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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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