On Feb 28, 6:42 pm, wrote:
I have an electrical/audio question. The intercom I'm installing has a
wiring diagram that shows the sleeve terminal of the phones and mic
connected to the shield at the radio end. This seems pretty typical.
My question: is it okay practice to use the wiring shield as one of
the conductors?

Yes. The fact that the return current from the phones is flowing down
the
shield does not adversely affect the electrostatic function
of the shield at all. If it did, the manufacturer of the phones
wouldn't do
it this way either! The return current in the shield and the outgoing
current in the center create cancelling fields; no external field is
generated. Nor is any external field able to create a voltage
difference
inside the shielded cable by reciprocity.

In other words, can I use 2-conductor shielded wire

Yes.

for stereo headphones (left, right, and shield for ground) or should I
buy 3-conductor so the sleeve terminal has its own wire inside the
shield? I ask because both the sleeve and the shield get tied to
ground at the radio end anyway.

3 wire not necessary, see above.


Also, should I use a separate wire for the mic audio, or can all of
the conductors run inside the same shielded wire using a common
ground?

Do NOT run the phone line in the same lines as the mic. Some
intercoms will oscillate if you do this. It is usually OK to run the
keyline in the same shield as the mic line, so have lots of the
2 conductor.

You can run both channels of the phones in a single shielded wire;
you can run the mic and key in the same shielded wire. But not all.

What is very important is to isolate the mic and phone jacks
where they mount to the airframe. Use shoulder insulating washers,
available from spruce. What WILL destroy the effect of the shield is
to ground the far end to something like the airframe. Creates a
ground
loop and alternator currents (which have an AC component) flowing
to things like the nav lights thru the airframe will have a small
drop
and flow in the shield of your
mic line which you absolutely do not want. You avoid this by
insulating
the jacks so no ground current from any source can flow into them.

Oh, and use the correct aircraft wire, not rat shack. There's lots
of
2 conductor 27500 out there.

The manufacturer's diagram will probably reflect all this.

Bill Hale EE A&P

Thanks,

Craig.