Jim- In reply to your questions: (references are to UL1950, 3rd ed, including new IEC950, 2nd ed., 4th Amd, requirements)
1) Per Table 0.1 of UL1950, Basic insulation is required between TNV
(TNV-2 or TNV-3) and unearthed SELV circuits. In other words, you don't
want the high voltage ring getting out onto the RS-232 pins that the
user is assumed to have access to. It doesn't matter that the ringing
voltage is supplied by a SELV/LPS source. (yes, you can touch the
ringing voltage if you stick your finger in the POTS line RJ-11/-14
modular jack, but the area of contact is very limited and considered ok
to touch).
2) Your POTS ports going to internal phones are considered TNV-2. They
do not connect to outside plant (and are therefore not exposed to
overvoltages), but do have TNV level voltages present.
3) Ringing voltages have to comply with Annex M, period. The ring has
to be a standard ring so that any analog phone will work properly when
connected to the POTS port..
4) TNV-1 and SELV circuits do not need to be separated by Basic
insulation, so no creepage and clearance distances need be specified.
But, the "separation" requirements of Sub-clause 6.4.1 do apply. You
don't want lightning transients getting through to accessible SELV
circuits. So you still need some sort of spacing that will pass the
electric-strength and/or impulse tests.
You do, however, need to have Basic insulation (creepage,
clearance) between the TNV-2 circuits (POTS) and your unearthed SELV and
TNV-1 (ISDN) circuits. Again, you can't allow the ringing voltage
getting on to circuits (SELV, TNV-1) that are operating at SELV
voltages.
hope this gives you some help...
Dwight Hunnicutt
VINA Technologies, Inc.
<[email protected]>
JIM WIESE wrote:
> 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]
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