On 4/27/2006, Mike Hopkins wrote:
Likewise, 5kV on a telecom line is also extremely unlikely since most of the connectors and cable used won't allow it to happen..... connectors will flash over at well below 5kV in most cases..... Hi Mike: I won't argue your point about peak surges on the AC mains because I have only limited data from actual field failures, and my 10 KV number may be too conservative. However, for telephone cables I have extensive data from actual field failures. The 5 KV number that I cited relates to situations where the primary protector at the service entrance is either missing or inoperative. Otherwise the peak voltages would be limited to about 1.5 KV. In the absence of a functioning primary protector, the limiting mechanism is usually internal breakdown within the cable bundle or at various junction points in the outside cabling. The majority of surges are common mode with respect to earth, so there is usually no opportunity for phone wiring connectors within the premises wiring to flash over. In the old days when most phone cables used paper insulation, the internal breakdown limit in the outside cabling was in the range of 3 to 4 KV. With the newer plastic insulation, the level has increased somewhat. This is one reason that the newer versions of ITU K.20 and K.21 now use a 6 KV test for the "enhanced" protection level. In my own experience I have found that equipment that withstands 5 KV rarely fails in the field, while equipment that withstands only 3 KV has significantly higher failure rates. Of course, the impact of these failure rates depends on the number of units deployed. If a manufacturer sells only 100 units per year, a 0.5% field failure rate might be acceptable. However, if the product is a modem or a fax machine that is being sold at the rate of several million per year, a 0.5% failure rate from lightning damage generates a huge number of returns. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) [email protected] http://www.randolph-telecom.com <http://www.randolph-telecom.com/> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
