Hi John and the group, Sometimes the shield can cause high frequency problems as it is just a thick wire. I documented this in a paper published at the EMC'94 Roma Symposium. One problem can happen when CM shield currents land on a PC chassis and then get into the internal circuits through seams in the chassis. This is what I described in the Roma '94 paper. UTP was shown to perform much better for both immunity and emissions compared to STP in a PC environment.
The link to the paper is: http://emcesd.com/pdf/roma94.pdf . For those who haven't been to my site, there are 160+ other papers and articles I have written are posted there as well. Doug On 11/24/10 12:04 PM, John Woodgate wrote: In message <8F3FCCF474B9484DAE45A95D68B5379E@christopher>, dated Wed, 24 Nov 2010, Chris Wells <radioactive55...@comcast.net> <mailto:radioactive55...@comcast.net> writes: We also use multipoint RS485 serial communication links in our power gear that can go thousands of feet between separate gear line ups. In this application we reference shield at each tap AC to ground through a surge rated cap or 4.7 to perhaps 10 nF to ground. Also the RS485 nodes are electrically isolated. This approach blocks power ground loops but helps us deal with Electrical Fast transient or RF conducted EMI type exposures. It appears to be most meaningful from around 100KHz up to perhaps 10MHz. You need better capacitors (lower inductance). Does this AC shield approach translate into Ethernet? It's the Laws of Physics, so it translates anywhere. How do others deal with the treatment of shields for Cat5 Ethernet in noisy wide spread applications like this? Are there IEEE Standards on Cat5 STP shield treatment that I should be looking at? The Audio Engineering Society has addressed this matter at length. Breaking the shield continuity *at the receiving end if the transmission is unidirectional* is legitimate but these days it has to work up to at least 2 GHz. To work at lower frequencies, 10 nF is about the minimum, so the inductance has to be reduced to a very small value. If you can afford them, use discoidal capacitors, but radial arrays of SMD caps, e.g. 1 nF x 10, can also work. There is a problem with surge rating, but in many cases, it's not necessary if you put a single thick conductor in parallel with your signal cable, connected to equipment enclosures at each end. Always check with UTP to see if you actually need to use STP. Often, the shield has very little effect. -- ------------------------------------------------------- ___ _ Doug Smith \ / ) P.O. Box 1457 ========= Los Gatos, CA 95031-1457 _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 / /\ \ ] / /\ \ Mobile: 408-858-4528 | q-----( ) | o | Email: d...@dsmith.org \ _ / ] \ _ / Website: http://www.dsmith.org ------------------------------------------------------- - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at http://product-compliance.oc.ieee.org/ Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.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...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher <j.bac...@ieee.org> David Heald <dhe...@gmail.com>
