>From an immunity point-of-view: A totally floated shield is at best useless, at worst worse than no shield. At the frequency at which it is a half-wavelength long, quite a bit of current will flow in the middle, and quite a bit of potential will occur at the ends. Both current and potential will couple to the underlying cable and the resultant circuit-induced noise will be at a higher level than if the shield was not there.
>From an emissions point-of-view: If the shield floats, it is as if it is not there. Was the braid not terminated because the connectors were plastic? Maybe the braid is supposed to be terminated by the user depending on specific conditions not controlled by the supplier? > From: "Hudson, Alan" <[email protected]> > Date: Thu, 15 Sep 2005 16:22:31 +0100 > To: "EMC-pstc (E-mail 2)" <[email protected]> > Subject: When is a cable shield not a cable shield? > > G'Day! > > Came across an oddity today which has failed to stimulate any memories (from > about the time the old King died!) of basic electromagnetic theory. A > well-known computer Company supplies what it calls "shielded ribbon cables". > When we examine one of these we find it's essentially a metallic braid > sleeve around a standard ribbon cable. The braid isn't connected to anything > (not even a drain wire at the connector ends). > > So what level of shielding is this going to give - if any? > > I could imagine signal lines being capacitively coupled to the braid and > then capacitively coupled to any 0V lines in the ribbon. But I can also > imagine signal lines being capacitively coupled to other signal lines. > Similarly with external signals hitting the braid - they could go to either > the signal or 0V lines - yes/no? > > I know I should know the answer to this, but obviously, today at least, if I > had a brain I'd be dangerous! > > :-) > > Regards, > > Alan > -- > Principal Engineer, > BAE Systems Limited, > UK > > > > ******************************************************************** > This email and any attachments are confidential to the intended > recipient and may also be privileged. If you are not the intended > recipient please delete it from your system and notify the sender. > You should not copy it or use it for any purpose nor disclose or > distribute its contents to any other person. > ******************************************************************** > > - > 2005 IEEE Symposium on Product Safety Engineering > 3-4 October Schaumburg, IL > http://www.ieee-pses.org/symposium > > ---------------------------------------------------------------- > 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 [email protected] > > Instructions: http://listserv.ieee.org/listserv/request/user-guide.html > > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > > Scott Douglas [email protected] > Mike Cantwell [email protected] > > For policy questions, send mail to: > > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc > - 2005 IEEE Symposium on Product Safety Engineering 3-4 October Schaumburg, IL http://www.ieee-pses.org/symposium 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 [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
