Jody Leber wrote:
>
> What working voltage for measuring clearances and creepages should be used
> for TNV (Telecommunication Network) circuits in the EU?
>
> If a ringing signal is generated from the output of an LPS (SELV) should the
> voltage generated be considered or just that the circuit is dirived from an
> LPS supply?
>
This is not clearly established in the current IEC 950. The third
edition draft
74/453/cd is attempting to clear some of this up. It includes the
following:
2.9.2.1 Clearances in Primary Circuits
...
For a WORKING VOLTAGE to be used in determining CLEARANCES for PRIMARY
CIRCUITS
in accordance with table 2.9A;
...
- the voltage of any ELV CIRCUIT, SELV CIRCUIT or TNV CIRCUIT shall be
regarded as zero.
...
2.9.2.3 Measurement of transient levels
...
If the NETWORK TRANSIENT VOLTAGE is not known for the TELECOMMUNICATION
NETWORK in question, it shall be taken as:
- 1500 Vpeak if the circuit connected to the TELECOMMUNICATION NETWORK
is a
TNV-1 CIRCUIT or a TNV-3 CIRCUIT; and
- 800 Vpeak if the circuit connected to the TELECOMMUNICATION NETWORK
is an
SELV CIRCUIT or a TNV-2 CIRCUIT.
...
2.9.2.2 Creepage distances
...
For an RMS WORKING VOLTAGE to be used in determining CREEPAGE DISTANCES:
...
- short term conditions (e.g. in cadenced ringing signals in TNV
CIRCUITS)
shall be ignored.
When determining RMS WORKING VOLTAGE for a TNV CIRCUIT connected to a
TELECOMMUNICATIONS NETWORK whose characteristics are not known, the
normal
operating voltages shall be assumed to be the following values:
- 60 Vdc for TNV-1 CIRCUITS
- 120 Vdc for TNV-2 CIRCUITS and TNV-3 CIRCUITS.
...
These are a long way from being adopted EN clauses, and difficult to use
out of context, but they reflect the current aims of the TC74
subcommittees.
Bob Johnson
