Joe, You are correct on all accounts. Unfortunately there has been a lot of work by a manufacturer of PTC's that is trying to drive interface design on telecom products such that these parts must be utilized. The problem is that current PTC technology is not really adequate for reliable telecom circuits (in my opinion). In analog circuits, the longitudinal balance can be significantly affected and in DSL circuits, the impedance is too great, especially for extended periods of time after a lightning strike. For instance we have seen PTC's that increase impedance from 5 to 70 ohms or more for 15 minutes and sometimes longer as a result of GR-1089 surges. This could take down a DSL circuits for long periods of time on long loops and drive crafts persons crazy trying to troubleshoot these circuits. There is not a requirement in GR-1089 or ITU that requires the product to work within a given period of time after the surges, yet in the real world this can be a significant issue. In my opinion the intent is that immediately after the surge the product should return to its original functional condition (not 15 minutes, 2 hours, or maybe never). The new GR-1089 issue 3 does now require that performance be checked at near the maximum rated loop length after surges and as such requires the circuit to return to normal performance at some point(although it technically could be minutes, hours, days). There are also other tests such as first level power faults 7, 8, 9 they may impact the ability to utilize PTC's in carrier class equipment. PTC's also tend to explode like roman candles if hit by a 600V power fault on multiple occasions or due to contact bounce. A close look at the data sheets will uncover a small note that states they are not intended for greater than a 3 amp surge current. ITU, UL, and GR-1089 call out power faults much greater than that, but only require a single surge. So during the lab testing process, the manufacturer can replace the PTC and do the next test. In the real world the PTC resets, the same PTC could be hit multiple times.
The thought with the GR-1089-CORE revision was that the product should not need to functionally survive a surge level greater than a primary protector is required to survive. The Telcordia primary protector spec is GR-974-CORE and only requires a primary protector to function after 10x1000uS, 1000V, 100 amp, lightning strikes. Therefore, if the product does not force the primary protector to fire yet can handle the 1000V or 100 amps at 10x1000 uS, it is as robust as the primary protector and coordination is not necessary. If larger surges occur, then either the primary protector or the product is probably going to fail anyway and a truck would need to be rolled. Since for all practical purposes, the product will have overcurrent protection such as fuses, a large strike would blow the fuses, and then the primary protector would fire and handle all the energy from the larger strike. At least that was our thought process. The reality is that until an impedance product can be developed that can truly meet GR-1089-CORE (especially the new issue 3)and also the real world demands of telecom circuits, I believe it is unwise to write spec's that require certain performance like that in ITU K.20 and K.21 2000. What is sad is that in order to meet the 2000 K.20 and K.21 criteria, the options are very limited, with PTC's being one of the few options. As a result, manufacturers will generally need to make the product work at shorter distances, reduce the reliability, and increase trouble call rates in order to simply meet the test requirements. As an FYI, we see very few instances of damage on our carrier class products due to lightning events. As such I doubt the coordination issue is really that significant. What seems to be the root cause in many or most instances of lightning damage is improper grounding of the telecom equipment. This is becoming a much bigger issue as the installers are tending to use more and more sub-contractors with limited interest or knowledge in proper installation and bonding/grounding practices. Please note that these are only my opinions and not necessarily that of my employer! Good Luck, Jim Jim Wiese NEBS Project Manager/Senior Compliance Engineer ADTRAN, INC. 901 Explorer Blvd. P.O. Box 140000 Huntsville, AL 35814-4000 256-963-8431 256-963-8250 fax [email protected] From: [email protected] [mailto:[email protected]] Sent: Thursday, April 17, 2003 2:54 PM To: [email protected]; [email protected] Subject: Lightning coordination in K.20 (2000) versus GR-1089 Hello All: I have been studying the new 2000 edition of K.20, "Resistibility of Telecommunication Equipment Installed in a Telecommunication Centre to Overvoltages and Overcurrents." There appears to be an important change from the previous edition that will have a big impact on line interface design. I would like to get some feedback on whether I am understanding this properly. The change that concerns me is that for test 2.1.2 (4000 volt surge on twisted pair phone lines), K.20 now requires that the primary protector *must* operate. If there is any kind of secondary overvoltage protection internal to the equipment under test (EUT), requirement 2.1.2 pretty much forces the EUT to contain series resistors in front of the internal protection. Otherwise, the internal protection will prevent the external primary protector from operating. The requirement for the primary protector to operate can be waived if the protection internal to the EUT itself meets the requirements for a primary protector. However, this includes passing the test of 2.1.5 with vaguely specified surges of 1000 amps per wire and (presumably) open circuit voltages of 4000 volts. I note that in Telcordia GR-1089, the requirement to coordinate with the primary protector can be waived if the EUT can survive a 10x1000 uS, 100 amp surge (clause 4.6.7.1 of the 2002 edition). This requirement is fairly easy to meet without using series resistors. I find it interesting that series resistors have never been required for compliance with GR-1089, which itself is a pretty rigorous standard, nor were they required for previous editions of K.20. Now, it appears that manufacturers must decide at the outset whether their GR-1089 compliant products might ever go into a market where K.20 compliance is required. If so, the resistors have to go in the design. The series resistors needed to pass the new K.20 requirement are not ordinary resistors. Typically, they are large, wirewound, surge tolerant, flameproof resistors with steady state ratings of several watts. Two of these per port on a high density, multiport board is a big hit on board area. Furthermore, the added resistance is very detrimental to some types of DSL transmission. In other words, this change in K.20 looks like it will have a big impact on line interface design. My questions are as follows: 1) Is my understanding of the new coordination requirement in K.20 correct? 2) Is there a simpler way to comply with the requirement other than using series resistors? 3) Has there been any industry feedback to the ITU complaining about the coordination requirement as presently written? 4) Is there evidence that the 10x1000 uS, 100 amp waiver in GR-1089 is inadequate, justifying the much more stringent waiver requirement in K.20? Any and all comments on the above would be most welcome. I'm just trying to make sense out of the new requirements. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 [email protected] http://www.randolph-telecom.com This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
