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Shielded wiring

D

Dorfie

Well Known Member
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.
 
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.....



Dorfie said:
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.
Click to expand...
 
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 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.
 
Shield used as ground

Again, not always true. Again, it depends on what is being powered... I would use the wing as a ground for landing lights (non-HID) and regular incadecent NAV lights. I would use the airframe to ground the incadecent tail light (with a ground wire coming from the tail light to the fuselage).
Yes, "Ground at the source" for shields means ground the shield to the airframe nearest the source of the noise. The HID frame should be connected to the wing structure, but the HID ground wire I would bring back to a ground nearer the battery/firewall IF the HID demonstrated a lot of radiated noise.... The HID power leads (+ & -) would be inside a shielded cable to prevent radiated noise. Some exposed, unshielded wire near the HID shouldn't be much of an issue. If, after wiring in this manner, I still got a lot of HID related noise in the radios, I would then put a power lead filter ( a 'PIE' network) near the HID, using the airframe for the filter ground.

Dorfie said:
[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
Click to expand...
 
Shield used as ground

Again, not always true. Again, it depends on what is being powered... I would use the wing as a ground for landing lights (non-HID) and regular incadecent NAV lights. I would use the airframe to ground the incadecent tail light (with a ground wire coming from the tail light to the fuselage).
Yes, "Ground at the source" for shields means ground the shield to the airframe nearest the source of the noise. The HID frame should be connected to the wing structure, but the HID ground wire I would bring back to a ground nearer the battery/firewall IF the HID demonstrated a lot of radiated noise.... The HID power leads (+ & -) would be inside a shielded cable to prevent radiated noise. Some exposed, unshielded wire near the HID shouldn't be much of an issue. If, after wiring in this manner, I still got a lot of HID related noise in the radios, I would then put a power lead filter (a 'PIE' network) near the HID, using the airframe for the filter ground.

Dorfie said:
[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
Click to expand...
 
A shield grounded at one end only shields the electric field component of any signals carried in the wires. If the shield is also used to carry the return current then it also helps to null out the magnetic field component since the magnetic field of the outbound and return current are equal and opposite. The only reason that the magnetic field isn't completely canceled is due to the geometry of the wire inside the shield not always producing magnetic fields centered on the same axis. Most shields have more wire strands than then wires they shield so they can handle at least as much current as the wires as long as they are terminated properly at the ends (not handing on by just a few strands).

Dean Wilkinson
CTO, AeroLEDs LLC (and B.S.E.E.)
 
Dean_aeroleds said:
A shield grounded at one end only shields the electric field component of any signals carried in the wires. If the shield is also used to carry the return current then it also helps to null out the magnetic field component since the magnetic field of the outbound and return current are equal and opposite. The only reason that the magnetic field isn't completely canceled is due to the geometry of the wire inside the shield not always producing magnetic fields centered on the same axis. Most shields have more wire strands than then wires they shield so they can handle at least as much current as the wires as long as they are terminated properly at the ends (not handing on by just a few strands).

Dean Wilkinson
CTO, AeroLEDs LLC (and B.S.E.E.)
Click to expand...

To elaborate a bit on what Dean said:

By using the shield to carry the return current, you're essentially using the shielded wire as a coaxial cable. And yes, as Dean describes, that helps reduce interference associated with the magnetic field component (or completely eliminate it in the ideal case).

However, it should also be noted that by using the shield to carry the return current, you sacrifice some of the suppression of interference associated with the electric field component. No free lunch. And in some cases you can introduce ground loop issues as well, which is a whole 'nother can of worms.

In summary, I just wanted to clarify that there is not one configuration that works well for all possible situations. The best solution depends on the type of source and load, and its particular emissions and susceptibility characteristics. But not everyone has the background to do this kind of analysis. And even for those of us who do, it's not always entirely straight forward because there are various trade-offs involved. Sometimes only testing will reveal the real-world effectiveness of a shielding/grounding scheme.

So what's the best general advice to an RV builder? Whenever possible, simply follow the manufacturer's recommendations when installing a device. They presumably know what they're doing, and presumably have also tested the configurations they recommend. So if you're installing AeroLED lights, your best bet is to follow Dean's recommendations. If you're installing someone else's gizmo, follow their recommendations. Etc.
 
I have one question on this topic:

When running a RS232 data cable, must I use a 3-wire shielded cable (TxD, RxD and ground + shielding) or can I just run a 2-wire shielded cable (TxD, RxD and shield) ? I wonder if the shield alone can be used as a signal ground connection or do I need to have a dedicated ground wire inside the cable and use the shield with it, tying both together ?
 
ricardoramos said:
I have one question on this topic:

When running a RS232 data cable, must I use a 3-wire shielded cable (TxD, RxD and ground + shielding) or can I just run a 2-wire shielded cable (TxD, RxD and shield) ? I wonder if the shield alone can be used as a signal ground connection or do I need to have a dedicated ground wire inside the cable and use the shield with it, tying both together ?
Click to expand...

You probably do not need to shield this type of connection but you would want to use twisted wires to eliminate cross talk (like CAT-5/6). If you want to use shielded wire to reduce the chance noise interfering with the signals, do not use the shield as a ground. It should be grounded at one end only to dissipate the surrounding noise.

+1 on what Fred said.
 
ricardoramos said:
I have one question on this topic:

When running a RS232 data cable, must I use a 3-wire shielded cable (TxD, RxD and ground + shielding) or can I just run a 2-wire shielded cable (TxD, RxD and shield) ? I wonder if the shield alone can be used as a signal ground connection or do I need to have a dedicated ground wire inside the cable and use the shield with it, tying both together ?
Click to expand...

You want a dedicated shield. The signal ground is carrying the return currents for the data signals. RS232 is not a transformer coupled differential interface like 10/100Base-T Ethernet. If you use twisted pair, twist each signal with a ground, i.e. TxD/gnd, RxD/gnd with overall shield terminated at both ends. Otherwise, TxD, RxD, gnd, overall shield terminated at both ends.
 
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