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From: INTERNET:[email protected], INTERNET:[email protected] To: , INTERNET:[email protected] List-Post: [email protected] Date: 5/19/99 9:53 AM RE: Re: overvoltage protectors to ground Dwight: As revealed in other responses to your inquiry, there are not so obvious return paths for the fault energy including the 'isolated' paths through optocouplers, transformers, etc. Once a 3-legged approach is decided upon, consideration should be given as to how it is implemented. Using item 2 below with 2 devices in series and 3rd leg to ground results in a higher protect voltage across T-R than on each leg to ground. Adding a 3rd device in the ground lead can even this out but this whole approach can result in unmatched breakover voltages as well as the expense and realestate of multiple parts. 3 leg devices are available that have balanced OVP built-in. Teccor comes to mind. Depending on your design, some of these later devices are capable of handling surges without a current limiting device. There is also a 3 leg device which combines the tremendous current sink capability of a gas tube and the speed/predictability of solidstate switching. Finding a good ground point can also be a challenge. Plug-in cards and portable equipment present special cases. Ideally, these surge currents should be directed to chassis/earth ground. If some other ground point has to be utilized, do the layout to avoid having the surge currents pass through other more sensitive circuitry ground returns. At 10:23 AM 5/18/99 -0700, you wrote: >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 > >**************************** >* <[email protected]> * >* (510) 771-3349 direct * >* (510) 492-0808 fax * >* VINA Technologies,Inc. * >**************************** Mark Sechler [email protected] General Dynamics - Advanced Technology Systems 3801 Boren Drive Greensboro, NC 27407 Phone: 336/547-3871 Fax: 336/547-7432 ----------------------- Internet Header -------------------------------- Sender: [email protected] Received: from europe.std.com (europe.std.com [199.172.62.20]) by hpdmgaaa.compuserve.com (8.8.8/8.8.8/HP-1.4) with ESMTP id IAA05374; Wed, 19 May 1999 08:53:29 -0400 (EDT) Received: by europe.std.com (STD1.2/BZS-8-1.0) id IAA21266; Wed, 19 May 1999 08:51:59 -0400 (EDT) Received: from world.std.com by europe.std.com (STD1.2/BZS-8-1.0) id IAA21250; Wed, 19 May 1999 08:51:56 -0400 (EDT) Received: from stingray.gdats.com (client.gdats.com) by world.std.com (TheWorld/Spike-2.0) id AA28360; Wed, 19 May 1999 08:51:51 -0400 Received: from dnoel.gdats.com ([172.17.17.107]) by stingray.gdats.com (8.8.8/8.8.8) with SMTP id IAA20024 for <[email protected]>; Wed, 19 May 1999 08:51:33 -0400 (EDT) Message-Id: <[email protected]> X-Sender: [email protected] X-Mailer: QUALCOMM Windows Eudora Pro Version 4.1 List-Post: [email protected] Date: Wed, 19 May 1999 08:53:24 -0400 To: [email protected] From: Mark Sechler <[email protected]> Subject: Re: overvoltage protectors to ground In-Reply-To: <[email protected]> Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="=====================_3082733==_.ALT" Sender: [email protected] Precedence: list Reply-To: [email protected]
