On 5/18/99 Dwight Hunnicutt writes: >Question regarding "two-legged" vs. "three-legged" overvoltage >protection circuitry: What are the pro & cons of the two? >Background: Typically, to protect against overvoltages on a telco >interface protection circuit (analog or digital, such as POTS, T1, HDSL, >etc.), one sees either: >1) an MOV/varistor type device across TIP/RING, or >2) two MOV/varistor/gas-tube type devices tied in series across >TIP/RING, with the center connection tied to earth ground. >Of course, there are also typically PTC's or fuses in line for >overcurrent protection. However, my interest is the pros/cons of the >overvoltage protection topology. >If the interface circuit has no path to earth (typically through >overvoltage protectors), then UL1950/UL1459 allows waiving of the >longitudinal(common) mode overvoltage tests, which makes sense, because >there is no return path for the fault energy. >Since this waiver eliminates about half of the overvoltage testing, why >does one see the "three-legged" topology being used? Are there some >advantages to shunting energy to earth, rather than just back out the >TIP/RING pair? Certainly, one has to provide overcurrent protection to >prevent building telco wiring from burning (tested via the MDQ 1-6/10A >fuse), but are there other reasons for preferring a three-legged >approach? What are you missing out on if you elect to use the simpler >topology of just an MOV across TIP/RING? >(To further stir things up, how about if we take into consideration >Bellcore GR-1089-CORE? Does that change things? I don't believe >GR-1089 specifically contains the same waiver as UL1950/UL1459, but >certainly the results are the same, and a test lab should consider >waiving for the same rationale.) >I'm sure many of you have seen both topologies described in application >notes for various interface components, and have had to deal with both. >Any light shed will be appreciated by all. There are steady-state overvoltage conditions (power cross/induction) and impulse conditions (lightning/ESD). Protection for one does not necessarily do both. If the T/R pair is truly floating, then yes, longitudinal protection is questionable. Many circuits, especially those in a telephone central office, do not float. There are ground start (not as many now) and battery feed circuits. The feed could could be either across T/R or simplexed across the T/R and T1/R1 pairs. These cicuits, despite secondary protection on the equipment side of the transformer, still need clamping to 230 Vac, 300 Vac, or whatever. Some circuits don't even have transformers; they come right off the SLIC. Makes OV protection and longitudinal balance design real interesting. So, "How well does the circuit float?" (for impulse AND steady-state) is the big question.... Eric Petitpierre Pulse Communications Herndon, VA eric.petitpie...@pulse.com
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