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Shield grounds

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matronics.list(at)gmail.c
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Posted: Wed Feb 25, 2015 2:27 pm Post subject: Shield grounds

I thought I understood how shield grounds worked and how to implement them but now looking at a Garmin G3X.  The garmin manual specifically has some of the shields terminated on both ends and grounded.  some of the shields are only terminated and grounder on one end and the other is floating.  So what is the proper termination of shields and what is the science behind the proper methods.



John Snapp

( excuse the typos. This email was sent from a mobile device!)

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Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Thu Feb 26, 2015 6:22 am Post subject: Re: Shield grounds

Here is the ultimate authority on the subject:



http://eeeic.eu/proc/papers/55.pdf



Also see my attachment on the subject.

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nuckolls.bob(at)aeroelect
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Posted: Thu Feb 26, 2015 6:52 am Post subject: Shield grounds

At 16:26 2015-02-25, you wrote:

 	  Quote: 	
		  

<matronics.list(at)gmail.com>



I thought I understood how shield grounds worked and how to 

implement them but now looking at a Garmin G3X.  The garmin manual 

specifically has some of the shields terminated on both ends and 

grounded.  some of the shields are only terminated and grounder on 

one end and the other is floating.  So what is the proper 

termination of shields and what is the science behind the proper methods.


	


    The 'shielding' feature of a conductor

    wrapped about a central wire or collection

    of wires is to break potential electro-static

    coupling between said wires and any other

    wires.



    Wires inside a shield can be EITHER a potential

    antagonist (like p-leads) or victim (audio

    wires). Shielding allows potential victims and

    antagonists to share the same wire bundles.



    Since the shield is also a conductor, the system

    designer may . . . and often does . . . use

    the shield for both electro-static barrier

    AND as some functional flow of energy from

    one item to another. Obviously, in this

    case BOTH ends of a shield may be terminated.



    Sometimes, shielded wire is used as two conductor

    cable and both ends of the shield are terminated

    even if wires within the shielding are neither

    potential victim or antagonist. I have used

    shielded wire to build flying leads off an LED

    annunciators to exploit the nice round characteristic

    of the shielded wire versus say, an open twisted

    pair. I have used shielded trios to wire a 4-conductor

    power/control path even tho the shielding is

    not REQUIRED . . . only handy.



    When the shield is applied ONLY for a barrier

    to electro-static coupling, the general practice

    is to terminate ONE END only. Terminating the

    other end may not be a bad idea but it is

    unnecessary and offers a low risk for creating

    a 'ground loop' that was not foreseen by the

    installer . . . or perhaps event the system

    designer.



    Bottom line: Don't second guess the system

    designer. Wire per installation instructions.

   Bob . . .

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user9253

Joined: 28 Mar 2008
Posts: 1938
Location: Riley TWP Michigan

Posted: Sat Apr 11, 2015 2:55 pm Post subject: Re: Shield grounds

Interesting discussion on VansAirforce



Posted by Richard

 	  Quote: 	
		  General rule is: If you want to keep EMI inside the cable, ground both ends. If you want to keep the EMI outside the cable, ground one end. 	




Reply by Louis 	  Quote: 	
		  This isn't really the case.  Terminating the shield at one point vs. multiple depends on the electromagnetic coupling mode you're trying to defend against (electric or magnetic)[1]. Electric coupling is a function of the capacitance across the insulator between the signal wire and the shield[2], while magnetic coupling is picked up by the loop formed by the cable and the ground plane[1].

 Low frequency signals interfere via magnetic coupling, and high frequency signals via electric[1][2]. Therefore, when trying to defend against low frequency interference (like when you're trying to protect a low frequency audio signal), you use single point shield termination to avoid creating a loop/current path for magnetic coupling to occur. For high frequencies, it's important to reduce the voltage accumulating on the shield, so multiple grounding points are used[2].



[1] Fröbel, Anke. "Cable Shielding to Minimize Electromagnetic Interference." Cotbus University of Technology, Germany.

http://eeeic.org/proc/papers/55.pdf



[2] Renner, E. NAVSO P-3181 Aircraft and Avionics Cabling E3 Design Manufacturing Guidelines. U.S. Navy, 1992 para 4.3

http://bmpcoe.org/library/books/navso%20p-3181/53.html

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Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Sat Apr 11, 2015 3:15 pm Post subject: Shield grounds

At 17:55 2015-04-11, you wrote:

 	  Quote: 	
		  



Interesting discussion on VansAirforce



Posted by Richard



 > General rule is: If you want to keep EMI inside the cable, ground 

 both ends. If you want to keep the EMI outside the cable, ground one end.




	


    "Keeping the EMI inside the cable" is the same task

    as "keeping the EMI from getting INTO the cable.:"

    Shielding can only break the electro-static coupling

    mode of transferring EMI from antagonist to victim . . .

    and shielding EITHER wire will do the task. Furhter,

    the currents that flow in the shield are exceedingly

    tiny . . . easy to calculate but very difficult to

    measure. Hence, the pig-tail that brings a shield to

    chassis ground COULD be a 40 AWG wire without degrading

    the shield's performance.



    Both of the documents cited use the term "EMI"

    and "shielded cable" but the discussions and

    energy propagation models immediately jump into

    the realm of transmission lines and frequencies

    of interest of a Mhz and up . . .



    Almost NONE of what is discussed in those papers

    applies to why wires may be shielded in light

    aircraft navigation and audio systems. If we

    were designing for resistance to nuclear EMP

    then the electro-magnetic coupling modes would

    have some significance . . . but understand

    also that the appliance designs and system integration

    problems go wAAAAYYyyyy beyond any fussing with

    shielded wires.



    I don't find much to argue with in those documents

    except that they don't apply to what we do. As

    cited many times in the past . . . follow the

    installation manual's instructions first. If

    there are doubts, determine IF the shield conductor

    is ALSO part of a signal/power path. If so

    attach both ends. If not, attach one end only.

   Bob . . .

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