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Surge Protection Device (SPD)
Questions & Answers
(Commonly Asked Questions)
1. What are Surges (also called Transients, Impulses, Spikes)?
An electrical surge (transient voltage) is a random, high energy, short duration electrical disturbance. As shown in Figure 1, it has a very fast rise time (1-10 microseconds). Surges, by definition, are sub-cycle events and should not be confused with longer duration events such as swells or temporary over-voltages. High-energy surges can disrupt, damage or destroy sensitive microprocessor based equipment.
Microprocessor failure results from a breakdown in the insulation or dielectric capability of the electronics. Approximately 80% of recorded surges are due to internal switching transients caused by turning on/off motors, transformers, photocopiers or other loads. The IEEE C62.41 surge standard has created the Category B3 ringwave and the B3/C1 combination wave to represent higher energy internal surges.
Externally generated surges due to induced lightning, grid switching or from adjacent buildings account for the remaining recorded surges. The Category C3 combination wave (20kV, 10kA) represents high-energy surges due to lightning. Refer to the CPS Technote #1 for more information on IEEE surge standards.
2. Is there a difference between a TVSS and a SPD?
No, Underwriters Laboratories (UL) uses the term Transient Voltage Surge Suppressor (TVSS), while NEMA,IEC, and IEEE use Surge Protection Device (SPD). An SPD/TVSS is a device that attenuates (reduces inmagnitude) transient voltages.
3. How does a SPD work?
- The design goal is to divert as much of the transient disturbance away from the load as possible. This is accomplished by shunting the energy to ground through a low impedance path (i.e. the surge suppressor).
Metal Oxide Varistors (MOVs) are the most reliable and proven technology to reduce transient voltages. Under normal conditions the MOV is a high impedance component. When subjected to a voltage surge (i.e. voltage is over 125% of the nominal system voltage), the MOV will quickly become a low impedance path to divert surges away from loads. The MOV reaction time is nanoseconds – 1000 times faster than the incoming surge.
In AC power applications, over 95% of SPDs use Metal Oxide Varistors (MOVs) because of their high-energy capability and reliable clamping performance. For added performance and SPD life expectancy, a filter element is used in conjunction with the MOVs.
Silicon Avalanche Diodes (SADs) are frequently used in dataline or communication surge protectors. They are not recommended for use in high exposure AC applications due to their limited energy capabilities. Selenium cells were once used in surge applications but are now an outdated technology. They were used in the 1920’s but were replaced in the 1960’s by the more efficient silicon and MOVs. One TVSS company continues to use selenium enhanced surge protection as a marketing ploy to create confusion with engineers. Selenium cells are metallic rectifiers (diodes) having a maximum reverse voltage of 25 VDC. Many selenium plates are stacked together to create sufficient voltage breakdown for use in AC power circuits. When mounted in parallel with MOV components, selenium offers no performance, cost or application advantages. In fact they are expensive and add considerable space (which makes installation more difficult). There are no patents on selenium cells.
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