Shielded wiring
 

Can the "shield" be used as the return ground? Meaning, if I have shielded wire containing 2 wires, can one wire supply the nav and the other the strobe, and use the outside shield as the return ground (neg)?
I understand the concept that each positive wire should have a ground that return to the "main ground buss", and not to use the airframe as the ground. Can I have a "common" ground at the outside wing rib that is isolated from the airframe to which all the negative wires of all the wingtip electrical stuff (landing light, taxi light, strobe, nav) connect, and from this common ground run one wire large enough to carry the combined current back to the central ground? If this can be done, I would think that the same concept can be used at different stations of the airframe?
Thanks.
Johan.

I don't think so
 

Most of the shields in my wiring required grounding only at one end. That would pretty much shoot that down.

The shield is not designed to carry the current. The shield is grounded to prevent noise.

Shield used as Ground
 

It all depends on the application. Shielding is commonly used to shield the shielded wires from radiated noise from other sources. When shielded wires are used in avionics wiring, one or both ends of the shield are commonly grounded, shielding the wiring from other radiated noise sources. When used in audio panel headset and mic wiring, the shield is commonly used to ground both jacks back to the audio panel (BOTH jacks MUST be insulated from the panel with fiber washers). Here the shield is part of the audio circuit. And in the case of magneto "P" lead wiring, the shield is commonly used as the ground return to BOTH magnetos. In this case the shield is grounded to BOTH magnetos, and to the GND terminal on the ignition switch. The ignition switch GND terminal should NEVER be grounded to the panel ground. Again, here the shield is part of the circuit.
If I were to install headlights or NAV lights that had high internal frequency switching (like HID lights), I would not utilize the shield as part of the power conducting circuit. I would then ground the shield at the end nearest to the source of potential radiated noise. In this case, the shield helps to minimize the radiated noise only. In the case of a strobe power supplies mounted inside the cabin, the strobe cable shields are generally only grounded near the power supply, not at the bulb end.
So it all depends on what you are trying to achieve with the shielding.... In some cases it's OK to use the shield as part of the power conduction circuit. In other cases, it?s not.....

As Jay mentioned the shield is normally not connected at both ends. The reason for this is to prevent current flow in the shield due to differences in the ground potential(voltage). If the shield is connected at both ends and there is a difference in potential and therefore current flow then the shield would act like an antenna of sorts and could create interference instead of preventing it. That is the rational of only connecting one end.

That does not mean of course that if you have both ends connected that you will have a problem.

You need to keep in mind that for any current flow in your power and return(Ground) wires there will be a small drop in both the power and the ground wires due to the fixed resistance of the wire. This is known as Ohm's Law to us EE types. Power sizing of the wire ensures that these voltages are small.

P leads
 

a search using "magneto P-lead grounding 8-27-08 will take you to an excellent post by the late Paul Lipps on this subject.

[I would then ground the shield at the end nearest to the source of potential radiated noise. In this case, the shield helps to minimize the radiated noise only. In the case of a strobe power supplies mounted inside the cabin, the strobe cable shields are generally only grounded near the power supply, not at the bulb end.]

Fred,
Thank you. Question on the above statement.
As I understand, we should never "ground" the negative return wire to the airframe and as such use the airframe for the negative. In stead use a dedicated negative return back to battery negative. (Got that) If you say "ground at the source", do you mean that the shield is contacted with the airframe near the HID? Is it contacted with the HID driver device and not the airframe? Does the HID unit stay insulated from the airframe? Out of necessity the inner wires will run exposed (without shield) for some distance. Does this distance matter?
Thanks.
Johan

Thanks, found it and now printed.
Johan

Thanks for all the responses.
 

Goes a long way!!
Just to pick up on another question. It has to do with the use of a single negative return large enough to carry ALL the negative returns. Reason I'm asking is that I want to use a multipin plug at the wingroot, and using a negative return wire for each positive basically doubles the number of pins needed.
Thanks.
Johan

Multiple wires in parallel are equivalent to one larger wire. The current carrying capabity does add when this is done, the only possible issue is that if one of the connections is lost then the remaining wires would need to carry the entire current. Sometimes an extra wire is added in wire bundles as a form of redundancy. A familar example is your PC power supply. It includes multiple wires for +12 and Ground as part of the connections to the mother board. While my experience is limited to non-aircraft applications I'm sure this is done in aircraft systems all the time.

Oh one more thing. Make sure you check the current rating of the connector contacts that you are using. Always stay below this rating and don't assume that you will get a perfect spilt of the current.