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Reading a wiring diagram and cable shielding/grounding

Sam D

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Petaluma, CA
I'm wiring an Artex ELT 345 and following this wiring diagram:

IMG_3221[1].jpg
The shielding/grounding is a little confusing to me.
1. At the Serial Nav Data source end, are they suggesting that one of the two Nav Data wires is wired directly to the shielding?
2. At the D-Sub end, they appear to suggest the shielding is wired into pin 7. Does it also need to be grounded?
3. Alternatively to #1 and #2, would I have a ground wire in the bundle and connect it to the shielding at the Nav Data source end? I did a weekend EAA electronics workshop and this is what we did with the intercom wiring harness project.

I have read that, as a general rule, shielding should not be used to carry current (Ausman). I'm not sure how this inter plays with ground. I also seem to recall reading that shielding should generally be grounded to the aircraft frame.

Any help is appreciated.
 

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I'm wiring an Artex ELT 345 and following this wiring diagram:

View attachment 51339
The shielding/grounding is a little confusing to me.
1. At the Serial Nav Data source end, are they suggesting that one of the two Nav Data wires is wired directly to the shielding?
2. At the D-Sub end, they appear to suggest the shielding is wired into pin 7. Does it also need to be grounded?
3. Alternatively to #1 and #2, would I have a ground wire in the bundle and connect it to the shielding at the Nav Data source end? I did a weekend EAA electronics workshop and this is what we did with the intercom wiring harness project.

I have read that, as a general rule, shielding should not be used to carry current (Ausman). I'm not sure how this inter plays with ground. I also seem to recall reading that shielding should generally be grounded to the aircraft frame.

Any help is appreciated.

One line at a time;

(1) Correct. Use a solder sleeve to terminate the shielding and add that pigtail wire. That pigtail wire and the wire listed as 'ELT RX' are used to connect to a GPS for RS232 data for position information. 'ELT RX' is connected to the data transmit line (TX) from the GPS. The pigtail wire is acting as the data lo (data ground).
(2) The easiest way to terminate this is to put two pigtail wires into a solder sleeve when terminating the shielding. Connect one pigtail wire to pin 7 on the D-sub and connect the other to airframe ground.
(3) Keep it simple. Do steps 1 and 2 as I described and you won't need an extra ground wire.

Were talking about low power circuits here. Using the shielding as ground is not a problem. I've used it in actual power circuits with absolutely no problems. Example of the two philosophies is wiring NAT and PS Engineering intercoms. NAT uses the shield as the lo and PS Engineering uses a wire for lo and has the shielding tied to ground at one end. They both work very well.

Give us a shout if you need anything.

Web
 
The method in the Garmin video is only for Teflon wire, you'll melt plastic insulated wire. With plastic strip back the outer jacket and push back the shield just enough to poke a hole thru so you can pull out the conductors. Then you have a nice shield only pigtail. If its short enough comb out the strands and retwist, you don't have to poke an extraction hole.
 
The fun part is when you need to add a shield termination to a wire and back shell that's behind the panel. Two big arthritic hands trying to do fine motor skill work in a 2 cubic inch area that's surrounded by fragile wires and sharp corners. Good times.
 
The method in the Garmin video is only for Teflon wire, you'll melt plastic insulated wire. With plastic strip back the outer jacket and push back the shield just enough to poke a hole thru so you can pull out the conductors. Then you have a nice shield only pigtail. If its short enough comb out the strands and retwist, you don't have to poke an extraction hole.

NEVER use plastic coated wire in an aircraft! It will burn and smoke! Use mil spec wire such as M27500- and M22759- or better.

I do not use the shield for it's own pigtail. If you use the solder sleeves, it's faster and you do not need a soldering iron. See my video in post #4 and MCS Mikes pic in #6.

Web
 
Thanks for the responses and video links. Lots of good stuff in here. I’ll post my results in my larger building thread.
 
Stewart -- Thanks for asking. I am a ways away. The interior is pretty much done. My current project is dealing with the windshield trim strips. Once those are complete and the windshield is fit, I'm going to take the project across town to the airport to hang the wings and related activities. Then I think I am pretty much ready for cover except for some odds and ends. Jay is currently building an O-360 for me. Once I start covering, I will probably also get going on ordering the avionics (your avionics tray pictures are gold for me)

I find that with many steps, I need to stop and learn the building process first before actually building on my plane. For example, with the windshield trim strips, I practiced quite a bit with stretching and shrinking aluminum (something I'd never done before). Now with the ELT, its basic electronics. I was pretty thrilled with myself the other night when I successfully wired the ELT buzzer to operate using the airframe as a ground. Last night, my big victory was wiring the jumper wire on the ELT. I think my wife is growing tired of hearing about these things! :)

Sam
 
Good stuff. It's fun, isn't it? Keep up the energy. Pretty soon a day's work will show a lot more progress!
 
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