Hi Richard:
My, you are working late tonight!
> Sorry to contradict you but...
>
> 1) Agreed that doorbells operate at ELV and are typically double insulated
> from mains circuits. Some standards would even call this SELV <grin>.
I don't believe you have contradicted me here.
> 2) The ringing voltage for a domestic telephone (POTS) is above the ELV
> limits. When such a Telecommunications Network is connected to an item of
ICT
> equipment then IEC 60950-1 would require you to apply the requirements for
a TNV-3
> Circuit to internally connected parts (IEC 60950-1 and its regional
> derivatives (being equipment standards) do not apply to the wiring in the
> installation). FYI, it is common for one pole of the telecommunications
network to be
> 'earthy' - i.e. connected to earth at the central office but having an
impedance to
> ground and a current-carrying capacity that would certainly not be
adequate
> for safety purposes.
Agreed.
The issue was that of protection of low-voltage
wiring from crossing to mains voltages within
the home due to the effects of alteration or
remodel of walls and such. I introduced
telephone wiring as such wiring would be much
the same as floating SELV in the event of a
cross -- which John feels is a dangerous situation.
I believe you agree because the impedance to
ground of telephone wiring is too high, and
because the current-carrying capacity of the
telephone wiring is too low to reduce the
voltage to ground.
My comment was to suggest that such crosses
within the walls and ceilings of homes in the
USA are rare, and seem not as dangerous as
John feels they are. He may have UK construction
in mind rather than USA construction.
Best regards,
Rich
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