Shielded Wire?

[Tbone]

Say for instance that your strobe drive need a power & ground. Could you use shielded wire? Center for power and outer shield for ground?

[MCA]

Terry, Strobes generally run about 9 amps at 14v. Recommend you get 2-conductor shielded wire (wires 16 ga). The two wires carry the load, and the shield should be grounded on one end.

[CESSNADON]

IMHO and at the risk of igniting a posting war, I suggest that you wire the strobe power supply as the manufacturer suggests. A single conductor for the + supply and the - side attached to the airframe at the mounting location. There are thousands of certificated airplanes flying today wired as above and without any noise problems.

[n2prise]

Terry,

The DC power going TO the strobe power supply is not the problem at 12 volts. The cables going from the strobe power supply to the lights is the thing to be careful about. The voltage is much higher, and therefore the kits come with a shielded power cable for each light. The ground wires in those cables should all be grounded together, ONLY at the end of the cables on the power supply hold down screws or bolts. The other end of the bare ground wire in those cables is NOT grounded at the wing tips and tail. I did that and have never heard a tick on the intercom or radios. Like the man said, follow the instructions from the strobe manufacturer, they work just fine!


Notice the silver bare wires above are all soldered together with the black wire that comes toward the camera. The end of that black wire has a ground lug crimped and soldered to it and gets grounded to the power supply when it is bolted in place. My strobe power supply is attached to the baggage wall, down low, inside the tail cone area on the right side.

Keep those shielded high-voltage strobe cables away from the 12-volt wires that go to other lights, servo motors, etc. The gray cable is the high-voltage strobe cable. This plug between the fuselage and the wing strobe cable wiring passes the ground wire through to the cable in the wing.


In this photo, the wing is upside down on the work table. The white 4-pin plug at the top of the photo is the high-voltage strobe cable. The gray outer jacket is still there, just hidding inside the conduit in the wing. The 12-volt white wires for the landing light and wing tip marker light are at the bottom of the photo. Those two 12-volt lights are grounded to the wing spar and the last rib at the end of the wing. The strobe wire only has three connections at the wing tip to the strobe fixture only, not any metal ground point out there.


You can see in the wing tip how the marker light has one wire tied to the end rib to complete the 12-volt light bulb circuit. The gray strobe cable has only the three wires going into the plug on the strobe fixture. The silver ground wire is cut off at the end of the gray outer sheath with no connection to anything.

[az_gila]

Jerry... at our VHF frequencies, the length of the exposed grounding shield connections should be in the 1.5 inch range.

Your long grounding lead might actually act as a receiving antenna and defeat the shielding purpose.

A short wire to the mounting screw just under the connectors would probably be better shielding.


If you can't sleep one night, you can try reading this NASA document....
Page 61/62 defines the general requirement....

OOPS... forgot the link.... here it is...

http://www.hq.nasa.gov/office/codeq/doctree/87394.pdf

[Tbone]

I guess I shouldn't have used the strobe driver as an example. Say for instance you wanted to wire up a radio or other 12v unit and were to use shielded wire for the power and ground. Is there a problem with that? It seems that it would save some weight and time.

[az_gila]

Originally Posted by Tbone

I guess I shouldn't have used the strobe driver as an example. Say for instance you wanted to wire up a radio or other 12v unit and were to use shielded wire for the power and ground. Is there a problem with that? It seems that it would save some weight and time.

The shield should not be used as a conductor - and it is typically connected to a grounding surface at one end only.

So if you power something (a radio) you would still need two wires, as well as the shield. No weight or time savings.

If you want to lead all grounds back to a single source, then a twisted pair is the way to go - but, with the exception of the avionics - a local ground to the airframe is more typical and is a proven method for all metal aircraft.

[SteinAir]

Gil's pretty much spot on. Do NOT use a shield for a conductor - ground or otherwise- lots of bad juju.

Also, regarding local grounds - some things are fine (like nav lights and other "non noisy" items), but any electrically sensetive or noisy items should be run back to the single airframe ground.

My 2 cents as usual.

Cheers,
Stein

[Mike S]

Originally Posted by SteinAir

Do NOT use a shield for a conductor - ground or otherwise- lots of bad juju.

My 2 cents as usual.

Cheers,
Stein

PTT???

Or other similar such as remote ident, etc.???

[az_gila]

Originally Posted by Mike S

PTT???

Or other similar such as remote ident, etc.???

In general - a shield as a conductor, only if it's in the manufacturer's Installation Manual.

In the cases you list, a twisted pair would be a solution - unless the manufacturer says (or to quote Tim Allen "suggests") otherwise....

Also, the cases you list are usually logic signals and much less susceptible to noise.