Hi John, Thanks for the info on audio systems in theaters. I was thinking more of safety problems of currents flowing. I have debugged a number of audio hum problems over the years, some from the magnetic fields of mains currents getting into sensitive audio inputs.
The use you cite below is not a coupled bonding conductor. More of a current shunt or bypass. When used for power mains current the voltage drop across the wire is mainly across the resistance of the wire not the inductance so you don't really have a canceling voltage induced by mutual inductance. If the audio use were truly a coupled bonding conductor there would be no problem with mains current flowing in the shield in the first place. The shield would induce a canceling voltage by mutual inductance eliminating the hum and an extra conductor would not be needed. Doug John Woodgate wrote: > In message <[email protected]>, dated Fri, 2 Feb 2007, Doug > Smith <[email protected]> writes > >> For the February Technical Tidbit, I discuss a technique that has been >> used in protection of telephone systems for many years, and has many >> potential uses in other settings, Coupled Bonding Conductors (CBS). > > > This technique is strongly advocated by some, but not all, audio > engineers for professional sound installations, as a better technique > than interrupting the shielding of a cable to eliminate a large > low-frequency current flowing in a ground loop. The interruption (at the > receiving end of the cable for preference) is bridged by a > low-inductance capacitor to preserve shielding for high-frequency > signals. This technique is also used for cables for the DMX data system > for controlling theatrical luminaires. > > You wrote in the article: > > I am not aware of any problems caused by mains current flowing in > coupled bonding conductors. > > The magnetic field created can cause serious interference with > audio-frequency induction-loop systems for assisted hearing in theatres > and cinemas. While the fundamental frequency is not very audible, the > current is usually rich in harmonics, especially odd-order, up to 1 kHz > and even beyond in some cases. Variable-speed drives, used for > scenery-handling equipment, can produce high-frequency ground currents, > unrelated to the mains frequency (so-called 'interharmonics'). > -- ___ _ 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: [email protected] \ _ / ] \ _ / Website: http://www.dsmith.org - 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/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: Jim Bacher: [email protected] David Heald: [email protected] 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 ______________________________________________________________________
