Food for thought (or Gas Bag time depending on your perspective.)
TNV circuit definition.
I must be missing an update somewhere - my page shows July 26 - be
gentle with me here if I'm way behind, but
>From UL 1950 Third edition.
1.2.8.8 Telecommunication Network Voltage (TNV) circuit: A circuit
that, under normal operating conditions, carries Telecommunication signals.
A TNV circuit is considered to be a secondary circuit in the meaning of this
standard.
>From EN 60950
1.2.8.8 TELECOMMUNICATION NETWORK VOLTAGE (TNV) CIRCUIT: A circuit
in the equipment to which the accessible area of contact is limited and that
is so designed and protected that, under normal operating and single fault
conditions, the voltages do not exceed specified limiting values.
While this is much more vague the defining sections add to this definition.
1.2.8.9, 1.2.8.10 and 1.2.8.11 all go on to define the different TNV
circuits and each of them carries the same note
- which is or is not (my words) subject to overvoltages from
TELECOMMUNICATIONS NETWORKS.
EN60950 2.3.1 (SELV general requirements.)
2nd paragraph. If an SELV circuit is intended to be connected to a
telecommunication network, consideration shall be give both to normal
operating voltages generated internally in the equipment and to those
generated externally, including ringing signals Earth potential rises and
induced voltages from power lines and from electric traction lines, that may
be received from the telecommunication network, shall not be considered.
I would contend that TNV lines can only be those lines which are
conductively connected to public telecommunications lines/and or which can
generate telephone ringing voltages. Whether or not a line runs outside of
the building and is subject to lightning is a separate issue and doesn't in
and of itself classify it as a TNV line. Protection must be provided for
any line regardless of its designation - TNV, SELV, or what ever.
An scoreboard at an outdoor stadium has data lines - good ol' RS-232,
differential drivers, or even coax lines running signals below ± 12 Vdc all
of the time. These are data lines not TNV because they have no connection to
the telecommunications network. But again it would possible to have
telecommunication lines running next to them that could cause inductive
overvoltage.
I can even have a phone conversation without ever hitting a
telecommunications network. Ethernet driven LAN systems are quit capable of
sending voice of classically defined digital lines. They only need some
intervening device to send the ring voltage to the handset. (Wherein TNV
lines may be located).
Digital lines can carry voice traffic and exit and enter buildings but they
are still not TNV lines. There is an interim black box that decodes a "ring
phone" packet or word and applies the appropriate ringing voltage etc. to
the telephone handset. Independent of the digital line. That just wakes the
guy at the other end up. Once its picked up the transfer of voice is
strictly a digital affair. The medium itself can be either copper or glass
fiber for this whole process of data transfer. Its almost always fiber if
its leaving a building because of transmission distance restrictions at the
high bandwidths need for voice/video transmission, but it could be copper.
The overwhelming idea of TNV lines and the intent is to protect the public
telecommunications network. My equipment cannot bring cripple MA Bell. Any
broader definition makes virtually any circuit TNV and starts unneeded and
unwanted additional regulation. The wires running through your cubical
probably runs next to your phone lines and a fault if big enough could, in
theory, provide an overvoltage situation through inductive couple etc to
your data lines. That does not make the digital lines a TNV circuit, as
clarified by the text of 2.3.1. (2.3.3 goes on to reference section 6 and
TNV lines but were getting into circular reasoning here because there is no
intention to connect to a telecommunications network).
The protection of digital lines which are subject to lightning etc are
typically done with grounded conduit transorbs and other types of things
without regard for physical separation between these lines and the rest of
the circuitry, again I'll refer back to the scoreboard scenario. This type
of protection my not protect my electronics from destruction but neither
does it pose a threat to a telecommunications network.
Even though this was just my 2 cents worth I have put on my asbestos
underware so fire away.
Gary
<<snip>>
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