Shield ... what to do

[bkervaski]

I have a kit harness that extends the wire shield forward by using a pin in a connector. Easy, get that. However, where it ends I just have the 3 wires to my avionics harness.

In these cases, what should I do with the shield pin, if anything? Ground it? If so, what if the other end of the shield is already grounded?

Thanks for any advice!

 

[wjb]

I have a kit harness that extends the wire shield forward by using a pin in a connector. Easy, get that. However, where it ends I just have the 3 wires to my avionics harness.

In these cases, what should I do with the shield pin, if anything?

What's the signal?

I'm reading your situation like this: that have 3 wires plus shield going into a connector, but only have the 3 wires (no shield) proceeding forward from there?

What to do ... it depends on the signals on the wire and the lenght of the unshileded run. If it is a noise-generating signal, such as a CAN bus or high speed serial, it can be a source of EMI to other signals or equipment (ie radios) so keeping EMI inside the wiring is a priority. If it is an analog signal (from a sensor, pot, etc) that's heavily filtered by the receving electroncis, no shield will likely not hurt. Or, if the run is especially short, your exposure to interference is lessened.

The short answer is that if the is a potential EMI generator or is particularly sensitive, it's good to have a shield end to en. Otherwise, just run the 3 wires and let the shield terminate in the connector.

 

[bkervaski]

What's the signal?

I'm reading your situation like this: that have 3 wires plus shield going into a connector, but only have the 3 wires (no shield) proceeding forward from there?

Exactly.

It's a wire trim motor position sensor located in the tail wing.

Thanks, that's helpful, much appreciated.

Follow up question, would you ground one end of this? It's easy to do at this point, not so much if I wait until I button it up.

 

[N804RV]

Ray Allen used to sell a 5-conductor cable for trim servo wiring. 2 wires are power and ground, the other 3 are for position sensing. The cable I bought a couple of years ago wasn't even shielded. And, I didn't think shielding was even specified.

 

[penguin]

In general a shield should be grounded at one end only, but there are exceptions (usually when the mfr demands something different).
Also look at what is being shielded and why, are you stopping something getting out or interference getting in?
I don't see much chance of EMI messing with the position feedback wires.
The trim power wires are only used infrequently so you may be able to tolerate a little noise when the trim motor operates (usually when the servo is slowed down using a chopper circuit).
Assuming the shield is on the power wires ground at one end only and leave open at the other.

 

[Avanza]

Grounding the shield in one end only prevents electric "noise" getting in/ out
of the wires inside the cable. It also prevents that a current (amps) may be traveling along the shield causing disturbance to the wires inside.

Grounding the shield in both ends is done when it is important to have the same ground (voltage) reference in both ends of the cable.
It will also prevent electric "noise" getting in/ out of the wires inside the cable.
This is typically pointed out in the installation instructions.

Good luck

 

[bjdecker]

Depends...

If the wires inside are differential pairs (ARINC 429), digital signals (RS-422, RS-232, etc.), terminate the shield at the backshell of the source and destination. If the wires inside are analog in nature (e.g. Audio), or low voltage capacitive, resistive loads, then ground the shield at the current source and leave it unconnected at the current sink.

Reference the G3X installation manual, Rev. AU - section 24. You'll note that most of the sensor wires/signals shields are grounded to the backshell of the GEA 24 and left open at the ends by the senders/sensors (position indicators.)

 

[Mtherr]

Data lines grounded on both ends; how?

Am installing Garmin avionics now. Garmin requires to have all data lines (Arinc, RS-232, CAN) to be shielded and terminated on both ends. They also state that the Power ground of the device at the other end may NOT be used for shield termination.

For Garmin devices, seems it is not an issue as we can terminate on the Garmin shell and I presume the device itself is considered attached to the ground though its frame.

What about for other non Garmin devices? I always have seen shield terminations connected to the ground wire/pin of the device… Good or bad practice? and what is the alternative?