Combining Gdts and Movs for Surge Protection of AC Power Lines

APPLICATION NOTES Combining GDTs and MOVs for Surge Protection of AC Power Lines

AC power line disturbances are the • 1.2/50µs-8/20µs Combination Wave 1000 cause of many equipment failures. (Figure 1B) — a unidirectional The damage can be as elusive as impulse waveform having a peak occasional data crashes or as open-circuit voltage of up to 6kV 0 (Note 1) with a rise time of 1.2µs

dramatic as the destruction of a (V) VOLTAGE and a duration of 50µs (Note 2) power supply, computer terminal, or AND a peak short-circuit current of 0.5µs to 100kHz television set. Power line distur- up to 3kA (Note 1) with a rise time 0.5 µs 10µs 100kHz bances go by many names -- TIME of 8µs and a duration of 20µs (Note transients, surges, spikes, glitches, etc. Figure 1a. IEEE C62.41 3). -- but regardless of the name, an Location Categories A through C, understanding of their characteristics dependent on the distance of the Test waveforms for evaluation of a and the operation of the various equipment from the service surge protection system should protection devices available is neces- entrance. Line cord-connected conform to these standard wave- sary to design an effective equipment will usually be covered forms as closely as possible to protection circuit.This Application by Location Category A or, occa- ensure valid results.The use of test Note will illustrate how to design sionally, Location Category B.There waveforms having slower rise times high performance, cost-effective or lower peak currents/voltages may surge protection for equipment 3000 result in a false sense of security connected to AC power lines.The concerning the level of protection role of gas discharge tube (GDT) actually provided under field condi- 50% tions. surge arresters specifically designed PEAK CURRENT (A)

for AC power line protection will 1000 also be discussed. METAL OXIDE VARISTOR 1000A 750

08 20 µ The first step in providing an effec- TIME ( s) 100A 500 tive defense against power line Figure 1b. IEEE C62.41 VOLTAGE (V) VOLTAGE 25A transients is to accurately character- are two standard waveforms which 250 0.1A ize the transients. One good define the types of transients 0 reference is IEEE C62.41-1991 enti- expected in these Location 0 510152025 tled “IEEE Recommended Practice TIME (µs) Categories: Figure 2a. MOV clamping voltage at various current levels on Surge Voltages in Low-Voltage AC Power Circuits” (formerly IEEE • 0.5µs-100kHz Ring Wave (Figure 1A) Standard 587).This standard defines — an oscillatory waveform having a The second step in designing an the open circuit voltage and short peak open circuit voltage of up to effective surge protection circuit is circuit current waveforms which can 6kV (Note 1), a risetime of 0.5µs, a to choose which type(s) of surge be expected to occur on AC power ring frequency of 100kHz, and a “Q” protector to use. Surge protection lines of 1000 Volts (RMS) or less. of three.Though a short-circuit devices can be divided into two current is not specified, peak currents The standard defines three levels of basic types: Crowbar-type devices of up to 0.5kA can be expected such as gas tube surge arresters, increasing transient activity, labeled (Note 1). spark gaps, and SCRs; and Clamp- thyristors are available which offer tions provide the low impedance type devices such as avalanche faster response times, their use is switching action and high peak diodes, transient absorption zener limited to telecom and signal line current capabilities of traditional gas diodes, and metal oxide varistors. applications due to their relatively tube surge arresters while optimiz- low peak current ratings. A major ing the ability to extinguish in the The clamp-type devices have faster benefit of the gas tube surge response times but are limited in arrester is that the voltage drop 1000 their current handling ability because across the device remains essentially 750 most of the energy of the transient constant (<20V) regardless of the must be dissipated by the clamping conducted current. 500 A

VOLTAGE (V) VOLTAGE C D device. Also, the voltage drop across B 250 a clamp-type surge protector The ideal surge protector would

1000 0 GAS TUBE SURGE ARRESTER overcome the current handling and 0 5 10 15 20 25 energy diverting characteristics of TIME (µs) 750 the crowbar type device with the Figure 3b.Transient response of hybrid circuit

500 speed of the clamp type device.This presence of AC follow-on currents

VOLTAGE (V) VOLTAGE approach has been difficult and in excess of 300A. In most applica- 250 expensive to realize with traditional tions, no additional components are crowbar type devices because their required other than those of the 0 0 5 10 15 20 25 designs were optimized for the abil- basic surge protection circuit. A TIME ( µs) Figure 2b.Transient response of a plasma surge arrester ity to turn off in the presence of a surge protection circuit with sub- low-current DC bias.While that is nanosecond response time, precise increases with the conducted appropriate for protecting a telecom control of transient energy let- current as shown in Figure 2A. line, additional components (such as through, and a peak current rating of 20,000A is now practical even for Crowbar-type devices such as gas cost-sensitive applications such as tube surge arresters have slightly HOT power supplies, home stereos, moni- slower response times but can tors, and printers. handle much higher current because LOAD they act as a low impedance switch NEUTRAL A typical installation is illustrated in which diverts the transient energy Figure 3A.This is a two stage hybrid away from the protected equipment circuit consisting of a gas tube surge GROUND PLASMA SURGE RFI FILTER METAL OXIDE to be dissipated externally.While the ARRESTERS VARISTORS arrester as the primary protector peak voltage experienced by the Figure 3a. Hybrid surge protection circuit and a Metal Oxide Varistor (MOV) protected circuit during the leading as the secondary protector.These edge of some transients may be a series resistor or parallel elements must be separated by an higher than with a clamp-type connected series-RC network) were isolating impedance.This impedance device; the duration, and thus the required to ensure that the gas tube may be either resistive (>10Ω) or total energy delivered to the surge arrester would extinguish inductive (>0.1mH) to ensure protected circuit, is much lower when placed across an AC power proper coordination of the protec- when using a crowbar-type device as line with its relatively low source tive devices. Most AC applications shown in Figure 2B. impedance and the resultant follow- utilize an inductive element to mini- on currents (Note 4).These mize power dissipation and voltage This peak voltage is a function of the components invariably decreased drop during normal operation. rise time of the leading edge of the the performance of the protector transient. Faster rise times will result while increasing its installed cost. The inductor used in this example is in higher peak voltages due to the part of the RFI filter already response time of the protector. Gas tube surge arresters specifically required by the design.The output Although Zener-gated SCRs and designed for AC power line applica- of the protection circuit during a transient is illustrated in Figure 3B. 3. A small diameter MOV (LA or too low, the MOV may clamp the UltraMOV™ varistors) is used transient voltage at a level that does The following sequence of events is since the gas tube surge arrester not allow the gas tube surge depicted in Figure 3B: handles the high-energy portion arrester to go into conduction.This of the transient. would result in energy beyond the A.The leading edge of the transient ratings of the MOV.Tests have been is clamped by the MOV to a value The example duplicates the circuit conducted using several common just above the normal operating between Neutral and Ground in RFI filters having inductances or 1- voltage. addition to the Hot-to-Neutral 2mH with excellent results. circuit.This provides protection B. As the current through the MOV against Common Mode (both lines Hybrid surge protection circuits increases, a voltage is developed surged relative to ground) as well as incorporating HAC Series gas tube across the inductor which causes Normal Mode (Hot-to-Neutral) surge arresters can provide cost- the gas tube surge arrester to fire. transients.This is important because effective protection against The energy of the transient is both types of transients are frequent transients that exceed even the now quickly shunted through the occurrences in the real world.The tough guidelines of IEEE C62.41- gas tube surge arrester and away failure to provide Common Mode 1991 for Location Categories A and from the protected circuit. protection is one of the leading B. causes of failure in many otherwise C.The gas tube surge arrester solid designs. Notes remains in full conduction for the 1.The exact value is a function of duration of the transient. The critical points in the selection of the Location Category and the gas tube surge arrester are the System Exposure level. See the D.When the transient has passed, minimum DC breakdown voltage IEEE spec for more detailed the gas tube surge arrester extin- (which must be higher than the information. guishes—ready for the next highest normal voltage expected on transient. the protected line) and the follow- 2.The rise time for an open- on current rating (which must be This circuit uses each component to higher than the expected fault circuit voltage waveform is do what each does best: the gas current of the incoming supply line). defined as 1.67 x (t90 - t30 ) tube surge arrester diverts the high- In this example, the minimum DC where t30 and t90 are the 30% energy portion of the transient and breakdown voltage is calculated by and 90% amplitude points on the the MOV provides the fast, accurate multiplying the normal line voltage leading edge of the waveform. clamping of the low energy leading (120VRMS ) by 1.414 to obtain the The duration is defined as the edge. peak voltage and then adding an time from the virtual origin, t0 appropriate guard band to allow for (where a line through t30 and t90 The cost effectiveness of this normal variations in the supply volt- intersects the zero voltage axis), protection circuit is enhanced by age. to the 50% amplitude point of three factors: the trailing edge of the wave- The MOV should be selected using 1.The use of an AC line gas tube the same formula.When used in a form, t50 . surge arrester eliminates the need properly designed hybrid circuit, a for additional components to 7mm or 10mm device is normally 3.The rise time for an short- ensure turnoff. adequate to handle the small lead- circuit current waveform is ing-edge currents until the gas tube defined as 1.25 x (t90 - t10 ) 2.The isolating impedance is surge arrester goes into conduction. where t10 and t90 are the 10% supplied by an existing component and 90% amplitude points on the (the RFI filter). The inductor should have a value of leading edge of the waveform. at least 0.1mH. If the inductance is The duration is defined as the time from the virtual origin, t0 (where a line through t10 and t90 intersects the zero current axis), to the 50% amplitude point of the trailing edge of the waveform, t50 .

4. If the current supplied by the AC power line exceeds the maximum follow-on current of the gas tube surge arrester (typi- cally, ~20A), the device will continue to conduct, even at a zero crossing of the AC voltage signal, causing the gas tube surge arrester to overheat and fail.

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