In a message dated 1/8/2004, Peter Merguerian writes:
> There should be no problem to find min. 400 V surge protectors - they are
readily available....
> I am sure that the fax, modems, PBXs out there which have been certified are
mounted with
> surge protectors with a breakover voltage of 1.6 Vrated of the equipment.
Hi Peter:
Actually, in the cases I referenced, the protection devices were in the range
of 300 volts. This seems to be fairly common. I think the typical reason is
that the designers were simply not aware of the 1.6x requirement, so they
chose breakdown voltages based on the maximum ringing voltage.
I know it is easy to get gas tubes with minimum breakdowns of over 400 volts,
but semiconductor devices such as sidactors are not typically available with
such high voltage ratings. I have seen several designs that used a 3-terminal
sidactor with a minimum breakdown in the range of 270 volts.
In my own designs, I avoid putting protection devices from TNV-3 to ground
unless it is absolutely necessary, such as on a FXS-type feed circuit that
must supply ground referenced DC feed and ringing. For the typical FXO-type
interfaces used in terminal devices like modems and fax machines, I only put
breakdown devices across tip/ring, and then I rely on a good isolation barrier
for protection from surges between tip/ring and ground.
The reason for my original posting was simply that I am curious about all the
products I have seen that had sub-400 volt protection devices from tip/ring to
ground and also had no provisions for a permanent earth connection. Either
these designs are slipping through the safety approval process by mistake, or
I am misinterpreting clause 6.1.2.1.
It sounds as though you would agree that any such designs must have slipped
through the safety approval process by mistake.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected]
http://www.randolph-telecom.com
