Hi Joe,
If you have unbalanced protection only, i.e. a surge protector between tip
and ring, then the conduction voltage for the device can be down as low as
270V, which is dictataed by the functional requirements of the circuit
rather than safety. For functional reasons, sometimes you have to have a
ground reference on the interface, so you have to usea balanced protection
scheme. If you have balanced protection, ie a surge protector between tip
and ground (T-G) , ring and ground (R-G), and tip to ring(T-R), AND you
don't have a permanently connected protective earth, then the T-G, and R-G
surge devices have to be 1.6 times the rated voltage, i.e. for 240V rated
the conduction voltage must be 400V. Obviously the T-R device does not have
to meet that requirement. As a bonus it turned out that the 400V devices
were the same price as the 300V devices. (Note: the surge protectors I am
talking about are semiconductor type, not gas discharge).
Regards
Doug
"Joe Randolph"
<jpr@randolph-telecom. To: "'peter
merguerian'" <[email protected]>,
com>
<[email protected]>
Sent by: cc:
owner-emc-pstc@majordo Subject: RE: EN60950-1,TNV
to Earth Insulation
mo.ieee.org
01/09/04 01:50 AM
Please respond to "Joe
Randolph"
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
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