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n76lima(at)mindspring.com
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 Posted: Mon Sep 24, 2012 7:19 am Post subject: Melted wire |
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| Quote: | From: "Hosler, John"
The OEM ran the ground wire to the airframe with a sheet metal screw???? John
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Yes, they did this at several locations. The instrument lights and compass light are grounded to the honeycomb via a ring lug on the wiring and a sheet metal screw.
The Magneto Ground at the ignition switch is also connected to the honeycomb via the same method.
--Bob Steward
Birmingham, AL
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teamgrumman(at)yahoo.com
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| Posted: Mon Sep 24, 2012 10:29 am Post subject: Melted wire |
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The wire in question, IS a direct short to ground. It's a ground wire through the Master switch. I can't see where it would draw enough current to get hot. Yet, when I cut the wire and inserted an ammeter in series, drawing just .65 to .7 amps, the wire got damn hot.
Don't you just love electrical problems?
From: flyv35b <flyv35b(at)minetfiber.com>
To: teamgrumman-list(at)matronics.com
Sent: Monday, September 24, 2012 6:35 AM
Subject: Re: TeamGrumman-List: Melted wire
--> TeamGrumman-List message posted by: flyv35b <flyv35b(at)minetfiber.com (flyv35b(at)minetfiber.com)>
On 9/23/2012 7:45 PM, n76lima(at)mindspring.com (n76lima(at)mindspring.com) wrote:
| Quote: | --> TeamGrumman-List message posted by: n76lima(at)mindspring.com (n76lima(at)mindspring.com)
[Gary] IF there is a significant corrosion in the areas where the plastic shield is, would there be enough resistance to make the wire heat more? I replaced the entire wire with #18 wire.
The problem is that the power is applied to the coil internally in the Master Relay, and then this wire is a GROUND. A #2 gauge jumper cable would not carry anymore current than the OEM wire, because the limiting resistance is at the SOURCE (power goes through the positive cable to the Master Relay, and then to the coil internally, and finally to a ground via the wire in question). One can't really lower the resistance of the circuit by fiddling with the ground wire. One can INCREASE the resistance, and thereby lessen the current, but no way to externally to the Master Relay LOWER the resistance. So "corrosion" on the wire would have no effect at increasing the current enough to heat it. I suppose if enough strands were corroded through that the current of ~0.8 amps would heat the wire locally, but the insulation would have to be off and the corrosion damage happen BEFORE the heating, and since the supposition is that the wire got hot FIRST, damaging the insulation, wh
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i!
| Quote: |
ch could then lead to the corrosion and the narrowing of the wire, the chain is completely backwards. And after the FIRST area was damaged (by whatever means), then no further damage could occur down stream, since the current would now be even MORE limited by the increased resistance at the first site damaged on the wire.
With several areas of damage, and wide spread lengths of heating, this is NOT a localized problem of the wire losing a few strands to corrosion.
Hence my suggestion that someone had replaced the Master Relay with a Starter Relay, or that the relay had some sort of internal fault that was shorting power to ground that did not go through the 15 ohm coil.
--Bob Steward
Bob, what's the current draw of the coil in a starter contactor? Would
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that even be enough to melt the insulation of an 18 ga wire? I assume
the wire was 18 ga. A single 18ga wire in free air wire should be able
to handle nearly 25 amp at conductor temp of around 150 C (100 C above
ambient). How would you get that kind of current flow without a direct
short to ground?
Cliff
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teamgrumman(at)yahoo.com
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| Posted: Mon Sep 24, 2012 10:38 am Post subject: Melted wire |
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No, they didn't. The ground from the Master goes to one of the screws that hold the instrument panel to the fuselage.
From: "Hosler, John" <JHOSLER(at)epri.com>
To: teamgrumman-list(at)matronics.com
Sent: Monday, September 24, 2012 8:11 AM
Subject: RE: Re: Melted wire
The OEM ran the ground wire to the airframe with a sheet metal screw????
John
From: owner-teamgrumman-list-server(at)matronics.com [mailto:owner-teamgrumman-list-server(at)matronics.com] On Behalf Of Rick Lindstrom
Sent: Monday, September 24, 2012 11:08 AM
To: teamgrumman-list(at)matronics.com
Subject: Re: Re: Melted wire
I'm guessing here, without a wiring diagram in front of me, but wouldn't a poor mechanical ground to the body of the starter solenoid result in all current going through the 18 ga. wire instead?
If so, that would overheat the wire for sure.
Rick
On Mon, Sep 24, 2012 at 7:50 AM, Bob Steward <n76lima(at)mindspring.com (n76lima(at)mindspring.com)> wrote:
--> TeamGrumman-List message posted by: Bob Steward <n76lima(at)mindspring.com (n76lima(at)mindspring.com)>
The coil in the Starter should have a resistance of ~2 ohms. So that means a current of ~6 amps (at) 12V.
Far below what the 18 ga. wire should safely carry.
The starter relay is usually wired differently than the Master. The Starter usually has the coil isolated from the power terminals, and is POWERED by the start button switch, rather than grounded.
It is also protected by the 10 amp fuse that powers the start/fuel pump/strobe circuit.
So the wire powering the Starter relay is not subject to a ground fault, since it would pop the 10 amp fuse before heating the wire.
The Master wiring might be subject a ground fault if some internal fault in the relay allowed a lower resistance in the relay to overload the ground wire.
--Bob Steward
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teamgrumman(at)yahoo.com
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| Posted: Mon Sep 24, 2012 10:39 am Post subject: Melted wire |
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The magneto ground should also be going to the instrument panel mount.
From: "n76lima(at)mindspring.com" <n76lima(at)mindspring.com>
To: teamgrumman-list(at)matronics.com
Sent: Monday, September 24, 2012 8:19 AM
Subject: RE: Re: Melted wire
--> TeamGrumman-List message posted by: n76lima(at)mindspring.com (n76lima(at)mindspring.com)
| Quote: | From: "Hosler, John"
The OEM ran the ground wire to the airframe with a sheet metal screw???? John
|
Yes, they did this at several locations. The instrument lights and compass light are grounded to the honeycomb via a ring lug on the wiring and a sheet metal screw.
The Magneto Ground at the ignition switch is also connected to the honeycomb via thtoshare, and much much ===
[quote][b]
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teamgrumman(at)yahoo.com
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| Posted: Mon Sep 24, 2012 10:40 am Post subject: Melted wire |
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on that subject, why is it (the magneto P-Lead shields) grounded at both ends?
From: "n76lima(at)mindspring.com" <n76lima(at)mindspring.com>
To: teamgrumman-list(at)matronics.com
Sent: Monday, September 24, 2012 8:19 AM
Subject: RE: TeamGrumman-List: Re: Melted wire
--> TeamGrumman-List message posted by: n76lima(at)mindspring.com (n76lima(at)mindspring.com)
| Quote: | From: "Hosler, John"
The OEM ran the ground wire to the airframe with a sheet metal screw???? John
|
Yes, they did this at several locations. The instrument lights and compass light are grounded to the honeycomb via a ring lug on the wiring and a sheet metal screw.
The Magneto Ground at the ignition switch is also connected to the honeycomb via the samhare, and much much ===
[quote][b]
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mel(at)becknet.com
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| Posted: Mon Sep 24, 2012 10:49 am Post subject: Melted wire |
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You never ground radio-frequency interference (RFI) shields at both ends. The objective of shielding is to absorb RFI, convert it to an electrical current, and drain the current to ground. If you ground the shielding at both end it won't work, because no current can be induced in a conductor entirely at ground potential. This is true for all RFI shielding, from ancient TV coax to the newest inter-building shielded Category-6 data cabling.
-Mel Beckman
On Sep 24, 2012, at 11:40 AM, Gary Vogt <teamgrumman(at)yahoo.com (teamgrumman(at)yahoo.com)> wrote:
[quote]on that subject, why is it (the magneto P-Lead shields) grounded at both ends?
From: "n76lima(at)mindspring.com (n76lima(at)mindspring.com)" <n76lima(at)mindspring.com (n76lima(at)mindspring.com)>
To: teamgrumman-list(at)matronics.com (teamgrumman-list(at)matronics.com)
Sent: Monday, September 24, 2012 8:19 AM
Subject: RE: TeamGrumman-List: Re: Melted wire
--> TeamGrumman-List message posted by: n76lima(at)mindspring.com (n76lima(at)mindspring.com)
| Quote: | From: "Hosler, John"
The OEM ran the ground wire to the airframe with a sheet metal screw???? John
|
Yes, they did this at several locations. The instrument lights and compass light are grounded to the honeycomb via a ring lug on the wiring and a sheet metal screw.
The Magneto Ground at the ignition switch is also connected to the honeycomb via the samhare, and much much ===
| Quote: |
===================================
t">http://www.matronics.com/Navigator?TeamGrumman-List
===================================
cs.com
===================================
matronics.com/contribution
===================================
|
[b]
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BARRY CHECK 6
Joined: 15 Mar 2011
Posts: 738
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| Posted: Mon Sep 24, 2012 11:18 am Post subject: Melted wire |
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Mel:
In response to your email... It depends on what you want to accomplish.
Example:
In an RF (Radio Frequency) transmitter where there is shielding involved you ground at both ends. An example of this would be a simple COAX cable going to an antenna. COAX is an unbalanced medium and by grounding at both ends (Radio & Antenna) you trap the signal and send it to ground via the shortest path.
In an AF (Audio Frequency) intercom where you want to shield a transmitted signal such as a RF transmission, you ground the shield (COAXIAL Shield around the twisted pair of audio lines) at one end. The end you ground is at the intercom, which is known as The Source <-- I know CRAZY since the source of the interference is the RF. I don't know why they depict it that way.
Think of it this way: In RF you don't want the signal to escape, you want it to say contained within the shield. In AF you want the shield to act as a fishing net and collect the signal and then send it to ground. And as you said: " ... no current can be induced in a conductor entirely at ground potential." <-- This is true in 99.9273% of cases. Where it gets foggy is when you are talking WHAT FREQUENCY of RF and HOW LONG A GROUND WIRE. And this stuff just starts to become a problem at our VHF frequencies and goes really crazy at Microwave frequencies.
Barry
On Mon, Sep 24, 2012 at 2:49 PM, Mel Beckman <mel(at)becknet.com (mel(at)becknet.com)> wrote:
[quote] You never ground radio-frequency interference (RFI) shields at both ends. The objective of shielding is to absorb RFI, convert it to an electrical current, and drain the current to ground. If you ground the shielding at both end it won't work, because no current can be induced in a conductor entirely at ground potential. This is true for all RFI shielding, from ancient TV coax to the newest inter-building shielded Category-6 data cabling.
-Mel Beckman
On Sep 24, 2012, at 11:40 AM, Gary Vogt <teamgrumman(at)yahoo.com (teamgrumman(at)yahoo.com)> wrote:
| Quote: | on that subject, why is it (the magneto P-Lead shields) grounded at both ends?
From: "n76lima(at)mindspring.com (n76lima(at)mindspring.com)" <n76lima(at)mindspring.com (n76lima(at)mindspring.com)>
To: teamgrumman-list(at)matronics.com (teamgrumman-list(at)matronics.com)
Sent: Monday, September 24, 2012 8:19 AM
Subject: RE: TeamGrumman-List: Re: Melted wire
--> TeamGrumman-List message posted by: n76lima(at)mindspring.com (n76lima(at)mindspring.com)
| Quote: | From: "Hosler, John"
The OEM ran the ground wire to the airframe with a sheet metal screw???? John
|
Yes, they did this at several locations. The instrument lights and compass light are grounded to the honeycomb via a ring lug on the wiring and a sheet metal screw.
The Magneto Ground at the ignition switch is also connected to the honeycomb via the samhare, and much much ===
|
| Quote: |
st" target="_blank">http://www.matronics.com/Navigator?TeamGrumman-List
tp://forums.matronics.com
_blank">http://www.matronics.com/contribution
|
[b]
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teamgrumman(at)yahoo.com
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| Posted: Mon Sep 24, 2012 11:24 am Post subject: Melted wire |
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Is the plane unairworthy if I remove the ground at one end? It would not longer meet type design.
From: Mel Beckman <mel(at)becknet.com>
To: "teamgrumman-list(at)matronics.com" <teamgrumman-list(at)matronics.com>
Sent: Monday, September 24, 2012 11:49 AM
Subject: Re: Re: Melted wire
You never ground radio-frequency interference (RFI) shields at both ends. The objective of shielding is to absorb RFI, convert it to an electrical current, and drain the current to ground. If you ground the shielding at both end it won't work, because no current can be induced in a conductor entirely at ground potential. This is true for all RFI shielding, from ancient TV coax to the newest inter-building shielded Category-6 data cabling.
-Mel Beckman
On Sep 24, 2012, at 11:40 AM, Gary Vogt <teamgrumman(at)yahoo.com (teamgrumman(at)yahoo.com)> wrote:
| Quote: | on that subject, why is it (the magneto P-Lead shields) grounded at both ends?
From: "n76lima(at)mindspring.com (n76lima(at)mindspring.com)" <n76lima(at)mindspring.com (n76lima(at)mindspring.com)>
To: teamgrumman-list(at)matronics.com (teamgrumman-list(at)matronics.com)
Sent: Monday, September 24, 2012 8:19 AM
Subject: RE: Re: Melted wire
--> TeamGrumman-List message posted by: n76lima(at)mindspring.com (n76lima(at)mindspring.com)
| Quote: | From: "Hosler, John"
The OEM ran the ground wire to the airframe with a sheet metal screw???? John
|
Yes, they did this at several locations. The instrument lights and compass light are grounded to the honeycomb via a ring lug on the wiring and a sheet metal screw.
The Magneto Ground at the ignition switch is also connected to the honeycomb via the samhare, and much much ===
| Quote: |
===================================
t">http://www.matronics.com/Navigator?TeamGrumman-List
==========
cs.com
==========
matronics.com/contribution
==========
|
|
[quote][b]
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N9681L(at)dicombox.net
Guest
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| Posted: Mon Sep 24, 2012 11:25 am Post subject: Melted wire |
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That's reversed. A conductor is not entirely at ground potential if there is a current induced in it. Only at the ground connection is it a zero potential. Everywhere else it differs by the product of the current and the conductor impedance. That is a good reason to ground it at both ends. I have seen shielded cables grounded every 6 inches to reduce the amount of energy re-radiated from the shields.
You ground at one end only to eliminate ground loops.
Curt
On 9/24/2012 11:49 AM, Mel Beckman wrote:
[quote] You never ground radio-frequency interference (RFI) shields at both ends. The objective of shielding is to absorb RFI, convert it to an electrical current, and drain the current to ground. If you ground the shielding at both end it won't work, because no current can be induced in a conductor entirely at ground potential. This is true for all RFI shielding, from ancient TV coax to the newest inter-building shielded Category-6 data cabling.Â
-Mel BeckmanÂ
On Sep 24, 2012, at 11:40 AM, Gary Vogt <teamgrumman(at)yahoo.com (teamgrumman(at)yahoo.com)> wrote:
[b]
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BARRY CHECK 6
Joined: 15 Mar 2011
Posts: 738
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| Posted: Mon Sep 24, 2012 11:38 am Post subject: Melted wire |
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Good question Gary...
But if it was supplied from the factory in working condition why would you have to remove a ground. It would be more like putting the item back into operational condition as supplied by the factory. Replace the wire back to original condition.
So in this case you would not be changing the Type Design.
Now, when it comes to the addition of new equipment such as a new GPS-Nav/Com or the classic case of Noise on the New Intercom [Problem usually being - Shield Wire Grounded AT the Headphone & Mic Jacks]... Then if you have a noise condition you do what is necessary to remove the noise. If that entails removing a ground, so be it.
Barry
On Mon, Sep 24, 2012 at 3:24 PM, Gary Vogt <teamgrumman(at)yahoo.com (teamgrumman(at)yahoo.com)> wrote:
[quote]Is the plane unairworthy if I remove the ground at one end? It would not longer meet type design.
From: Mel Beckman <mel(at)becknet.com (mel(at)becknet.com)>
To: "teamgrumman-list(at)matronics.com (teamgrumman-list(at)matronics.com)" <teamgrumman-list(at)matronics.com (teamgrumman-list(at)matronics.com)>
Sent: Monday, September 24, 2012 11:49 AM
Subject: Re: Re: Melted wire
You never ground radio-frequency interference (RFI) shields at both ends. The objective of shielding is to absorb RFI, convert it to an electrical current, and drain the current to ground. If you ground the shielding at both end it won't work, because no current can be induced in a conductor entirely at ground potential. This is true for all RFI shielding, from ancient TV coax to the newest inter-building shielded Category-6 data cabling.
-Mel Beckman
On Sep 24, 2012, at 11:40 AM, Gary Vogt <teamgrumman(at)yahoo.com (teamgrumman(at)yahoo.com)> wrote:
| Quote: | on that subject, why is it (the magneto P-Lead shields) grounded at both ends?
From: "n76lima(at)mindspring.com (n76lima(at)mindspring.com)" <n76lima(at)mindspring.com (n76lima(at)mindspring.com)>
To: teamgrumman-list(at)matronics.com (teamgrumman-list(at)matronics.com)
Sent: Monday, September 24, 2012 8:19 AM
Subject: RE: TeamGrumman-List: Re: Melted wire
--> TeamGrumman-List message posted by: n76lima(at)mindspring.com (n76lima(at)mindspring.com)
| Quote: | From: "Hosler, John"
The OEM ran the ground wire to the airframe with a sheet metal screw???? John
|
Yes, they did this at several locations. The instrument lights and compass light are grounded to the honeycomb via a ring lug on the wiring and a sheet metal screw.
The Magneto Ground at the ignition switch is also connected to the honeycomb via the samhare, and much much ===
|
| Quote: |
st" target="_blank">http://www.matronics.com/Navigator?TeamGrumman-List
tp://forums.matronics.com
_blank">http://www.matronics.com/contribution
|
[b]
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teamgrumman(at)yahoo.com
Guest
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| Posted: Mon Sep 24, 2012 11:39 am Post subject: Melted wire |
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Thanks Barry. That was very well said and even I understood it. I've always wondered why both ends were grounded.
From: FLYaDIVE <flyadive(at)gmail.com>
To: teamgrumman-list(at)matronics.com
Sent: Monday, September 24, 2012 12:18 PM
Subject: Re: Re: Melted wire
Mel:
In response to your email... It depends on what you want to accomplish.
Example:
In an RF (Radio Frequency) transmitter where there is shielding involved you ground at both ends. An example of this would be a simple COAX cable going to an antenna. COAX is an unbalanced medium and by grounding at both ends (Radio & Antenna) you trap the signal and send it to ground via the shortest path.
In an AF (Audio Frequency) intercom where you want to shield a transmitted signal such as a RF transmission, you ground the shield (COAXIAL Shield around the twisted pair of audio lines) at one end. The end you ground is at the intercom, which is known as The Source <-- I know CRAZY since the source of the interference is the RF. I don't know why they depict it that way.
Think of it this way: In RF you don't want the signal to escape, you want it to say contained within the shield. In AF you want the shield to act as a fishing net and collect the signal and then send it to ground. And as you said: " ... no current can be induced in a conductor entirely at ground potential." <-- This is true in 99.9273% of cases. Where it gets foggy is when you are talking WHAT FREQUENCY of RF and HOW LONG A GROUND WIRE. And this stuff just starts to become a problem at our VHF frequencies and goes really crazy at Microwave frequencies.
Barry
On Mon, Sep 24, 2012 at 2:49 PM, Mel Beckman <mel(at)becknet.com (mel(at)becknet.com)> wrote:
| Quote: | You never ground radio-frequency interference (RFI) shields at both ends. The objective of shielding is to absorb RFI, convert it to an electrical current, and drain the current to ground. If you ground the shielding at both end it won't work, because no current can be induced in a conductor entirely at ground potential. This is true for all RFI shielding, from ancient TV coax to the newest inter-building shielded Category-6 data cabling.
-Mel Beckman
On Sep 24, 2012, at 11:40 AM, Gary Vogt <teamgrumman(at)yahoo.com (teamgrumman(at)yahoo.com)> wrote:
| Quote: |
st" target="_blank">http://www.matronics.com/Navigator?TeamGrumman-List
tp://forums.matronics.com
_blank">http://www.matronics.com/contribution
|
|
[quote][b]
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n76lima(at)mindspring.com
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| Posted: Mon Sep 24, 2012 11:56 am Post subject: Melted wire |
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Guess it depends on the year model. There are certainly early 70's models that are grounding to the honeycomb just ahead of the instrument panel, by means of a sheet metal screw. And there are other places where they did that (eyebrow lights), so this isn't an isolated case.
I'd agree that a machine screw such as the instrument panel mount would probably be better.
--Bob Steward
| Quote: | No, they didn't. The ground from the Master goes to one of the screws that >hold the instrument panel to the fuselage. The magneto ground should also be >going to the instrument panel mount.
|
| Quote: | >"Hosler, John" <JHOSLER(at)epri.com>
>The OEM ran the ground wire to the airframe with a sheet metal screw????John
|
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mel(at)becknet.com
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| Posted: Mon Sep 24, 2012 12:33 pm Post subject: Melted wire |
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Gary,
I have to disagree. In GA applications, you do not ground RF antenna cables at both ends. If you review any avionics installation guide, you'll see that they invariably caution that only one end of the cable be grounded (the transmitter end), even if this means cutting the ground path at the antenna end (which I always do, because even so-called "insulated" antenna mounts often manage to conduct somehow).
The fact that COAX is unbalanced is the reason why you ground at only one end. Balanced conductors (such as twisted pairs) have intrinsic shielding due to an inverse of the original signal being imposed on one conductor, creating an evenly balanced EMF (hence the term "balanced"). It relies on a sum and difference principal, where sum and difference is the combining (summing) of two signals that are out of phase from each other. Whatever doesn’t cancel out is noise that can be rejected at the receiving end.
Coax, being unbalanced, requires a shield. To function at the frequencies present in GA aircraft, that shield has to be grounded at only one end. As an A&P, Ham radio operator, and electrical engineer, I've had to deal with many gnarly RFI problems. 99% of the time the cause is double-ended grounding.
The grounding practice for aircraft shielded cables is well specified as single-ended in almost all cases. The only exception would be RF emitters at well above 1 GHz, in which shielded antenna cables are grounded at both ends. These have limited application in aircraft (mostly radar systems), but for completeness I'll explain the difference.
Shielding effectiveness at low frequencies is a function of the impedance of the cable shield compared to the impedance of the shielded circuit. When grounded at both ends, not only does external RFI readily pass through the shield, the shield itself becomes an emitter as a result of any potential difference between the two ends of the cable -- the so-called "ground loop". One motive of grounding at one end is to break that loop to eliminate a shield-induced magnetic field coupling into the conductor. It doesn't make any difference if you're trying to contain the shielded cable's signal or protect it from outside RFI.
At high frequencies, the coupling mode is via an electric rather than magnetic field. As I mentioned earlier, we're talking here about frequencies well above 1 GHz (so transponders, operating right at 1GHz, aren't included). Voltage on the shield then couples to the circuit within the cable based upon the capacitance between the shield and the wire inside. In GA aircraft you can see this on remotely mounted radar antennas, although most manufacturers try to put the radio right behind the antenna to keep the cable length short. In this scenario, you want to minimize the voltage on the shield. Terminating the shield at both ends and at each point where the cable penetrates a bulkhead is the best practice here, but it's seldom done on GA aircraft, due to the short cable lengths. In large aircraft that's the recommended practice, though (see "Aircraft and Avionics Cabling E3 Shield Termination Recommendations, http://www.bmpcoe.org/library/books/navso%20p-3181/53.html).
In GA, at least until we start operating microwave transmitters, the general rule is ground at one end. Unless, of course the manufacturer says otherwise. 
-mel
On Sep 24, 2012, at 12:39 PM, Gary Vogt wrote:
| Quote: | Thanks Barry. That was very well said and even I understood it. I've always wondered why both ends were grounded.
From: FLYaDIVE <flyadive(at)gmail.com (flyadive(at)gmail.com)>
To: teamgrumman-list(at)matronics.com (teamgrumman-list(at)matronics.com)
Sent: Monday, September 24, 2012 12:18 PM
Subject: Re: Re: Melted wire
Mel:
In response to your email... It depends on what you want to accomplish.
Example:
In an RF (Radio Frequency) transmitter where there is shielding involved you ground at both ends. An example of this would be a simple COAX cable going to an antenna. COAX is an unbalanced medium and by grounding at both ends (Radio & Antenna) you trap the signal and send it to ground via the shortest path.
In an AF (Audio Frequency) intercom where you want to shield a transmitted signal such as a RF transmission, you ground the shield (COAXIAL Shield around the twisted pair of audio lines) at one end. The end you ground is at the intercom, which is known as The Source <-- I know CRAZY since the source of the interference is the RF. I don't know why they depict it that way.
Think of it this way: In RF you don't want the signal to escape, you want it to say contained within the shield. In AF you want the shield to act as a fishing net and collect the signal and then send it to ground. And as you said: " ... no current can be induced in a conductor entirely at ground potential." <-- This is true in 99.9273% of cases. Where it gets foggy is when you are talking WHAT FREQUENCY of RF and HOW LONG A GROUND WIRE. And this stuff just starts to become a problem at our VHF frequencies and goes really crazy at Microwave frequencies.
Barry
On Mon, Sep 24, 2012 at 2:49 PM, Mel Beckman <mel(at)becknet.com (mel(at)becknet.com)> wrote:
| Quote: | You never ground radio-frequency interference (RFI) shields at both ends. The objective of shielding is to absorb RFI, convert it to an electrical current, and drain the current to ground. If you ground the shielding at both end it won't work, because no current can be induced in a conductor entirely at ground potential. This is true for all RFI shielding, from ancient TV coax to the newest inter-building shielded Category-6 data cabling.
-Mel Beckman
On Sep 24, 2012, at 11:40 AM, Gary Vogt <teamgrumman(at)yahoo.com (teamgrumman(at)yahoo.com)> wrote:
| Quote: |
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beltz6
Joined: 31 May 2008
Posts: 10
Location: Goleta, CA
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| Posted: Mon Sep 24, 2012 6:27 pm Post subject: Re: Melted wire |
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Just for reassurance, I went out to the plane today, kicked on the master switch, and measured .67 amps running down the 5PA3 wire that Gary recently replaced. I used one of those wrap-around meters, which probably aren't that accurate, but it's about the same as what Gary measured in the shop with a real ammeter. And in the ballpark of what Bob mentioned it ought to be several posts ago (consistent with an internal resistance in the master relay of 15 ohms). If I understand correctly, that probably means the appropriate relay is installed (i.e., not a starter relay).
Also, no discernible heat in said wire - as expected for that amperage.
So the only thing I have to watch for now is if there is an -intermittent- short in the relay that only appears when I'm at 8500 feet, at night, not near any suitable landing sites :-/
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BARRY CHECK 6
Joined: 15 Mar 2011
Posts: 738
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| Posted: Mon Sep 24, 2012 11:15 pm Post subject: Melted wire |
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Beltz:
Which make and model meter did you use?
Barry
On Mon, Sep 24, 2012 at 10:27 PM, beltz6 <beltz6(at)yahoo.com (beltz6(at)yahoo.com)> wrote:
[quote]--> TeamGrumman-List message posted by: "beltz6" <beltz6(at)yahoo.com (beltz6(at)yahoo.com)>
Just for reassurance, I went out to the plane today, kicked on the master switch, and measured .67 amps running down the 5PA3 wire that Gary recently replaced. I used one of those wrap-around meters, which probably aren't that accurate, but it's about the same as what Gary measured in the shop with a real ammeter. And in the ballpark of what Bob mentioned it ought to be several posts ago (consistent with an internal resistance in the master relay of 15 ohms). If I understand correctly, that probably means the appropriate relay is installed (i.e., not a starter relay).
Also, no discernible heat in said wire - as expected for that amperage.
So the only thing I have to watch for now is if there is an -intermittent- short in the relay that only appears when I'm at 8500 feet, at night, not near any suitable landing sites :-/
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=383940#383940
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mel(at)becknet.com
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| Posted: Mon Sep 24, 2012 11:59 pm Post subject: Melted wire |
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Actually, in the case if an aviation radio, I got the source backwards. The source is the antenna, not the radio, so you would ground the coax shield at the antenna end. But you would still just ground one end.
-mel via cell
On Sep 24, 2012, at 1:33 PM, Mel Beckman <mel(at)becknet.com (mel(at)becknet.com)> wrote:
[quote]Gary,
I have to disagree. In GA applications, you do not ground RF antenna cables at both ends. If you review any avionics installation guide, you'll see that they invariably caution that only one end of the cable be grounded (the transmitter end), even if this means cutting the ground path at the antenna end (which I always do, because even so-called "insulated" antenna mounts often manage to conduct somehow).
The fact that COAX is unbalanced is the reason why you ground at only one end. Balanced conductors (such as twisted pairs) have intrinsic shielding due to an inverse of the original signal being imposed on one conductor, creating an evenly balanced EMF (hence the term "balanced"). It relies on a sum and difference principal, where sum and difference is the combining (summing) of two signals that are out of phase from each other. Whatever doesn’t cancel out is noise that can be rejected at the receiving end.
Coax, being unbalanced, requires a shield. To function at the frequencies present in GA aircraft, that shield has to be grounded at only one end. As an A&P, Ham radio operator, and electrical engineer, I've had to deal with many gnarly RFI problems. 99% of the time the cause is double-ended grounding.
The grounding practice for aircraft shielded cables is well specified as single-ended in almost all cases. The only exception would be RF emitters at well above 1 GHz, in which shielded antenna cables are grounded at both ends. These have limited application in aircraft (mostly radar systems), but for completeness I'll explain the difference.
Shielding effectiveness at low frequencies is a function of the impedance of the cable shield compared to the impedance of the shielded circuit. When grounded at both ends, not only does external RFI readily pass through the shield, the shield itself becomes an emitter as a result of any potential difference between the two ends of the cable -- the so-called "ground loop". One motive of grounding at one end is to break that loop to eliminate a shield-induced magnetic field coupling into the conductor. It doesn't make any difference if you're trying to contain the shielded cable's signal or protect it from outside RFI.
At high frequencies, the coupling mode is via an electric rather than magnetic field. As I mentioned earlier, we're talking here about frequencies well above 1 GHz (so transponders, operating right at 1GHz, aren't included). Voltage on the shield then couples to the circuit within the cable based upon the capacitance between the shield and the wire inside. In GA aircraft you can see this on remotely mounted radar antennas, although most manufacturers try to put the radio right behind the antenna to keep the cable length short. In this scenario, you want to minimize the voltage on the shield. Terminating the shield at both ends and at each point where the cable penetrates a bulkhead is the best practice here, but it's seldom done on GA aircraft, due to the short cable lengths. In large aircraft that's the recommended practice, though (see "Aircraft and Avionics Cabling E3 Shield Termination Recommendations, http://www.bmpcoe.org/library/books/navso%20p-3181/53.html).
In GA, at least until we start operating microwave transmitters, the general rule is ground at one end. Unless, of course the manufacturer says otherwise.
-mel
On Sep 24, 2012, at 12:39 PM, Gary Vogt wrote:
| Quote: | Thanks Barry. That was very well said and even I understood it. I've always wondered why both ends were grounded.
From: FLYaDIVE <flyadive(at)gmail.com (flyadive(at)gmail.com)>
To: teamgrumman-list(at)matronics.com (teamgrumman-list(at)matronics.com)
Sent: Monday, September 24, 2012 12:18 PM
Subject: Re: TeamGrumman-List: Re: Melted wire
Mel:
In response to your email... It depends on what you want to accomplish.
Example:
In an RF (Radio Frequency) transmitter where there is shielding involved you ground at both ends. An example of this would be a simple COAX cable going to an antenna. COAX is an unbalanced medium and by grounding at both ends (Radio & Antenna) you trap the signal and send it to ground via the shortest path.
In an AF (Audio Frequency) intercom where you want to shield a transmitted signal such as a RF transmission, you ground the shield (COAXIAL Shield around the twisted pair of audio lines) at one end. The end you ground is at the intercom, which is known as The Source <-- I know CRAZY since the source of the interference is the RF. I don't know why they depict it that way.
Think of it this way: In RF you don't want the signal to escape, you want it to say contained within the shield. In AF you want the shield to act as a fishing net and collect the signal and then send it to ground. And as you said: " ... no current can be induced in a conductor entirely at ground potential." <-- This is true in 99.9273% of cases. Where it gets foggy is when you are talking WHAT FREQUENCY of RF and HOW LONG A GROUND WIRE. And this stuff just starts to become a problem at our VHF frequencies and goes really crazy at Microwave frequencies.
Barry
On Mon, Sep 24, 2012 at 2:49 PM, Mel Beckman <mel(at)becknet.com (mel(at)becknet.com)> wrote:
| Quote: | You never ground radio-frequency interference (RFI) shields at both ends. The objective of shielding is to absorb RFI, convert it to an electrical current, and drain the current to ground. If you ground the shielding at both end it won't work, because no current can be induced in a conductor entirely at ground potential. This is true for all RFI shielding, from ancient TV coax to the newest inter-building shielded Category-6 data cabling.
-Mel Beckman
On Sep 24, 2012, at 11:40 AM, Gary Vogt <teamgrumman(at)yahoo.com (teamgrumman(at)yahoo.com)> wrote:
| Quote: | on that subject, why is it (the magneto P-Lead shields) grounded at both ends?
From: "n76lima(at)mindspring.com (n76lima(at)mindspring.com)" <n76lima(at)mindspring.com (n76lima(at)mindspring.com)>
To: teamgrumman-list(at)matronics.com (teamgrumman-list(at)matronics.com)
Sent: Monday, September 24, 2012 8:19 AM
Subject: RE: TeamGrumman-List: Re: Melted wire
--> TeamGrumman-List message posted by: n76lima(at)mindspring.com (n76lima(at)mindspring.com)
| Quote: | From: "Hosler, John"
The OEM ran the ground wire to the airframe with a sheet metal screw???? John
|
Yes, they did this at several locations. The instrument lights and compass light are grounded to the honeycomb via a ring lug on the wiring and a sheet metal screw.
The Magneto Ground at the ignition switch is also connected to the honeycomb via the samhare, and much much ===
|
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BARRY CHECK 6
Joined: 15 Mar 2011
Posts: 738
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| Posted: Tue Sep 25, 2012 12:18 am Post subject: Melted wire |
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Incorrect Mel:
Please read my post on this item. The type of radio does NOT make any difference. It can be from SLF to SHF the shielding and grounding remain the same. Even application does not make a difference... AM, FM, Radios or TV - Aircraft, Car or Fixed Station. No difference. Better Yet! Go look at the wiring in your plane. If you did have the COAX shield open at the radio end you will be sending it out for repairs due to a very high VSWR which will burn up your output transistor.
Barry
SSBSATSOAE
On Tue, Sep 25, 2012 at 3:59 AM, Mel Beckman <mel(at)becknet.com (mel(at)becknet.com)> wrote:
[quote]Actually, in the case if an aviation radio, I got the source backwards. The source is the antenna, not the radio, so you would ground the coax shield at the antenna end. But you would still just ground one end.
-mel via cell
On Sep 24, 2012, at 1:33 PM, Mel Beckman <mel(at)becknet.com (mel(at)becknet.com)> wrote:
| Quote: | Gary,
I have to disagree. In GA applications, you do not ground RF antenna cables at both ends. If you review any avionics installation guide, you'll see that they invariably caution that only one end of the cable be grounded (the transmitter end), even if this means cutting the ground path at the antenna end (which I always do, because even so-called "insulated" antenna mounts often manage to conduct somehow).
The fact that COAX is unbalanced is the reason why you ground at only one end. Balanced conductors (such as twisted pairs) have intrinsic shielding due to an inverse of the original signal being imposed on one conductor, creating an evenly balanced EMF (hence the term "balanced"). It relies on a sum and difference principal, where sum and difference is the combining (summing) of two signals that are out of phase from each other. Whatever doesn’t cancel out is noise that can be rejected at the receiving end.
Coax, being unbalanced, requires a shield. To function at the frequencies present in GA aircraft, that shield has to be grounded at only one end. As an A&P, Ham radio operator, and electrical engineer, I've had to deal with many gnarly RFI problems. 99% of the time the cause is double-ended grounding.
The grounding practice for aircraft shielded cables is well specified as single-ended in almost all cases. The only exception would be RF emitters at well above 1 GHz, in which shielded antenna cables are grounded at both ends. These have limited application in aircraft (mostly radar systems), but for completeness I'll explain the difference.
Shielding effectiveness at low frequencies is a function of the impedance of the cable shield compared to the impedance of the shielded circuit. When grounded at both ends, not only does external RFI readily pass through the shield, the shield itself becomes an emitter as a result of any potential difference between the two ends of the cable -- the so-called "ground loop". One motive of grounding at one end is to break that loop to eliminate a shield-induced magnetic field coupling into the conductor. It doesn't make any difference if you're trying to contain the shielded cable's signal or protect it from outside RFI.
At high frequencies, the coupling mode is via an electric rather than magnetic field. As I mentioned earlier, we're talking here about frequencies well above 1 GHz (so transponders, operating right at 1GHz, aren't included). Voltage on the shield then couples to the circuit within the cable based upon the capacitance between the shield and the wire inside. In GA aircraft you can see this on remotely mounted radar antennas, although most manufacturers try to put the radio right behind the antenna to keep the cable length short. In this scenario, you want to minimize the voltage on the shield. Terminating the shield at both ends and at each point where the cable penetrates a bulkhead is the best practice here, but it's seldom done on GA aircraft, due to the short cable lengths. In large aircraft that's the recommended practice, though (see "Aircraft and Avionics Cabling E3 Shield Termination Recommendations, http://www.bmpcoe.org/library/books/navso%20p-3181/53.html).
In GA, at least until we start operating microwave transmitters, the general rule is ground at one end. Unless, of course the manufacturer says otherwise.
-mel
On Sep 24, 2012, at 12:39 PM, Gary Vogt wrote:
| Quote: | Thanks Barry. That was very well said and even I understood it. I've always wondered why both ends were grounded.
From: FLYaDIVE <flyadive(at)gmail.com (flyadive(at)gmail.com)>
To: teamgrumman-list(at)matronics.com (teamgrumman-list(at)matronics.com)
Sent: Monday, September 24, 2012 12:18 PM
Subject: Re: Re: Melted wire
Mel:
In response to your email... It depends on what you want to accomplish.
Example:
In an RF (Radio Frequency) transmitter where there is shielding involved you ground at both ends. An example of this would be a simple COAX cable going to an antenna. COAX is an unbalanced medium and by grounding at both ends (Radio & Antenna) you trap the signal and send it to ground via the shortest path.
In an AF (Audio Frequency) intercom where you want to shield a transmitted signal such as a RF transmission, you ground the shield (COAXIAL Shield around the twisted pair of audio lines) at one end. The end you ground is at the intercom, which is known as The Source <-- I know CRAZY since the source of the interference is the RF. I don't know why they depict it that way.
Think of it this way: In RF you don't want the signal to escape, you want it to say contained within the shield. In AF you want the shield to act as a fishing net and collect the signal and then send it to ground. And as you said: " ... no current can be induced in a conductor entirely at ground potential." <-- This is true in 99.9273% of cases. Where it gets foggy is when you are talking WHAT FREQUENCY of RF and HOW LONG A GROUND WIRE. And this stuff just starts to become a problem at our VHF frequencies and goes really crazy at Microwave frequencies.
Barry
On Mon, Sep 24, 2012 at 2:49 PM, Mel Beckman <mel(at)becknet.com (mel(at)becknet.com)> wrote:
| Quote: | You never ground radio-frequency interference (RFI) shields at both ends. The objective of shielding is to absorb RFI, convert it to an electrical current, and drain the current to ground. If you ground the shielding at both end it won't work, because no current can be induced in a conductor entirely at ground potential. This is true for all RFI shielding, from ancient TV coax to the newest inter-building shielded Category-6 data cabling.
-Mel Beckman
On Sep 24, 2012, at 11:40 AM, Gary Vogt <teamgrumman(at)yahoo.com (teamgrumman(at)yahoo.com)> wrote:
| Quote: | on that subject, why is it (the magneto P-Lead shields) grounded at both ends?
From: "n76lima(at)mindspring.com (n76lima(at)mindspring.com)" <n76lima(at)mindspring.com (n76lima(at)mindspring.com)>
To: teamgrumman-list(at)matronics.com (teamgrumman-list(at)matronics.com)
Sent: Monday, September 24, 2012 8:19 AM
Subject: RE: TeamGrumman-List: Re: Melted wire
--> TeamGrumman-List message posted by: n76lima(at)mindspring.com (n76lima(at)mindspring.com)
| Quote: | From: "Hosler, John"
The OEM ran the ground wire to the airframe with a sheet metal screw???? John
|
Yes, they did this at several locations. The instrument lights and compass light are grounded to the honeycomb via a ring lug on the wiring and a sheet metal screw.
The Magneto Ground at the ignition switch is also connected to the honeycomb via the samhare, and much much ===
|
|
[/b]
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==========
cs.com
==========
matronics.com/contribution
==========
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tp://forums.matronics.com
_blank">http://www.matronics.com/contribution
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flyv35b(at)minetfiber.com
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| Posted: Tue Sep 25, 2012 5:50 am Post subject: Melted wire |
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On 9/24/2012 7:27 PM, beltz6 wrote:
| Quote: | So the only thing I have to watch for now is if there is an -intermittent- short in the relay that only appears when I'm at 8500 feet, at night, not near any suitable landing sites :-/
|
A good reason to have an emergency buss directly off the battery,
bypassing the master switch and relay.
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Dan.Schmitz(at)calix.com
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| Posted: Tue Sep 25, 2012 6:03 am Post subject: Melted wire |
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[quote] --
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JHOSLER(at)epri.com
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| Posted: Tue Sep 25, 2012 6:21 am Post subject: Melted wire |
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And two flashlights in your lap.
John
--
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