From: "Patterson,Gordon" <[email protected]>
Subject: RE: safety question regarding CPE equipment that provide rin
Jim,
First, your power supply does not meet the Cl. 2.4 requirements for
a
Limited Current Circuit, 2mA thru a 2K resistor.
The ringing circuit becomes a TNV-2 circuit and requires Basic
insulation to SELV, unless your SELV is grounded (conditions
applicable to table 19, Cl. 6.2.1.2), and must meet the
requirements
of Annex M.
To answer question 4), you must pass the dielectric test of Cl.
6.4.1,
between SELV and TNV-1. (table 19, Cl. 6.2.1.2).
Best regards,
Gordon Patterson
Racal-Datacom, Inc.
______________________________ Reply Separator
_________________________________
Subject: safety question regarding CPE equipment that provide ringing
Author: JIM WIESE [SMTP:[email protected]] at usa
List-Post: [email protected]
Date: 2/27/98 9:32 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.
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
[email protected]
