Unused wires in shielded cables  - Page 1
Forum section: electronics / projects

author: pgross
author_rank: Contributor
timestamp: September 24, 2014, 01:05:34 pm
content: How to deal with unused wires in a shielded cable.The case:A cable comprised of 9 conductors (AWG28) with an overall braided shield is used for carrying both microphone and speaker signals to a headset. Because of the utilized termination, the signals are restricted to run in single wires only – leaving 3 or 4 wires unused.The cable and connected equipment must be able to operate in relative high RF environments where radios are used for communication. Both the headset and the associated cable must be highly immune to EMI.I know that in the perfect world, the cable should be specifically designed to the application with only the right amount of wires and so forth. This is unfortunately not an option here.The Questions:1. Should the unused wires be grounded together with the cable shield at both ends, or one end only?2. Should the unused wires be left unterminated at both ends?Your suggestions and comments are highly appreciated.Best RegardsPeter

author: macboy
author_rank: Super Contributor
timestamp: September 24, 2014, 03:22:16 pm
content: Personally I would ground the unused conductors at the terminating end, and leave them open at the headset end and be done with it. I'd like to hear what others have to say.Obviously you will be putting a ferrite on the terminating end of the cable to prevent it acting like an antenna in your high RFI environment. Obviously.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 24, 2014, 03:44:48 pm
content: I don't think it will really matter. I would connect them to your signal ground at both ends in parallel with your existing wires for return current. You could also connect them at one end, or neither, and I doubt it would make a difference.

author: pgross
author_rank: Contributor
timestamp: September 24, 2014, 08:30:15 pm
content: Thanks a lot for your contributionActually, my first thought was to use the one end termination, until I remembered something about parasitic capacitance. Lets imagine the wires which are terminated to the shield in one end only, they can still behave as antennas – picking up RF from the cable shield etc.Maybe some of this RF noise can be capacity coupled to the signal wires. Even if the wires areterminated to the shield in both ends, they may still be able to carry some RF – or?Remember, that I’m restricted to utilize only one wire to each termination – unfortunately that also applies to the signal ground in the connector and headset.I just don’t know whether the shielding performance of a cable be can diminished by adding wires to the screen.Best RegardsPeter

author: German_EE
author_rank: Super Contributor
timestamp: September 25, 2014, 11:12:18 am
content: Either ground at one end or use spare cores in parallel with used cores (such as power)

author: made2hack
author_rank: Frequent Contributor
timestamp: September 26, 2014, 03:19:57 pm
content: Excuse my ignorance, but why must only 1 cable be terminated at the ends? Why can't you pair the cables?Because, if you need 3 - 4 connections only, this would be the perfect use for shielded ethernet cable, no? I mean, you can get 4 pairs + the shielding to use as GND that wraps the entire length of the cable.Any reason why this won't work?

author: PointyOintment
author_rank: Frequent Contributor
timestamp: September 27, 2014, 08:56:03 am
content: Quote from: pgross on September 24, 2014, 08:30:15 pmThis is a really strange requirement.