The tests are intended to deal with power system faults on the network. These are known to happen and publications like UL1459, CSA225, UL1950/CSA950 and Bellcore GR-1089-CORE all have requirements that address this. In UL1459 the only option is testing. For your info, the test you mention does have limiting....to 40A. I believe that 15ohm resistors are used in series with the 600V source to accomplish this.
Prior to the merging of the telecom and ite standards, modems typically were not evaluated for overvoltage conditions. Even though the installed base was not as large as telephones, and a lot were connected behind pbxs etc., there still appeared to be little to suggest ite connected to teleco lines were having problems. This led the standards development group to look at the construction typically employed by modems at the time and the result was a set of "construction requirements" that most felt would deliver an equivalent level of safety as passing the overvoltage tests. These made their way into the 3rd edition of Ul1950/CSA950. Another significant change is that the UL1950 philosophy of being only an equipment standard eliminated the "simulator fuse" that was intended to represent (if it opened) damage to the network wiring. Only the equipment cord is a concern and this can be addressed by using a min 26AWG size telecom wire. If you look at the flowchart included with the standard you can see the left path is "construction" and the right path is "testing" and there are various interconnecting paths allowing some construction and some testing. FYI, GR-1089 still requires testing and even requires the unit to be functional after some of the tests. Randy Ivans Underwriters Laboratories Inc. Business Sector Coordinator Telecommunications Industry email: [email protected] ------------- Original Text From: "Duane J. Marcroft" <[email protected]>, on 1/26/99 1:16 PM: Hi Treg'rs I don't have a copy of UL 1459 and I'm having a problem trying to understand why a particular UL test is being performed. Could I impose on someone to explain what UL1459,59.5,b),1) is testing and why? The test is being performed on a modem DAA by a piece of automated equipment. As a consequence the person performing the test is unable to explain much about the test. The TIP & RING circuit being tested consists of a 300V Sidactor, two 4.7 ohm resistors and a RJ-11. The resistors are series with T/P and are there to limit current when Sidactor hits break-over point. The UL test is applying a longitudinal 600V @ 40A to TIP and RING via the RJ-11, with modem on-hook. There is no current limiting in the test equipment except a fuse. Heres whats happening: When the test is run the Sidactor, one of the resistors fails and current limit fuse in test equipment blows. But, the DAA circuit does pass the Part 68 transient test. Is this a valid test? I have designed many modems and never had to pass this test before. Can someone please advise? Thank you in advance. Duane Marcroft Telecom Consultant
