Thanks Peter for the excellent response. I'll take the challenge and extend the thread.
Suppose you are network equipment, and your customer is a U.S telco or you want to sell the product to an international telco. Your equipment is strictly Central Office equipment, not CPE. Your customer requires a Listing to UL 1950 3rd Edition or EN 60950 (for Europe). They are also requiring the use of amp champ telco connectors like those used on most channel banks. Secondly suppose that your equipment meets basic (2.0mm at 200 VDC, Bellcore maximum for Class A3) everywhere except at the pins of an amp champ connector. All circuits have basic insulation (except the champ connector) and a fault of deficient spacings at the amp champ connector results in a fault between a 190 VDC HDSL circuit and a ISDN TNV 1 circuit. The ISDN card TNV 1 has basic insulation to SELV and the HDSL card has basic insulation to SELV. Since they are in adjacent slots of the equipment, The TNV 1 and the 190VDC HDSL cards have their tips and rings appear on adjacent pins of the amp champ connector. A fault of the 190VDC HDSL onto the TNV 1 ISDN line would only appear on the network going toward the end customer. The ISDN customer now gets something unexpected (i.e. 190VDC). The telco employees are protected as the basic insulation protects their SELV. Is the amp champ connector required to provide basic insulation onto the network eventhough the MDF's, splices crossconnects etc. do not provide 2.0mm of insulation? If so what is the rational? Thanks, Jim Jim Wiese ADTRAN, INC. 901 Explorer Blvd. P.O. Box 140000 Huntsville, AL 35814-4000 256-963-8431 256-963-8250 fax jim.wi...@adtran.com >---------- >From: Jon D Curtis[SMTP:j...@world.std.com] >Sent: Wednesday, April 29, 1998 12:24 PM >To: nebs@world.std.com >Subject: BOUNCE nebs@world.std.com: Non-member submission from ["Peter >Tarver" <peter.tarver.ptar...@nt.com>] (fwd) > >From: "Peter Tarver" <peter.tarver.ptar...@nt.com> >Subject: RE: UL 1950 3rd, and IEC 950 > >As with many things in life, there is no simple answer to your whole >question list, but I'll address what I can in a short form. > >A telco is a utility and normally a monopolistic entity. The systems and >equipment the telco use are maintained (supposedly) to a much higher degree >(at least inside the CO) than equipment in a users premises (software >blunders, fires in battery rooms and ignored alarms of recent years aside). >This is because the utility, at least in the US, is required to maintain >highly reliable service to remain exempt from antitrust laws. Service >interruptions are inevitable, but you get the drift. Likewise, in the US, >the telco is exempt from complying with the NEC. I don't know how their >relationship with OSHA works > >In cases where the telco might still be owned by the government, it's no >surprise that they exempt themselves from the same laws, edicts, etc., that >they subject industry or the public at large to. > >Both TNV-1 and TNV-3 are subject to the same assumed 1.5kV transient level. >The primary difference lying in the presence of alerting signals for TNV-3. >The alerting signals are desired to remain off of TNV-1 and SELV circuits, >where it is expected to have only safe to touch (in the former sense of the >950 based standards) normal operating voltages. The TNV-3 should then be >separated by some level of insulation. Since the voltage levels for TNV-3 >far exceed those normally present in TNV-1 and would at one time have been >called hazardous in the context of the 950 based standards, the level of >insulation called for would be at least Basic for reliably earthed circuits >and Supplementary and/or Reinforced for cases where one or more circuits are >unearthed. > >Knowing that the typical current levels are low for alerting signals when >compared to the unrestricted current levels that can be associated with >Hazardous Voltages, there is reasonable rationale to allow a reduction of >the otherwise required insulation for the general case of unearthed or >"unreliably" earthed circuits to Basic. Where earthing and other criteria >are met, even Basic insulation can be waived (refer to the Conditions >applicable to Table 19). > >As to your last question, I don't think the standards writers assumed >miswiring by the telco, though they may have recognized the possibility >(still, I could be wrong). The EST isolation requirements between TNV-1 and >TNV-3 are more likely based on TNV-1 being most often derived from SELV >circuits. It seems preferable to require Basic between TNV-1 derived from >an unprescribed source voltage and TNV-3 than to require Basic insulation >between SELV and TNV-1. > >Don't forget, Pluggable Equipment Type A not installed by Service Personnel >used to need Supplementary insulation from TNV of any type to earthed parts >(and still does in Sweden). At least Basic doesn't have through insulation >thickness requirements. > >Maybe I haven't satisfactorily answered your question; maybe I've added a >little grist to the mill. It will be interesting to see how this thread >progresses. > >Regards, > >Peter L. Tarver >Nortel >ptar...@nt.com > >> ---------- >> From: JIM WIESE[SMTP:jim.wi...@adtran.com] >> Sent: Wednesday, April 29, 1998 6:01 AM >> >> I am looking for some guidance in understanding the separation >> (creepage/clearance) requirements with regard to TNV circuits in UL 1950 >> 3rd and IEC 950 and its derivatives. >> >> Why is there a requirement that TNV circuits have basic insulation >> between TNV1 and TNV3 circuits? >> >> I am asking the question for the following reasons: >> >> The telco network provider is generally exempt from listing requirements >> altogether. They have no restrictions on separation of circuits and >> have TNV 1, TNV 3 and span powered circuits (200 VDC HDSL, T1 etc.) >> intermingled at the central office in channel banks, cross connects etc. >> They provide these services via cables that are spliced, cross >> connected and intermingled in the Outside Plant. They are also >> intermingled without regard to spacings at the distribution and >> demarcation points. Since it is a fact that these services are not >> separated by "basic" insulation, why would it be important to separate >> TNV1 and TNV 3 in the terminating equipment. To me it seems like a >> chain made of paper, and at the end of the chain the standards are >> requiring a steel link. >> >> Secondly, do the standards assume that faults of TNV 1, TNV 3, and other >> high voltage telco services could be faulted by the telco provider >> inadvertently and therefore this type of fault is accounted for by the >> dielectric test (and other restrictions) between SELV/chassis and TNV 1, >> and TNV 3. >> >> Any input would be appreciated and helpful. >> >> Thanks, >> >> Jim >> >> Jim Wiese >> ADTRAN, INC. >> 901 Explorer Blvd. >> P.O. Box 140000 >> Huntsville, AL 35814-4000 >> 256-963-8431 >> 256-963-8250 fax >> jim.wi...@adtran.com >> > >
