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Common Grounding

 
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rtitsworth(at)mindspring.
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PostPosted: Wed Jan 11, 2006 8:21 am    Post subject: Common Grounding Reply with quote

Ok, here comes perhaps a silly/stupid question (flamesuit in place)...

I'm building a composite airplane with AFT batteries (Lancair ES).

This would normally result in two large wires/cables running from the aft
batteries/contactors up to the firewall/engine (alt and starter) - one live
(+) and one ground (-).

If it were a metal aircraft the airframe itself would/could provide the
return ground circuit - hence only 1 "wire".

Q: Why not incorporate/embed a "solid" electrical conductor (ground) which
runs the length of the fuselage. For example, this could be an
appropriately sized aluminum bar/rod/strap. It could be properly
fabricated/shaped and molded/bonded to the fuselage to reduce/remove
vibrational stresses/hardening. The ends could be fabricated/welded to
interface/bolt directly with the contactors (aft) and ground buss (fwd) to
minimize connections. This might save a few lbs, $, and wire ties (andel
clamps). If nothing else, it would/could be a very "clean" looking
installation with no added risk/cost/weight/etc.

If properly designed/routed, it could perhaps also be used as the ground for a few other (not ground loop sensitive) devices/circuits which might lie
along its path (flap motor, boost pump, seat heaters, etc). Thus,
economizing on some ground wiring.

Rsk/concern: failure of the insulation on the +wire could result in a short
if contact is made - same risk as with a metal aircraft. Routing proximity
could reduce this risk and/or the rod/bar/strap could be covered with a thin
layer of glass/epoxy.

Risk/concern: durability/corrosion - My 30yr old aluminum Cessna still looks brand new under the inspection panels. Epoxy/glass and/or
paint/zinc-chromate to environmentally protect it.

Risk/concern: serviceability/inspection - 1-2 bid E-glass and epoxy is
nearly transparent. I could see it's condition (something impossible inside
wire insulation)

Risk/concern: if it gets too hot/melts it might damage the fuselage due to
the proximity (bolded in). Must be properly sized. Best if it is routed
away from sensitive stuff if possible (wing spar, fuel lines, etc). Similar
risk with cable (at the extreme), but the wire/cable insulation provides
some teat transfer resistance/insulation. .

Insanity continued.

If it works for the ground, why not do the same with the main live (+)
wire/cable. Since I'm early in the build process, this "big" electrical
conduit (wire/cable) doesn't need to be "flexible". I can shape it and
build the plane around it.

Thoughts? (Commence firing)!

Rick


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nuckollsr(at)cox.net
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PostPosted: Wed Jan 11, 2006 10:45 am    Post subject: Common Grounding Reply with quote


At 11:19 AM 1/11/2006 -0500, you wrote:
Quote:

<rtitsworth(at)mindspring.com>
Bob, etal,
Ok, here comes perhaps a silly/stupid question (flamesuit in place)...
I'm building a composite airplane with AFT batteries (Lancair ES).
This would normally result in two large wires/cables running from the aft
batteries/contactors up to the firewall/engine (alt and starter) - one live
(+) and one ground (-).
If it were a metal aircraft the airframe itself would/could provide the
return ground circuit - hence only 1 "wire".
Q: Why not incorporate/embed a "solid" electrical conductor (ground) which
runs the length of the fuselage. For example, this could be an
appropriately sized aluminum bar/rod/strap. It could be properly
fabricated/shaped and molded/bonded to the fuselage to reduce/remove
vibrational stresses/hardening. The ends could be fabricated/welded to
interface/bolt directly with the contactors (aft) and ground buss (fwd) to
minimize connections. This might save a few lbs, $, and wire ties (andel
clamps). If nothing else, it would/could be a very "clean" looking
installation with no added risk/cost/weight/etc.

Depends on how much time you want to spend on it. 2AWG copper is
4 oz per foot. Assuming 8' of cable from battery to firewall
gives you 2# of installed weight.
2AWG is about .25" diameter or .05 square-in of cross section.
Alum is 63% conductivity of copper so you'll need about
0.08 square-in of alum. A piece of 0.050" x 1.6" wide fills
the bill. 96" x .05 x 1.6 is 7.7 cu" of aluminum. 0.1 pound
per cubic inch is about 0.8 pounds.
Expected weight savings is about 1.2 pounds. You could bring
the strap up the inside of the firewall and clamp it up to
the firewall ground bus. If you used copper strap, you could
solder it to the ground strap for a really good joint.
Quote:
Risk/concern: failure of the insulation on the +wire could result in a short
if contact is made - same risk as with a metal aircraft. Routing proximity
could reduce this risk and/or the rod/bar/strap could be covered with a thin
layer of glass/epoxy.

Not significant . . .
Quote:

Risk/concern: durability/corrosion - My 30yr old aluminum Cessna still looks
brand new under the inspection panels. Epoxy/glass and/or
paint/zinc-chromate to environmentally protect it.

Corrosion of the surface of any conductor is not a big
driver for electrical performance. All the important stuff
happens in the gas-tight joints between various components.
Quote:

Risk/concern: serviceability/inspection - 1-2 bid E-glass and epoxy is
nearly transparent. I could see it's condition (something impossible inside
wire insulation)
Risk/concern: if it gets too hot/melts it might damage the fuselage due to
the proximity (bolded in). Must be properly sized. Best if it is routed
away from sensitive stuff if possible (wing spar, fuel lines, etc). Similar
risk with cable (at the extreme), but the wire/cable insulation provides
some teat transfer resistance/insulation. .

It's in LESS danger of overheating and melting than a 2AWG
wire. Wire has less surface area and rejects heat poorly
compared to a flat strap.
Quote:
If it works for the ground, why not do the same with the main live (+)
wire/cable. Since I'm early in the build process, this "big" electrical
conduit (wire/cable) doesn't need to be "flexible". I can shape it and
build the plane around it.

It's been done. I've had canard pusher builders lay parallel
straps up under a layer of glass in the fuselage. Most of them
used copper for integrity of soldered joints at the ends.
Bob . . .


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Mark Phillips in TN



Joined: 10 Jan 2006
Posts: 431
Location: Columbia, TN

PostPosted: Wed Jan 11, 2006 5:04 pm    Post subject: Common Grounding Reply with quote


In a message dated 01/11/2006 10:28:27 AM Central Standard Time,
rtitsworth(at)mindspring.com writes:
Thoughts? (Commence firing)!
Quote:
>>
Considering all the Risk/concerns you seem to have pretty carefully

considered, seems going conventional just might be preferable to a more "interesting"
approach. Is "interesting and different" the attraction? Don't get me wrong,
I admire deviant behavior (see my plane!) but there is always a point where
cost/benefit/risk/cool factor intersect.
Also don't think you'd save much weight- possibly add more just to implement-
Maybe not relevant, but you asked...
Mark Phillips RV-6A FWIW- my batts up front and airframe is AL so what do I
know?
do not archive


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PostPosted: Thu Jan 12, 2006 7:52 am    Post subject: common grounding Reply with quote

hi all,
did i miss the answer to the original question because i am also interested...''will a 300 mm run of #10 wire to the forest of tabs from the grounded firewall create a groundloop? thanks all,
bob noffs


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Dan.Beadle(at)hq.inclines
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PostPosted: Thu Jan 12, 2006 8:20 am    Post subject: common grounding Reply with quote

It is important to keep high loads, especially noisy loads, like motors,
away from the forest of tabs. The #10 wire has some small resistance;
when you put a noisy high current through it, it will spill over to the
other equipment. (the high current would cause the tabs to momentarily
rise above ground)

I would be comfortable with a 1 foot run of #10 to forest of tabs which
were used to ground avionics and instruments.

Most noisy loads like motors should ground back at the engine mount.
Alternatively, you could use a separate remote ground point for all the
noisy returns in the cockpit, and ground them back through an
appropriately sized wire to the main ground point.

Hope this helps.

Dan

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Eric M. Jones



Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

PostPosted: Thu Jan 12, 2006 4:57 pm    Post subject: Re: Common Grounding Reply with quote

Check: http://www.periheliondesign.com/fatwires.htm

This is Copper Clad Aluminum Fatwire and it may solve your problem.

Eric M. Jones


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rtitsworth(at)mindspring.
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PostPosted: Thu Jan 12, 2006 6:24 pm    Post subject: Common Grounding Reply with quote

Eric,
Thanks. I am familiar with the product referenced.

However, assuming I don't need flexibility/formability, does this offer any
advantage over a long aluminum bar? Seems aluminum would be slightly
lighter, cheaper, dissipate heat better, easier to secure (bonded in),
easier to terminate (aka a hole), and isn't toxic if it ever burns. Better
in most/all aspects?

Rick

--


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Eric M. Jones



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Location: Massachusetts

PostPosted: Fri Jan 13, 2006 7:23 am    Post subject: Re: Common Grounding Reply with quote

Rick

Snip...>does this offer any advantage over a long aluminum bar? Seems aluminum would be slightly lighter, cheaper....

The electric power company uses aluminum drops to your house, but for the grounding they use copper/zinc/etc. This is because aluminum is fine for higher voltages with the proper joints and anti-oxide goops, but to guarantee the ground won't float a couple volts is impossible with aluminum connections, and a real mess with low voltage, high current connections. The right way to do it would be to use aluminum bar/strap/whatever and a welded on aluminum-copper transition endpiece. (Google "aluminum copper transition"). Alternatively, copper or copper-clad-aluminum is your best bet.

If you use Perihelion Design's Super-CCA for ground, you can strip off the insulation to save a bit more.

If you choose to use plain aluminum...expect mysterious ground problems and the retrofit of a real ground at your first annual....
Eric M. Jones


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PostPosted: Fri Jan 13, 2006 7:45 am    Post subject: Common Grounding Reply with quote

Eric M. Jones wrote:

Quote:


Rick

Snip...>does this offer any advantage over a long aluminum bar? Seems aluminum would be slightly lighter, cheaper....

The electric power company uses aluminum drops to your house, but for the grounding they use copper/zinc/etc. This is because aluminum is fine for higher voltages with the proper joints and anti-oxide goops, but to guarantee the ground won't float a couple volts is impossible with aluminum connections, and a real mess with low voltage, high current connections. The right way to do it would be to use aluminum bar/strap/whatever and a welded on aluminum-copper transition endpiece. (Google "aluminum copper transition"). Alternatively, copper or copper-clad-aluminum is your best bet.

If you use Perihelion Design's Super-CCA for ground, you can strip off the insulation to save a bit more.

If you choose to use plain aluminum...expect mysterious ground problems and the retrofit of a real ground at your first annual....
Eric M. Jones


I've often wondered if the 'aluminum brazing' products hyped at flyins &
available at welding shops could be used to transition from AL bar stock
to a copper tab for mechanical connections. They are supposed to work on
'most non-ferrous metals'. This might accomplish Rick's goal of light,
cheap, simple, safe. Is that worth exploring?

Charlie


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nuckollsr(at)cox.net
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PostPosted: Fri Jan 13, 2006 4:03 pm    Post subject: Common Grounding Reply with quote

At 07:23 AM 1/13/2006 -0800, you wrote:

Quote:


Rick

Snip...>does this offer any advantage over a long aluminum bar? Seems
aluminum would be slightly lighter, cheaper....

The electric power company uses aluminum drops to your house, but for the
grounding they use copper/zinc/etc. This is because aluminum is fine for
higher voltages with the proper joints and anti-oxide goops, but to
guarantee the ground won't float a couple volts is impossible with
aluminum connections, and a real mess with low voltage, high current
connections. The right way to do it would be to use aluminum
bar/strap/whatever and a welded on aluminum-copper transition endpiece.
(Google "aluminum copper transition"). Alternatively, copper or
copper-clad-aluminum is your best bet.

If you use Perihelion Design's Super-CCA for ground, you can strip off the
insulation to save a bit more.

If you choose to use plain aluminum...expect mysterious ground problems
and the retrofit of a real ground at your first annual....

We need to make a distinction between aluminum strip having
the necessary cross section to replace an equal length of other
wire and the classic aluminum wire used by the power distribution
industry.

A strip of alclad aluminum is no more difficult to work with than
to simply use your aluminum airframe as a ground system. We use
airframe grounds all over the airplanes at RAC and this is not
an automatic recipe for problems. It's really coarse/soft
aluminum in the old wire that make it difficult to achieve
gas tight joints and the coarse stranding is particularly vulnerable
to vibration issues.

The fine stranding of Eric's copper-clad wires combined with the
solder and crimp friendly copper surface alleviates all the issues
with the older wire but it doesn't preclude consideration of
a continuous, flat aluminum strip especially when they can be covered
up with glass and epoxy.

Bob . . .


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Eric M. Jones



Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

PostPosted: Fri Jan 13, 2006 8:38 pm    Post subject: Re: Common Grounding Reply with quote

Rick said: Snip...>>does this offer any advantage over a long aluminum bar? Seems
aluminum would be slightly lighter, cheaper....

Eric Said: >>The electric power company uses aluminum drops to your house, but for the grounding they use copper/zinc/etc. This is because aluminum is fine for higher voltages with the proper joints and anti-oxide goops, but to guarantee the ground won't float a couple volts is impossible with aluminum connections, and a real mess with low voltage, high current connections. The right way to do it would be to use aluminum
bar/strap/whatever and a welded on aluminum-copper transition endpiece. (Google "aluminum copper transition"). Alternatively, copper or
copper-clad-aluminum is your best bet.

If you choose to use plain aluminum...expect mysterious ground problems and the retrofit of a real ground at your first annual....

Bob N, Said>>
We need to make a distinction between aluminum strip having
the necessary cross section to replace an equal length of other
wire and the classic aluminum wire used by the power distribution
industry.

A strip of alclad aluminum is no more difficult to work with than
to simply use your aluminum airframe as a ground system. We use
airframe grounds all over the airplanes at RAC and this is not
an automatic recipe for problems. It's really coarse/soft
aluminum in the old wire that make it difficult to achieve
gas tight joints and the coarse stranding is particularly vulnerable
to vibration issues.

Eric Says: Bob, It's true that metal aircraft use the skin as a conductor, for lighting and antennas, but I would guess where grounding is a big concern, like sensors and complex electronics a copper ground is added. That's also why remote industrial sensors are usually current loops, not voltage levels.

The forest of tabs is such a good idea...single point grounds.

Earlier Charlie Asked: >>I've often wondered if the 'aluminum brazing' products hyped at flyins & available at welding shops could be used to transition from AL bar stock to a copper tab for mechanical connections. They are supposed to work on 'most non-ferrous metals'. This might accomplish Rick's goal of light, cheap, simple, safe. Is that worth exploring?

Eric Says: The melting points of aluminum and copper are so different that successfully joining the two is not a matter for the amateur. I'm guessing that the required tools are generally outside the range of homebuilders.


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PostPosted: Sat Jan 14, 2006 7:26 am    Post subject: Common Grounding Reply with quote

On Fri, 13 Jan 2006 20:38:25 -0800
"Eric M. Jones" <emjones(at)charter.net> wrote:

<snip>
Quote:

Earlier Charlie Asked: >>I've often wondered if the 'aluminum brazing' products hyped at flyins & available at welding shops could be used to transition from AL bar stock to a copper tab for mechanical connections. They are supposed to work on 'most non-ferrous metals'. This might accomplish Rick's goal of light, cheap, simple, safe. Is that worth exploring?

Eric Says: The melting points of aluminum and copper are so different that successfully joining the two is not a matter for the amateur. I'm guessing that the required tools are generally outside the range of homebuilders.

--------
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net



Hi Eric,

This stuff doesn't operate at the melting point. Brazing is a
soldering process rather than welding. I've made a couple of test
joinings of aluminum to aluminum but never actually used it for
anything yet. (It seemed like I would use it more when I saw it at a
trade show.) It takes a little practice, but isn't that hard to do. I
think Charlie's suggestion would be worth a try.

Bob W.
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Custom Cables for your rotary installation -
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PostPosted: Sat Jan 14, 2006 8:40 am    Post subject: Common Grounding Reply with quote

Quote:

If you choose to use plain aluminum...expect mysterious ground problems
and the retrofit of a real ground at your first annual....

Bob N, Said>>
We need to make a distinction between aluminum strip having
the necessary cross section to replace an equal length of other
wire and the classic aluminum wire used by the power distribution
industry.

A strip of alclad aluminum is no more difficult to work with than
to simply use your aluminum airframe as a ground system. We use
airframe grounds all over the airplanes at RAC and this is not
an automatic recipe for problems. It's really coarse/soft
aluminum in the old wire that make it difficult to achieve
gas tight joints and the coarse stranding is particularly vulnerable
to vibration issues.

Eric Says: Bob, It's true that metal aircraft use the skin as a conductor,
for lighting and antennas, but I would guess where grounding is a big
concern, like sensors and complex electronics a copper ground is added.
That's also why remote industrial sensors are usually current loops, not
voltage levels.

The forest of tabs is such a good idea...single point grounds.

We're mixing apples, and grapes here. Yes, SENSORS that generate small
signals are NEVER locally grounded. They'll wave around on the end of a
shielded pair or trio and all outbound power, control and inbound signals
and grounds are carried in the wire. This is irrespective of the aircraft's
structural materials.

The idea of using an embedded strip in a glass/epoxy airplane to carry
(+) and (-) the length of the airplane in a canard pusher or between
firewall
and aft battery in a tractor airplane is no different electrically than
using
wires of any kind. The technology concerns are only for crafting long
lived, gas tight,
low resistance joints at the ends. Again, this matters not what the
conductor
material is, the requirements are the same so methodology has to be adjusted
to accommodate characteristics of the materials chosen. In the case of
coarse stranded, soft aluminum favored by the power distribution industry,
folks were unwilling or unable to develop satisfactory termination
techniques
so the brief terms of duty at Piper (and briefer at Cessna) were abandoned.
Availability of fine aluminum strands that behave pretty much like copper
in the joint makes the flexibility, vibration resistance and joining
technology issues pretty much go away. I believe we're using copper-clad
aluminum in Premier and Horizon. I need to check that.

None of the above has anything to do with architecture of the ground
system. Appliances other than the small-signal devices described above
are quite often grounded locally in metallic airframes. This picture . . .

http://www.aeroelectric.com/Pictures/Grounding/MVC-699X.JPG

. . . is but one of many examples of airframe grounding found on all of our
aluminum aircraft at RAC/Beech.

Quote:

Earlier Charlie Asked: >>I've often wondered if the 'aluminum brazing'
products hyped at flyins & available at welding shops could be used to
transition from AL bar stock to a copper tab for mechanical connections.
They are supposed to work on 'most non-ferrous metals'. This might
accomplish Rick's goal of light, cheap, simple, safe. Is that worth exploring?

Eric Says: The melting points of aluminum and copper are so different that
successfully joining the two is not a matter for the amateur. I'm guessing
that the required tools are generally outside the range of homebuilders.

The "brazing" rods I saw at OSH were a basically a soft solder
technique. Joining happens by alloying the surface of joined materials
with the thin interface between material and the liquid solder. IF
these same materials will alloy with copper too, then they're
obviously suited to joining of copper and aluminum. Certainly the
joints would be gas tight! That's what solder does best.

I used to have some of this stuff, I'll see if I can find it
and do some experiments in the shop. If anyone else has some,
we'd be pleased to hear the results of your experiments
too. But in any case, effective use of these joining technologies is
not influenced by the melting temperatures of the materials to be
joined. One may join copper to steel with tin/lead solders at
temperatures much below the melting points of either copper or
steel.

See chapter 9 of the 'Connection

Bob . . .


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PostPosted: Sat Jan 14, 2006 9:16 am    Post subject: common grounding Reply with quote

At 08:16 AM 1/12/2006 -0800, you wrote:

Quote:

<Dan.Beadle(at)hq.InclineSoftworks.com>

It is important to keep high loads, especially noisy loads, like motors,
away from the forest of tabs. The #10 wire has some small resistance;
when you put a noisy high current through it, it will spill over to the
other equipment. (the high current would cause the tabs to momentarily
rise above ground)


Quote:
I would be comfortable with a 1 foot run of #10 to forest of tabs which
were used to ground avionics and instruments.

Most noisy loads like motors should ground back at the engine mount.
Alternatively, you could use a separate remote ground point for all the
noisy returns in the cockpit, and ground them back through an
appropriately sized wire to the main ground point.

Not so. The forest of tabs ground block is not a potential
ingress point for noise into other systems that share the
ground. This is what SINGLE POINT GROUNDING is all about.
EVERY grounded article shares the same reference.

Standing in my front yard talking, we would be unaware
of our earth rotation velocity on the order of
1000 mph, our earth orbit velocity on the order of
70,000 mph or the universe expansion velocity of who knows
how much . . . because we're standing on the same hunk
of turf that carries us along together.

Single point electrical system grounds function in the
same manner. It's not essential that the ground system
not move electrically, it IS essential that all systems
sharing the ground be unaware of the movement . . . i.e.
no noise coupled from one system to another in spite of
a great deal of noise being present.

Bob . . .


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PostPosted: Sat Jan 14, 2006 9:29 am    Post subject: common grounding Reply with quote

At 09:48 AM 1/12/2006 -0600, you wrote:

[quote]
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Dan.Beadle(at)hq.inclines
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PostPosted: Sat Jan 14, 2006 12:02 pm    Post subject: common grounding Reply with quote

I guess I disagree with Bob a little on this.

As an analog engineer, I have designed a lot of low-signal systems. The
key to good analog performance is keeping noise out of the analog
system. The way we do that is actually to split the grounds - a quiet
ground where it matters and a noisy (digital) ground where it doesn't.
The key is to keep high currents out of the analog ground, where a high
current will actually create a voltage drop from one end of the ground
to the other. The digital and analog grounds are considered separate
sub-systems which are then tied together at a common grounding point.
So Bob and I are saying the same thing, but different things....

Extending this to airplanes, I plane to keep all my avionics and
instrumentation on a separate forest of tabs. That ground will tie back
to a common point at the battery main lead. Other grounds, like for
starter and servo motors will tie not to the forest of tabs, but to the
main battery single point ground. High currents will not flow over the
1" or so from the forest of tabs to the battery single point. (I am sure
Bob agrees with this too; I doubt he proposes grounding the starter to
the single point forest of tabs.)

Any current in the 1" ground from the forest will cause a voltage rise
V=IR. Given, R is very low. But some analog signals out of sensors are
only 15mV, so any significant current over even a small R can cause
sensor errors. The avionics and instruments going to the field of tabs
will have few amps flowing, keeping the V=IR rise very low. And, other
than keying the transmitter, they will be very stable. I won't have to
deal with a high current motor feeding back and injecting noise into my
ground bus.

For what its worth...

Dan
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