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. 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)? 2.) Excluding the ISDN port which would be TNV 1, are the POTS ports; TNV 2, SELV, limited current circuits, something else? 3.) Is there a limit on the ringing voltage if everything in the unit is considered a limited current circuit? 4.) Are creepage and clearance an issue if the unit is comprised of limited current circuitry, SELV, and TNV 1? Any information would be helpful, Thanks, Jim James Wiese Regulatory Compliance Engineer ADTRAN, Inc. 205-963-8431 205-963-8250 FAX jim.wi...@adtran.com
