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A Surge Protection Device (SPD), also called TransientVoltage Surge Suppressor (TVSS), is used to protectsemi-conductor loads from powerline transients. SPDsare installed in the AC power system at the serviceentrance and panelboards, and sometimes at the load.SPDs are also required on data communication lines toprevent ground loops and induced surges which candamage equipment.

In AC power applications, over 95% of SPDs use MetalOxide Varistors (MOV) because of their high energycapability and reliable clamping performance. For addedperformance, hybrid designs (MOVs and capacitive filter)are typically specified.

A small number of SPD manufacturers still promote theuse of Silicon Avalanche Diodes for AC applications.These companies attempt to scare customers into buyinga premium priced unit by publishing misleadinginformation about MOV surge components. Thefollowing Tech Note summarizes the marketing claimsand technical insights regarding SADs suppressors.

SAD Myths and Reality

SADs have a faster response time (i.e. 5 picosecond

compared to 1 nanosecond for MOVs). The faster SADresponse time results in improved SPD performance.

1. NEMA LS-1 and IEEE committees do not mention theuse of response time as a SPD specification. All SPDshave sufficient response time to turn on and shuntsurges. The response time of an MOV is 1000 timesfaster than the time it takes for a surge to reach fullcurrent (i.e. 8 microseconds). Response time is not anappropriate criteria to use when specifying SPDs.

2. The response time for an SAD device is equivalent tothat of an MOV device. Response time of the device isaffected more by the internal wiring/connection than thespeed of the SAD (or MOV). For example, a SAD mayreact in 1 picosecond but the internal wiring andconnecting leads within the SPD add inductance (about 1to 10 nanohenrys per inch). This inductive effect is the

dominating factor in overall response time - not the SADreaction time.

3. Note that hybrid filters (MOVs combined withcapacitive filtering) react the fastest because thecapacitors activate instantaneously to any high frequencysurge.

MOVs degrade resulting in short life expectancy of theSPD and unsafe failures. SADs do not degrade and aresafer to use

1. Life expectancy of SADs is much lower than that of anMOVs (see figure 1). A single SAD will be damaged by asurge under 1000 Amps. Given that IEEE C62.41requires SPDs to withstand 10,000 Amp surges, SADsdo not have sufficient energy capabilities for serviceentrance or branch panel applications. To hide thisweakness, SAD devices often publish Joule ratings or Wattage instead of publishing Surge Current Capacityper Phase (a more reflective performance criteria). Notethat IEEE and NEMA do not recommend the use of Jouleratings for SPD comparison

MOVs are rated to, 6500 Amps to 40,000 Amps, makingthem more reliable for AC power systems.

Quality SPDs often parallel MOVs to achieve Surge

Current ratings in excess of 250,000 Amps per phase.These results can be verified through independent testingat lightning labs. At these ratings, the SPD will operateeffectively for over 25 years in IEEE classified highexposure environments.

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1 10 100 1000 10000Surge Current (A)

V o

l t a g e

( V )

small MOV (20 mm)

large SAD (5 kW)

Silicon Avalanche Diode: note 52 SADs areequivalent surge current rating as the 1 small MOVillustrated. For a complete device, a significantnumber of SADs are required.

X

X

MOVFailure

SADFailure

Figure 1: SADs have limit ed energy capability

TECH NOTE #: CPS-12:WHY S ILICON A VALANCHE DIODES ARE NOT RECOMMENDED

FOR AC P OWERLINE S UPPRESSORS

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Paralleling SADs is more difficult than with MOVs.Suppressors using parallel SADs require a significantamount of components which reduce the overall devicereliability.

Given the limited energy ratings of SADs, these devicesare not recommended for panelboard or switchboardapplications.

Similarly, hybrid designs using MOVs and SADs do notachieve component synergies. In high energyapplications for example, the SADs are the weak linkbecause the SADs and MOVs cannot be coordinated towork together.

2. Failure mode. SAD manufacturers claim that their units do not degrade. Rather than degrade, the SAD failsin a short circuit mode at much lower energy levels thana MOV. A properly constructed MOV suppressor will notdegrade, even when exposed to 1000s of high energy

strikes. Ask your supplier to provide independent testingto guarantee the device achieves the published surgecurrent ratings (and thus the required life expectancy).

Degradation problems do exist with the very inexpensivesurge bars. These devices are usually manufacturedoffshore and are poorly constructed utilizing under-ratedMOVs. These low quality devices should not be

compared to the SPDs typically used at panelboards or service entrance locations.

SADs Provide Tighter Clamping than MOVs

When exposed to IEEE defined test waveforms and UL1449 test results, both MOV and SAD devices have thesame Suppression Voltage Ratings. Accordingly, ULdoes not regard SAD devices as providing any better clamping than MOV based SPDs.

Summary

There are a number of myths in the SPD industry. Whenevaluating SPDs, it is important to evaluate theperformance of the suppressor unit and not compareindividual internal elements. In other words, SPDconstruction methods and internal wiring/fusinglimitations are critical to overall performance.Independent testing is essential when comparing the

performance of these units.

Based on the proven track record of performance, MOVbased suppressors are highly reliable. That is whyalmost all suppressors still employ MOV components.For service entrance or panelboard locations, SADs arenot recommended because of their limited energycapability. SADs are primarily used to protect datalineand communication wires.

FIGURE 2: COMPARISON OF COMPONENTS USED IN SURGE PROTECTION DEVICES

SPD Component Advantages and DisadvantagesMetal OxideVarsior (MOV)

Highest energy capability, excellent reliability and consistent performance, better mechanicalconnectivity for paralleling multiple components. Non-liner clamping curve, gradually degrades over repeated use (only at high surge levels), moderate capacitance.

Silicon AvalancheDiode(SAD)

Flatter clamping curve, excellent reliability and consistent performance. Very low energy capability,expensive.

Selenium Cells Moderate to high energy capability. Very high leakage current, high clamping voltage, bulky, expensive,obsolete components.

Gas Tubes High energy capability, very low capacitive (requirement for data line applications). Unpredictable andunstable repetitive behavior, crow-bar to ground (unsuitable for AC systems), expensive.

Hybrid SPD MOV/Filter is most common hybrid; incorporates the advantages of other components while overcomingthe problems associated with each individual component (achieves long life expectancy, faster response, better clamping performance. Inherent problems with hybrids SPDs using MOV and SAD, or devices using selenium cells (inability to have the various components work together).