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. D -- DWIGHT HUNNICUTT Sr. Compliance Engineer **************************** * <dwi...@vina-tech.com> * * (510) 771-3349 direct * * (510) 492-0808 fax * * VINA Technologies,Inc. * **************************** --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
