I think there are others on this forum who understand this better than I do; in fact I learned this from someone else on this forum. Ethernet can run a long way, and if it connects two pieces of equipment whose enclosures are at slightly different mains frequency potentials, due to their green wires going to different grounds, that can cause current to flow through an Ethernet shield grounded at both ends. Hence the capacitive decoupling which is a high impedance at the mains frequency. This answer assumes that your digital ground is tied to chassis ground, which I believe is typical.
> From: "Jim Eichner" <jim.eich...@xantrex.com> > Date: Wed, 7 Mar 2007 12:05:08 -0800 > To: <emc-p...@ieee.org> > Subject: Ethernet connector shield decoupling capacitor > > We're being hit with a requirement that I don't understand. > > We are using a shielded Ethernet connector that contains the typical 2kV > cap from the common point of the 75 ohm resistors to the shield. This > is used on the output of an off-the-shelf Ethernet transceiver module, > and the mfr of that module is telling us we need to connect the > connector shield to their digital ground through a 1nF 250V Y-cap, not > directly. > > 1. Why not directly connect the shield to digital ground? > > 2. Why would a Y-cap be required? The cap would be joining two parts of > the same circuit, so it would not be across an isolation boundary. I > can't think of a reason to require either the 250V rating or the Y > rating. > > Any thoughts anyone? > > Thanks, > > Jim Eichner, P.Eng. > Manager - Compliance Engineering > Xantrex Technology Inc. > e-mail: jim.eich...@xantrex.com > web: www.xantrex.com > > Confidentiality Notice: This email message, including any attachments, > is for the sole use of the intended recipient(s) and may contain > confidential and privileged information. Any unauthorized review, use, > disclosure or distribution is prohibited. If you are not the intended > recipient, please contact the sender by reply e-mail and destroy all > copies of the original message. > > - > ---------------------------------------------------------------- > 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 - 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 ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
