I have a problem with this as well. A non-magnetic shield material protects against magnetic field radiation by providing a path for eddy currents to flow in. The eddy currents flow in such a way as to cause a magnetic field that opposes the originating field (Lenz' law). In order for the eddy currents to have sufficient magnitude to cause field cancellation, the shield must provide a very low impedance short circuit. An open-circuited shield braid cannot perform this function.
> From: "Robert A. Macy" <[email protected]> > Date: Thu, 15 Sep 2005 09:16:09 -0700 > To: "Hudson, Alan" <[email protected]> > Cc: [email protected] > Subject: Re: When is a cable shield not a cable shield? > > Alan, > > Think also in terms of magnetic fields, or "area loops". > > The area defined by the small loop between signal lines and > ground lines now have rather opaque metal plates over them. > Starting around 10KHz the conductive "shield" kills all > the magnetic fields coming out. Which were probably more > significant than the E-Field contributions. > > - Robert - > > On Thu, 15 Sep 2005 16:22:31 +0100 > "Hudson, Alan" <[email protected]> wrote: >> 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 > > - > 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
