 million W) requires appropriately sized components.

Do your homework

No matter what the actual technology for the product, if you rely on the U.S. government classification system, you have a powerful new tool that can give you a high level of confidence in the products you purchase or specify. But you must use this tool effectively, as with any other tool you have at your disposal.

In 1996, Series Mode technology has passed to the grade A. class 1, mode 1 performance level of the new U.S. government performance classification system, withstanding 1,000 surges of 3,000 A with a suppressed voltage of less then 300 V.

As with most products, superior performance and endurance justifies a higher price. Now you have a rational basis for your selection and can invest your money or your customers' money wisely. Before this classification system and certification by UL, you could pay a lot for a surge suppressor without receiving any assurance of the superior performance and endurance your equipment may need. S&VC

References:

U.S. Government performance verification CID (Commercial Item Description) number A-A-55818, issued July 9, 1996, titled "SURGE SUPPRESSOR, TRANSIENT VOLTAGE" For a copy, circle number 250 on the Rapid Facts Card.

The Development of a Government Procurement Specification for TVSS," by John Mercer, POWER QUALITY ASSURANCE magazine, Nov/Dec 1996, page 23.

UL1449 second edition, Underwriters Laboratories. "What Changes to UL 1449 Standard For Safety Transient Voltage Surge Suppressors May Mean to You," by J.R. Harford, Power Quality Assurance magazine, Jan/Feb 1996.

UL 1449 Adjunct Classification Service testing, Underwriters Laboratories.

Surge Suppression Report from New Frontier Electronics, 1996 issue.

"Surge Suppressors: How Efficient Are They?" by J. R. Harford, ITEM (Interference Technology Engineers' Master) 1997 annual reference, page 83.

Glossary of surge suppression terms

• Clamping level (clamping onset level): This generally is used by manufacturers to describe the voltage level that causes the surge diversion device to start diverting surge energy (usually at 1 mA of current, well below the maximum rated current). Related but more important parameters are the suppressed voltage (the term used by UL), effective clamping level (when discussing shunt-mode suppressors) and let-through voltage (when discussing Series Mode or shunt mode suppressors).
• Effective clamping level: This is the residual surge voltage left after voltage clamping when measured at the suppressor output for the specified surge current (1,000 A, 2,000 A, 3,000 A, etc.).
• Filter: An electronic device that allows only certain frequencies to pass from the input to the output.
• Ground: 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. Most electrical equipment chassis are electrically connected to this wire when a three-wire plug is used to connect the equipment to the electrical power receptacle. This third, non-current-carrying conductor, which is connected to earth ground at the service entrance of a building, is often referred to as ground, equipment ground or electrical ground wire.
• Ground contamination: The designers of electrical equipment assume that equipment connected to the electrical ground wire is at ground potential, that is, all at the same voltage. When equipment is electrically interconnected with audio, video or data cables, and surges or other noise is diverted into the ground wire, ground wire contamination occurs, which can show up as hum, buzz or noise in audio systems, or noise or horizontal bars in video systems.
• Inductor: An electrical component that opposes the flow of electric current. An inductor has the property of impedance, the opposition to the flow of electric current. Impedance changes value with the frequency of the applied electricity. When properly designed, an inductor can permit power frequencies to pass unabated while opposing surge frequencies, which are much higher frequencies than the power frequency.
• Let-through voltage: The peak voltage that is let through a surge protection device from a surge to the protected equipment. The theoretical lower limit is about 180 V peak for standard 120 VAC power. Lower is better. Also called SV (suppressed voltage) by UL. An important measure of suppressor performance.
• MOV: metal oxide varistor. This is an electrical component that is essentially an open circuit for lower voltages but becomes highly conductive after a certain voltage is exceeded for a certain length of time. The common 14 mm and 20 mm MOVs are about the size of a quarter and have high one-time surge current ratings. These components wear out with use and should be replaced periodically.
• Noise: An undesirable signal that rides on the desired power signal and, although generally irregular and of relatively low amplitude, occurs frequently enough or is large enough to be of concern.