T1 is clearly a TNV circuit. Most T1 networks provide sealing current that has an open circuit voltage of -48VDC and around 5 to 20mA of loop current. In addition T1 lines can have T1 repeater powering modules at the C.O. end that provide over 120 VDC at >43mA. If a span repeater is removed and the powering module is not turned off or replaced with a non powering module, that voltage appears at the customer premise. This is also true for DDS, HDSL and ISDN. In some of these technologies, the span powering is expected at the customer premise to power the remote equipment. All of them use outside plant wiring that can be exposed to lightning and power faults. This is why Bellcore GR-1089-CORE has such an exhaustive testing procedure for Lightning and Power Faults. It is far more comprehensive than UL 1950, IEC 950, EN60950 or UL 1459.
Jim Wiese ADTRAN, Inc. [email protected] ---------- From: treg-approval To: treg Subject: Re: SAFETY, UL1950 Amd.3, T1 service status List-Post: [email protected] Date: Friday, August 09, 1996 2:34AM In a message dated 96-08-08, Edward Fitzgerald writes: >I want to confirm the safety status of the T1 service offered in the USA / >Canada. Specifically when applying UL 1950 Amd.3 to a piece of Terminal >Equipment. > >Probably because there are now the issues of SELV, TNV-1, TNV-2 and TNV-3 >I am trying to understand if the service is either TNV or SELV. To >further complicate matters I am anticipating there to be a differnt >classifaction for the point of provision, i.e. DSU/CSU scenario ? Edward: There have been several postings in the last day or two on this subject (some of them under the thread "Digital Lines"), but I thought I would add my two cents. It is a complicated topic because several related standards are in various stages of revision. Also, the specific isolation requirements that apply depend somewhat on the type of equipment, how it is grounded, and who installs it. Under UL 1950, third edition (which I presume you meant instead of "Amd. 3"), a "telecommunications network" is a metallically connected circuit that may be subject to atmospheric discharges and/or power line failures. Most T1 lines would meet this definition, and be viewed as part of a "telecommunications network". Please be aware that UL 1950 makes subtle distinctions among the terms "telecommunications network" (1.2.14.7), "telecommunications signal" (1.2.14.8), and a "TNV circuit" (1.2.8.8 and 6.2). To determine whether a particular clause applies to your situation, you must pay close attention to the exact term which is being used. My reading of UL 1950 (third edition) is that a standard T1 line is considered to be a TNV circuit, even though the normal operating voltages happen to fall within the SELV limits. As a practical matter, however, this distinction may not be as important as it first appears. Most of the requirements in clause 6 (subclauses 6.3, 6.4, 6.6) reference connection to a "telecommunication network", not a "TNV circuit." As I stated above, I think that a standard T1 line is clearly part of a "telecommunication network." On a separate note, your reference to TNV-1, TNV-2, and TNV-3 appears to refer to the proposed Amendment 4 to IEC 950. In UL 1950 (third edition), as in the present versions of IEC 950 and EN 60950, there is only one kind of TNV circuit. The ambiguity in IEC 950 and its derivatives has allowed room for people to argue about whether an ISDN ot T1/E1 line is TNV. Again, though, the killer subclause in the present version of IEC 950 is 6.3.3, which refers to a "telecommunication network," not a "TNV circuit." For many types of equipment this clause invokes the requirement for supplementary insulation rated at 250 volts. The proposed Amendment 4 to IEC 950 attempts to clarify the TNV status of digital lines, and also relaxes the requirements of subclause 6.3.3. The definition of TNV-1 is targeted directly at digital lines. The net result is that TNV-1 circuits must be separated from SELV circuits by a 1000 VRMS dielectric barrier (no creepage or clearance distances apply to this barrier). Eventually, I expect IEC 950, EN 60950 and UL 1950 to be modified in accordance with the proposed Amendment 4. I have heard that parallel voting is being used for both IEC 950 and EN 60950, and that passage may be imminent. I do not know the timetable for UL 1950. In fact, now that I think about it, there were recent postings on treg that Amendment 3 to EN 60950 was just made official. I didn't pay much attention because I thought this was just EN 60950 catching up with Amendment 3 to IEC 950. Could it be that Amendment 3 to EN 60950 actually incorporates the contents of Amendment 4 to IEC 950? Perhaps someone else on treg can provide some insight about when the TNV-1, TNV-2, TNV-3 distinction is likely to become official for each of these three documents. I'm sure that all of us in the telecom compliance area will be relieved to have the TNV situation clarified and harmonized. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc.
