FWIW some of the things I've learned:
I think of the battery as the thing that dampens voltage fluctuations.
The alternator and varying loads make the voltage fluctuations,
especially the alternator that outputs DC but with a fair bit of voltage
ripple. Since the alternator ground current will travel through the
airframe to the battery then different points in the airframe will have
slightly different voltages and a corresponding amount of electrical
noise. Chances are you will have no difficulty but any remotely located
battery will have a higher chance of noise problems regardless of
whether an airframe ground is used as far as I know. All grounds going
to the battery (the thing that dampens voltage fluctuations) will help
but that is not practical with a remote battery.
Shielding the alternator leads will accomplish essentially nothing as
there isn't any significant radiated noise from this lead to shield. It
would be a waste of effort and money as I understand it. I think
Bingelis just gave an example of something that one of the SPAM CAN
manufacturers did at one time. Note that many expensive aircraft do not
shield this wire.
The second source of noise is just voltage generated by current flowing
across the resistance of a conductor. A heavy gauge of wire will help
reduce the voltage (noise) generated by the current since heavy wire has
less resistance and the voltage differences are proportional to the
resistance. I used 8awg even for my 40 amp alternator and I kept the
batteries on the firewall.
The third form of noise is magnetic coupling which is proportional to
the current flowing. Shielded wire won't help that either but two wires
twisted together or lying beside each other to and from the battery (or
to and from any device) will cancel out the magnetic effect generated.
The current going one way and its magnetic field is cancelled by the
opposite current and its magnetic field going in the other direction.
Thus a ground cable lying beside the positive cable from the battery can
eliminate the magnetic field and magnetically coupled noises. I believe
that is the main advantage of running a separate ground cable to the
battery. Similarly I did run my alternator feed beside the engine ground
cable but as it happens I nullified some of that advantage because my
redundant second engine ground strap is located elsewhere. A broken
ground cable would stop my Subaru so of course I have two ground cables ;)
Lastly I'd be amazed if there was any problem conducting starting
current through a metal rivetted airframe. Every rivet is going to be a
connection regardless of whether chromate is present or not. They do
recommend connecting to primary structure though to spread the current
over lots of rivets. An ohmeter measurement is going to be pretty much
meaningless as the resistances of concern for 200 amps of starter
current are down in the miliohm range. One hundreth of an ohm would drop
2 volts at 200 amps which would be a loss of at least 20% of your
starter power.
My 3 magnetic pickup wires are indeed all shielded mostly because that's
the way they were in the car. They can be conducting fairly small
signals (especially during cranking) and I don't want them to pick up
noise from elswhere especially since they are bundled with a lot of
other wires.
Ken
Kim Kimball wrote:
I also wondered about the epoxy chromate reducing or eliminating
conductivity between panels. So I measured the resistance between my
outer wing skin and inner wing skin prior to, and after adding a ground
strap between the panels. The resistance was very low, and unchanged
when adding the ground strap. Thanks for the responses about the
battery ground. I plan to jump to the floor stringer as was suggested.
I also have a ground bus on the firewall where I will connect all the
grounds for the stuff in the panel. That ground bus is connected to the
engine and I added a ground strap from the ground block to the airframe
since the stainless steel firewall is not the best conductor.
My other electrical question comes from looking at Tony Bingelis'
drawing again. He shows shielded wire being used from the alternator to
the main power bus, and also from the ignition switch to the magnetos.
However, he also shows the shield for those wires grounded on both ends.
I thought that was a no-no. I thought your were only supposed to
ground on one end. I plan to use shielded wires on my alternator cable
and the wires going to my ignition coils (MSD electronic ignition). I'm
using a single wire alternator so I don't have a field wire to worry
about. Does anyone know if the magnetic pickup wires to the distributer
need to be shielded? Does anyone know where one can by tubular wire
shielding in small quantities? Allied Electric only sells it in 100
foot rolls. I'd rather not have to spend $180 for six feet of
shielding. For that matter, does anyone know where one can buy shielded
8 AWG wire a bit cheaper than the almost $5 a foot price at Aircraft Spruce?
Mike
----- Original Message -----
From:
rognal@dcsol.com
Date: Friday, February 18, 2005 10:53 am
Subject: RE: Battery Cable question
John,
If Epoxy Zinc Chromate is used religiously between all metal
mating
surfaces and all rivets are dipped wouldn't this break the
continuity of
ground?
I have even less knowledge of electrics than you, but had the same
question.
I had chromated all the mating surfaces and dipped each rivet
also, and I
wanted to run local grounds for the Aeroflash strobe power
supplies mounted
on the wing tip ribs.
Using a multi-meter, I made several checks for continuity between
the main
spar and the tip rib and I always got a tone, indicating (to me) a
complete
circuit. The wiring is not completed so I can't tell you if it
will (or
won't) work yet.
Roger Hoffman #687R
Eugene, OR USA!
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