A magnetic field will couple through the shield even if grounded at both ends, albeit in attenuated form. It's really just a matter of how much attenuation can be achieved. I've been working on an instrument with a pulsed magnetic field powerful enough to cause robust electrostimulation of any flesh in the near vicinity. An interposed sheet of ordinary kitchen grade aluminum foil reduces but does not eliminate the shock sensation. This is at 5 KHz. Turning to the matter of audio cables, hum levels even 60 to 80 dB below the program content will prove annoying, and the standard braided shield or even braid over foil can reduce the hum, but at power frequencies the answer is never as much as one would wish for. Absent resort to mu-metal shielding, the key to audio hum rejection is, as always, balance rather than shielding per se.
The physics is straightforward enough. In magnetically transparent metals such as aluminum or copper, magnetic shielding is achieved by eddy current effect rather than flux shunting. Most shields are rather too thin to be efficient at this for power frequencies. For instance, the skin depth for pure copper at 60 Hz is around 8.5mm. It takes 4 skin depths to reduce ambient magnetic fields by 70 dB, easy at RF but rather impractical for hum reduction in an audio cable routed past a power transformer, or control wiring routed near 'cabling carrying large currents'. I completely agree that UTP is satisfactory for ethernet even in noisy industrial environments. It's not just that the system is well balanced, but also that the signals are transformer coupled and galvanically isolated from the equipment with insulation good past a kilovolt. Adding a shield is just a means to violate galvanic isolation. At RF a properly grounded shield can help suppress CM radiation, but if that is a problem then the balance assumption has been blown and ferrites are probably a better solution. Why are shielded CAT-5 cables available? Because people are willing to buy them, and to keep EMC consultants busy when it doesn't help. Orin Laney On Wed, 24 Nov 2010 21:34:10 +0000 John Woodgate <j...@jmwa.demon.co.uk> writes: > In message > <FCA549BE3ECF9D4CB8CB8576837EA4890A7744@ZEUS.cetest.local>, > dated Wed, 24 Nov 2010, "ce-test, qualified testing bv - Gert > Gremmen" > <g.grem...@cetest.nl> writes: > > >Magnetic fields from will couple > >through the shield. > > Well, not if it's grounded at all frequencies at both ends, but then > you > get all the circulating currents problems. > > We all seem to agree - use UTP unless you find you can't, but using > STP > may be difficult anyway. > -- > OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and > www.isce.org.uk > John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK > If at first you don't succeed, delegate. > But I support unbloated email http://www.asciiribbon.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> > > - ---------------------------------------------------------------- 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>
