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grosseair(at)comcast.net
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 Posted: Sat Jul 02, 2011 8:48 am Post subject: Ground loops |
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I think I understand about avoiding ground loops by providing only a
single path to ground, but I'm confused by the fact that the avionics
manufacturers (Garmin and PS Engineering in particular) tell you to
ground both ends of a shielded cable. Specifically, when going from a
mic or phone input to the audio panel or going from a com radio to the
audio panel all the wiring diagrams show the shield grounded at both
ends. Doesn't this create a ground loop?
Can someone please enlighten me.
John Grosse
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nuckolls.bob(at)aeroelect
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| Posted: Sat Jul 02, 2011 10:08 am Post subject: Ground loops |
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At 11:43 AM 7/2/2011, you wrote:
| Quote: |
I think I understand about avoiding ground loops by providing only a
single path to ground, but I'm confused by the fact that the
avionics manufacturers (Garmin and PS Engineering in particular)
tell you to ground both ends of a shielded cable. Specifically, when
going from a mic or phone input to the audio panel or going from a
com radio to the audio panel all the wiring diagrams show the shield
grounded at both ends. Doesn't this create a ground loop?
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If they are grounded to different locations . . . absolutely.
I am increasingly mystified by the instructions
from folks who should know better are mis-using shields over
wires . . . and especially 'grounding both ends' when the
shield is NOT being used as a signal path.
Here's an example of an audio system using NO shielded
wires. It also takes potential for ground loops into
account with note 2.
http://www.aeroelectric.com/PPS/Audio/2000014C.pdf
Here's a drawing I produced where shields ARE part
of the signal pathways. Note specific treatment
callouts for shields at both ends.
http://www.aeroelectric.com/PPS/Audio/DualCommAudio.pdf
Same thing here . . .
http://www.aeroelectric.com/PPS/Audio/hv1_760vhf.pdf
| Quote: | Can someone please enlighten me.
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If the shield over a wire is ALSO a signal path,
i.e. the 'ground' connection for a remotely mounted
mic or headset jack, then that shield goes to ground
at ONE END ONLY which will be the "lo" side of that
feature AT THE INTERCOM or RADIO's ground . . . which
in turn is taken to the instrument panel ground for
extension to aircraft ground as illustrated in Z-15.
You can have LOTS of grounds if they are strung out
in a planned, linear fashion. The "loop" happens when
a conductor operating at ground is tied to the airframe
in more than one place . . . when that structure
carries potentially antagonistic 'noise currents'.
Suggest you review
http://www.aeroelectric.com/articles/richter/response_1.pdf
http://www.aeroelectric.com/articles/richter/response_2.pdf
by searching on the word 'shield' in these two documents
and then review the context in each case.
The 'Richter Affair' was some years back but it seems
that many individuals have the same mis-conceptions
in common about shields. It's a sad day when the installers
instructions are not well thought out.
Bob . . .
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Sat Jul 02, 2011 10:14 am Post subject: Ground loops |
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At 11:43 AM 7/2/2011, you wrote:
| Quote: |
I think I understand about avoiding ground loops by providing only a
single path to ground, but I'm confused by the fact that the
avionics manufacturers (Garmin and PS Engineering in particular)
tell you to ground both ends of a shielded cable.
|
Can you point me to wiring diagrams on the 'net that
speak to this?
Bob . . .
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ceengland(at)bellsouth.ne
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| Posted: Sat Jul 02, 2011 10:15 am Post subject: Ground loops |
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On 07/02/2011 11:43 AM, John Grosse wrote:
| Quote: |
<grosseair(at)comcast.net>
I think I understand about avoiding ground loops by providing only a
single path to ground, but I'm confused by the fact that the avionics
manufacturers (Garmin and PS Engineering in particular) tell you to
ground both ends of a shielded cable. Specifically, when going from a
mic or phone input to the audio panel or going from a com radio to the
audio panel all the wiring diagrams show the shield grounded at both
ends. Doesn't this create a ground loop?
Can someone please enlighten me.
John Grosse
Hi John,
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One thing to remember is that for signals in an unbalanced circuit,
'ground' is usually referring to the return path to complete a circuit.
The return is usually at 'ground' potential, but in some circuits, it
might not be.
A ground 'loop' is actually having 2 different paths for the ground, as
you say. In most audio circuits, the shield is used for the return path.
With the mic circuit, the mic jack is almost always specified as
isolated from the airframe to avoid that 2nd return path.
The radio to audio panel issue is a bit harder to reconcile with the
'rule', but signal levels are higher there & the shield path is
typically so short that secondary return paths don't seem to be as much
of an issue. If you allow one end of the shield to float, and let the
return find its own path through the airframe, I'd bet that you'd be
creating the kind of problems that the 'rule' is trying to prevent.
That's not a very satisfying answer, I know. Maybe someone else can give
a more technically specific answer.
Charlie
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grosseair(at)comcast.net
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| Posted: Sat Jul 02, 2011 12:17 pm Post subject: Ground loops |
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Thanks for the responses.
I misspoke about PS Engineering. They ground the shield at one end, but
here's the diagram for a Garmin SL40: You'll notice that they
specifically say to ground both ends of the shield to the case.
John
| Quote: | ------------------------------------------------------------------------
Robert L. Nuckolls, III <mailto:nuckolls.bob(at)aeroelectric.com>
July 2, 2011 1:11 PM
<nuckolls.bob(at)aeroelectric.com>
Can you point me to wiring diagrams on the 'net that
speak to this?
|
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nuckolls.bob(at)aeroelect
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| Posted: Sat Jul 02, 2011 2:35 pm Post subject: Ground loops |
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At 03:13 PM 7/2/2011, you wrote:
| Quote: | Thanks for the responses.
I misspoke about PS Engineering. They ground the shield at one end,
but here's the diagram for a Garmin SL40: You'll notice that they
specifically say to ground both ends of the shield to the case.
John
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Okay, I'll bet you a dollar to a donut that
the "audio grounds" connected with wires are internally
grounded to chassis on both devices. Adding a shield
over a twisted pair adds no benefit for two devices
located next to each other. In this case, adding shields
and grounding at both ends is only redundant to the
ground-to-ground wire . . . hence, no risk.
You could just as easily wire these boxes up with twisted
pairs and no shields.
<ceengland(at)bellsouth.net>
The radio to audio panel issue is a bit harder to reconcile with the
'rule', but signal levels are higher there & the shield path is
typically so short that secondary return paths don't seem to be as
much of an issue. If you allow one end of the shield to float, and
let the return find its own path through the airframe, I'd bet that
you'd be creating the kind of problems that the 'rule' is trying to prevent.
I think the potential is already there with the 'audio
grounds' If the radio were, say, in a rear cockpit and
the audio panel in front . . . and the battery grounds were
in the tail, then there is potential for ground loop coupling
whether or not a shield is present.
That's not a very satisfying answer, I know. Maybe someone else can
give a more technically specific answer.
Most general aviation systems make prolific use
of chassis ground for both power and signals within
any given device. This generally not a problem for
small aircraft . . . especially when we go to some
pains to avoid noisy circuits circulating on the airframe.
Military stuff uses chassis ground only for shielding
where LOCAL bonding to airframe is desirable. RFI filters
grounded to chassis bring signal and power wires
in to dedicated, floating grounds within the device.
This makes is MUCH easier to manage ground systems.
So I guess I'd have to continue to say there are no
hard 'rules' for handling shields because we seldom
are privy to wiring details inside the black boxes.
If the schematic John provided us was for some really
'advanced' equipment, those paired audio lines would
be true balanced pairs and only one "low side" would
be grounded internally to inside the source box. The load
end would be totally floating with a ton of common mode
rejection. Shielding would still be of no particular benefit.
All we can do is the best guess and then go after a
noise problem should it happen to pop up. Our best guess
is adequate probably 99% of the time.
What we can say for sure is that all the schematics
showing shields that are not part of a return path
could easily be replaced with a wire. Shielding has
essentially zero probability of added value in our
working environment.
Bob . . .
Bob . . .
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stearman456
Joined: 14 Aug 2010
Posts: 49
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| Posted: Wed Aug 29, 2012 3:07 pm Post subject: Re: Ground loops |
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In the example above with the SL-40 and the audio panel, would it be acceptable to go to pin 13 & 14 with a shielded double wire and ground the shielding per Note 1 at the radio rack, leaving the shielding at the audio panel end free? I'm about to do a stand alone SL-40 installation with no audio panel and the install drawing for it shows the shielding also grounds at the jack ends as well as per Note 1.
Dan
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nuckolls.bob(at)aeroelect
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| Posted: Wed Aug 29, 2012 3:16 pm Post subject: Ground loops |
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At 06:07 PM 8/29/2012, you wrote:
--> AeroElectric-List message posted by: "stearman456" <warbirds(at)shaw.ca>
In the example above with the SL-40 and the audio panel, would it be acceptable to go to pin 13 & 14 with a shielded double wire and ground the shielding per Note 1 at the radio rack, leaving the shielding at the audio panel end free? I'm about to do a stand alone SL-40 installation with no audio panel and the install drawing for it shows the shielding also grounds at the jack ends as well as per Note 1.
Dan
[img]cid:7.1.0.9.0.20120829181047.01e1cff8(at)aeroelectric.com.0[/img]
There is no value in connecting both ends unless
the shield is also a signal/power conductor. Since
this is not the case, connection at one end only is
sufficient to reap the benefits of shielding but
connection at both ends in this case is unlikely
to be troublesome.
Bob . . .
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stearman456
Joined: 14 Aug 2010
Posts: 49
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| Posted: Wed Aug 29, 2012 3:31 pm Post subject: Re: Ground loops |
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Thanks, Bob. I don't know where you get the time to answer all our questions but we sure appreciate it.
Dan
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