Jim,
Network equipment is installed and operated by trained craftspeople who
are well aware of the associated hazards.
950, ITE, type equipment is installed and operated by common folk who
may or may not have any idea of the possible hazards.
950 attempts to reduce these hazards by placing barriers between SELV
circuits and sources of shock hazards,
dielectric withstand between TNV-1 and SELV
basic insulation and dielectric withstand between TNV-3 and SELV.
Then it follows that if you have TNV-3, TNV-1, and SELV you need basic
insulation between TNV-3 and TNV-1.
As to your second question, no RBOC will guarantee the safety of the
network connection to your home or anywhere else. That's what started
UL1459.
Regards,
Gordon Patterson
Racal-Datacom
-----Original Message-----
From: JIM WIESE [SMTP:[email protected]]
Sent: Wednesday, April 29, 1998 6:02 AM
To: 'emc-pstc'; 'TREG'; 'NEBS FORUM'
Subject: UL 1950 3rd, and IEC 950
I am looking for some guidance in understanding the separation
(creepage/clearance) requirements with regard to TNV circuits in
UL 1950
3rd and IEC 950 and its derivatives.
Why is there a requirement that TNV circuits have basic
insulation
between TNV1 and TNV3 circuits?
I am asking the question for the following reasons:
The telco network provider is generally exempt from listing
requirements
altogether. They have no restrictions on separation of circuits
and
have TNV 1, TNV 3 and span powered circuits (200 VDC HDSL, T1
etc.)
intermingled at the central office in channel banks, cross
connects etc.
They provide these services via cables that are spliced, cross
connected and intermingled in the Outside Plant. They are also
intermingled without regard to spacings at the distribution and
demarcation points. Since it is a fact that these services are
not
separated by "basic" insulation, why would it be important to
separate
TNV1 and TNV 3 in the terminating equipment. To me it seems
like a
chain made of paper, and at the end of the chain the standards
are
requiring a steel link.
Secondly, do the standards assume that faults of TNV 1, TNV 3,
and other
high voltage telco services could be faulted by the telco
provider
inadvertently and therefore this type of fault is accounted for
by the
dielectric test (and other restrictions) between SELV/chassis
and TNV 1,
and TNV 3.
Any input would be appreciated and helpful.
Thanks,
Jim
Jim Wiese
ADTRAN, INC.
901 Explorer Blvd.
P.O. Box 140000
Huntsville, AL 35814-4000
256-963-8431
256-963-8250 fax
[email protected]
