On 8/29/2008, Neven wrote:
In telco interfaces, e.g. FXS, FXO, xDSL, if I have a clamp device for
lightning protection connected from lines to the safety GND, is the safety
isolation requirement still met? Assume that without the clamp device all the
clearances and hi-pot are OK.
Hi Neven:
This is a messy area, and there can be differences among FCC Part 68, UL
60950, and some international standards.
The simplest case is when the equipment has a permanent, hard-wired ground
connection (such as most central office equipment and most large PBXs). In
this case, the use of protection from tip/ring to ground is always okay. In
fact, isolation from ground is not required at all for this case, so the
question of tying the protection to ground becomes moot.
Under FCC Part 68 and UL 60950, it is generally permissible to connect
protective devices from tip/ring to ground, even if the ground in question is
only the "Pluggable Type A" ground found on a normal, 3-wire AC power cord.
The protective devices need to meet certain leakage requirements at the
maximum AC mains voltages, but they can be removed for the formal hipot
testing of the isolation barrier. I have always regarded this as a bit
contradictory ("You must have a 1500 VRMS isolation barrier, but you can
intentionally bridge this barrier with a 400 volt gas tube.....").
However, this is what the standards allow. The rationale makes sense as long
as the "ground" you connect the surge devices to is reliably connected to a
safety ground. The rationale fails if for any reason the connection to safety
ground is missing.
It could be argued that in many residential applications, a connection to
ground that is dependent on the 3-wire AC mains plug is not a reliable ground,
since users frequently defeat the ground connection with a "cheater adapter"
or a 2-wire extension cord. I believe this is why Finland, Norway, and Sweden
do not allow protection devices to be connected to ground via a Pluggable Type
A connector except for very limited applications in central offices and
Restricted Access Locations.
In general, I try to avoid connecting protection devices to ground unless the
ground is permanently connected. For FXO and DSL interfaces, this is usually
easy to accomplish by implementing an isolation barrier that can stand off the
highest expected lightning surge (5000 volts is a good target). For FXS ports
that connect to outside lines, it is usually difficult and/or expensive to
provide a good isolation barrier, so the permanent ground option is usually
the simplest path.
Keep in mind that if you connect protection devices from tip/ring to "ground,"
you are usually directing lightning surges to the chassis and circuit ground
of your equipment. If your connection to earth ground relies on the 3-wire
plug on the AC mains cord, and for some reason that ground is not connected,
lightning surges will seek another exit from the equipment. Typically, this
will be through another port that looks like "ground" to the surge. The exit
point could be a USB port, an Ethernet port, or even another phone line port.
In theory, the exit path could even be through the user, although that
scenario is very unlikely for most types of equipment.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com <http://www.randolph-telecom.com/>
- ---------------------------------------------------------------- This
message is from the IEEE Product Safety Engineering Society emc-pstc
discussion list. Website: http://www.ieee-pses.org/
To post a message to the list, send your e-mail to emc-p...@ieee.org
Instructions: http://listserv.ieee.org/request/user-guide.html
List rules: http://www.ieee-pses.org/listrules.html
For help, send mail to the list administrators:
Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org
For policy questions, send mail to:
Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com
All emc-pstc postings are archived and searchable on the web at:
http://www.ieeecommunities.org/emc-pstc
