Confused on grounding sheilded wire!

I've referred to Bob's Aeroelectric book and the AC43.131b on how to ground shields. 11-89 indicates that it is typical to ground both ends of the shield..

Can someone confirm if I got the following right please?

All audio wires should have their shield grounded at the source end (audio panel in this case) and the jack end is terminated and not grounded?

All antennae cables are grounded at both ends of the shield?

I have an SL40, GTX327 and am purchasing a PMA5000. I've reviewed the PMA5000 installation manual and it indicates that the shielded wires coming from the audio source (SL40) should be grounded at the PMA5000 end. I get that, the SL 40 installation manual indicates on page 17, note 4, to connect the shields to the chassis ground at both ends? I don't see note 4 on the wiring diagram though.

The install manual for the GTX327 doesn't address shielded wire...should it be used there?

I'm installing a VP100, D180 (HS34 and AP74), and a G496. If I got this right, I should only be using shielded wire for the SL40, PMA5000, audio jacks, and possibly the nav lights, strobes and landing lights?

I hope someone can steer me right!

Much thanks


Don

did you read this ?

download the power point presentation from :

http://www.ps-engineering.com/support.shtml

Yes sir

Thanks Luke,

I do have the powerpoint and get most of it, but still see contradictory information in some places.

I've also read the "aircraft wiring for smart people" at the EAA.org site. In that document, it indicates that anything with a display should have shielded wiring. So I should assume that the GTX327 should also have shielded wire?

I've noticed with the D180 that only the EDC uses shielded wire. If the above is true, why isn't all the wire used in the installation of the D180 with the HS34, AP74, and so on shielded since there is a display attached?

Thanks.

Don

General suggestion

You will get many answers to this question on the forum. The thing to remember is what you are trying to accomplish with the shield.
Are you trying to keep the signal inside (audio mic) from being corrupted, or are you trying to keep a large or digital signal from radiating out (Tx Ant).
To keep signals from radiation out, connect both ends to ground. To keep from corrupting small analog signals ground only one side*.

*Grounding both sides of a shield can cause a 'ground loop' when this happens, current flowing through the shield can induce noise into the internal wires. This is why grounding at both ends is seldom done for audio lines.

Great tip, JD ...

...that's a great way to remember this otherwise-confusing wiring detail. Thanks!

Coaxial cable (RG400, etc) is designed to have current flow in the shield, so both ends of the shield must be attached to ground. For your phone jacks, you want 3 conductors inside a shield cable (assuming stereo). One wire goes to the tip jack, one to the ring jack, and the third to the jack's ground. The jack itself must not be grounded, use ribbed fiber washers to insulate. Connect the shield only to ground at the audio panel. Same for the mike jack except only two wires are needed (no tip connection, unless you plan to use a portable PTT in the jack).
For avionics, this is harder. If they have an "audio low" output then use 2 wires inside a shield. Connect one wire to audio high, the other to audio low, ground shield only at audio panel. If there is no audio low the best you can do is to attach the audio low to the avionic's ground. If possible route the cable along the ground wire to the ground buss, then back along the audio panel's ground wire to the audio panel.

For audio signals (not antenna feeds) sometimes the shield is used as the earth return for weight saving reasons. On our airplanes the wiring runs are not very long, so use wires inside the shield for conducting and the shield to stop unwanted interference on the audio signals. Connect the shield per manufacturers recommendation, which is usually to connect the screen at the audio panel end only. For the transponder the only shielded wire required is the antenna.

Conventional nav lights use straight DC, and so don't need screened cable. The strobe connection from the supply to flash box can be plain cable also, however the high voltage lines from flash box to strobe head are often screened (and perhaps supplied with strobe kit) with the aim to keep the energy in. That's not always the case - Aeroflash for example sites the box at the wing tip and uses plain cable for the short run to the strobe head.

Some use screened cable for LED nav light feeds - perhaps this is in case the power conditioning required for the LEDs generates noise into the audio system - it may depend on which LED lights you use whether screened cable is recommended or not.

Pete

What about P-lead ? should it be grounded at both ends ?

check App. Z

Don check the Appendix Z diagrams in the Aero-electric book. Good info on grounding shielded P-lead wires. A little different if you are using toggle switches or a key type switch but the idea is the same. The P-lead is nothing more than an on-off switch wire for the magneto. The App. Z method uses the shield as the ground path for the P-lead and connects it to the engine to ground out and "kill" the magneto. No airframe ground or panel ground is used, the shield is terminated at the magneto end. Slick mags have a screw to use for this, Bendix mags use a nut like a smaller sparkplug harness nut. It's supposed to keep mag noise out of the airframe. Like a previous poster said grounding at both ends can create ground loops and introduce noise into other systems. By not grounding the P-lead to the panel there is no ground loop. It's connected to the engine only. I've used it for two airplanes and it works fine. Hope this explanation helps.
Phil

Thanks

Thanks folks,

I think I'm getting it!

We'll see if I get it if there's no smoke and noise!

Cheers,


Don