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To: lml@lancaironline.net
Date: Sun, 16 Apr 2006 11:00:45 -0400
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From: "Tom Gourley" <tom.gourley@verizon.net>
Subject: Re: [LML] Re: Avionics=Black Art
Reply-to: "Tom Gourley" <tom.gourley@verizon.net>
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Angier,

This is my take on grounding cable shields.  It's based on general 
electrical engineering guidelines so I don't know if it agrees with what the 
avionics guys do, although I think it should.  What you want to avoid is 
having current flow through the shield.  If you do you probably have a 
ground loop, i.e. current is flowing in a ground wire, or wires, in the 
cable as well as in the shield.  For example, if you have a remotely mounted 
box and you connect the shield to a ground pin in the box's connector and to 
the avionics ground bus back at the panel you will have current flowing 
through the shield.  In this case I would only ground the shield at the 
avionics ground bus (which should have a very low impedance path back to the 
battery ground).  Some boxes have a pin in the connector specifically 
designated as a ground shield connection.  In this case you probably want to 
connect the shield at both ends.  Behind the panel grounding the shield at 
both ends should be ok as long as a common ground point is used and not the 
power ground pins in the connectors of the various boxes.  Hopefully none of 
the folks you mentioned are intentionally using the shield as a return 
current path.  I consider that to be a poor design practice.

Having said all that I'd like to hear from some folks who wire up avionics 
for a living.

Tom Gourley

----- Original Message ----- 
From: "Greenbacks, Ltd." <N4ZQ@comcast.net>
To: "Lancair Mailing List" <lml@lancaironline.net>
Sent: Friday, April 14, 2006 9:22 PM
Subject: [LML] Re: Avionics=Black Art


> Before igniting a spirited discussion on the subject of wiring a  panel, 
> I'll cheerfully disclose that I have no formal training and  have a hard 
> time trying to explain how an electron moves from one  place to another. 
> This said, how do we resolve an apparent conflict.  The technical wizards 
> at Garmin say in no uncertain terms that with  shielded cables (comm 
> primarily) the shield must be grounded at both  ends. Other technical 
> wizards at L3/Goodrich Avionics(WX500  Stormscope) are adamant that 
> shielded cables (Tx/Rx) must only be  grounded at one end. In the case of 
> the Wx500 talking the the MX20,  the Tx cable is to the shielded at the 
> processor end only and the Rx  cable shielded at the MX20 end only. Never 
> mind the fact that the  WX500 install manual calls out for these cables to 
> be shielded at  both ends.
>
> I suspect there may be a ground loop potential in here somewhere and 
> would welcome feedback from anyone one on the list who actually makes  a 
> living do'in this stuff day to day.
>
> I've got the WX500 up and running on the bench and haven't blown the 
> thing up yet so maybe there's hope. ):
>
> Angier Ames
> N4ZQ
> N3ZQ
>
> --
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