Jim - I offer the following. I hope you'll take the mild chiding as my own asininity, rather than anything serious.
Regards, Peter L. Tarver Nortel [email protected] >---------- >From: JIM WIESE[SMTP:[email protected]] >Sent: Friday, February 27, 1998 6:32 AM > >Dear Colleagues, > >I have a question regarding safety of a customer premise device that is >typically owned by a residential customer. It is an ISDN device with a >U-interface, a RS-232 data port, and two independent POTS ports. The >POTS ports are used to provide analog service to a standard phone via >the ISDN line. The POTS ports are not intended to leave the customer >premise or hook up to outside plant connections. Assume the device is >powered by a Listed Class 2, 9-12 VDC wall transformer. > >I would assume that most products of this type are Listed to UL 1950 2nd >or 3rd edition or UL 1459. My question specifically relates to UL-1950 >3rd edition with the assumption that UL now recognizes clauses from >amendment 4 of IEC 950 (TNV 1-TNV3 specifically), which they do. Unless using UL1459, CSA950/UL1950, 3rd ed. would apply. And you're right, they do use the Amendment 4 changes, in a semiunofficial way, of course. > >Assume that an engineer wanted to take the 9-12 volt input, and step it >up to 65 or 70 volts DC, then feed the 65 or 70 Volts DC into an off the >shelf SLIC (subscriber loop interface circuit) IC. The SLIC then turns >that into 65 or 70 Vrms for ringing, which will go out on the POTS >interface. > >Questions: >1.) Since the unit is powered by a class 2 wall transformer and >therefore everything inside the unit would be considered a limited >current circuit (except the ISDN port) would these voltages that exceed >SELV limits be allowed without isolation between the 65-70 VDC circuits >and the user accessible circuits (RS-232)? Everything inside would _not_ be considered a "limited current circuit." I suspect you mean "limited power circuit," per Subclause 2.11, but a Class 2 input wouldn't necessarily mean the 65-70Vdc would meet the requirements for limited power circuits either. For the circuit voltages that are converted at other than SELV voltage limits, the parts of those circuits not at SELV must remain dc (</= 150Vdc) and comply with Table 8 or 9 with regard to power and current limits to be considered limited power circuits. As soon as the voltage is modulated beyond the +/-10% limits accepted for a circuit to be considered dc (interruption by, say, a relay is not addressed in Subclause 2.11), the circuits are not and cannot be treated as SELV or as limited power circuits. As a side reference for the last sentence, see Note 4 of Tables 11(a) and 11(b) of the 1996 US NEC. After the 65-70Vdc is modulated out of dc, TNV-2 would seem to fit the circuit/application definition. > >2.) Excluding the ISDN port which would be TNV 1, are the POTS ports; >TNV 2, SELV, limited current circuits, something else? This fits the definition of TNV-2. > >3.) Is there a limit on the ringing voltage if everything in the unit >is considered a limited current circuit? The ringing voltage/current will still have to comply with Clause 6 and Annex M. Although it may be somewhat novel to try to treat it as a limited current circuit (again, I don't think that's what you really meant). [If it did comply with Subclause 2.4, would there be enough energy available to drive the ringer of a 500 or 2500 set (an electronic set might be able to respond)?] [If the output can meet the requirements for a limited current circuit, would it be difficult to comply with Subclause 2.2.6, Clause 6 and Annex M, etc.? I suspect not and it may be easier, since there aren't the capacitive circuit impedance limitations as there are for limited current circuits. A limited power circuit may need additional treatment.] Is your concern that you can't or don't comply with the ringing cadencing of Annex M, that you want to apply limited current (power) circuit criteria? If so, another SLIC or tweaking of the supporting circuitry is called for. > >4.) Are creepage and clearance an issue if the unit is comprised of >limited current circuitry, SELV, and TNV 1? There are specific cases in Amendment 4 where creepages and clearances may not be a difficult issue to comply with. Read Subclauses 2.2.6 and 6.2, if you use TNV-1 and TNV-2. I'll leave the limited current circuit aspect as an exercise for others. > >Any information would be helpful, > >Thanks, > >Jim > > >James Wiese >Regulatory Compliance Engineer >ADTRAN, Inc. >205-963-8431 >205-963-8250 FAX >[email protected]
