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grounding

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prkaye

Well Known Member
I've had some minor issues and behaviours that suggest I might have a grounding problem (fluctuating sensor values, EFIS generating an audible hum on my radio). I have two forests of tabs on each side of the firewall, with brass bolts between them. I have braided cables from the negative of the battery to one of these bolts, and another braided cable from the bolt to the engine.
A local builder (of a composite aircraft) suggested I run dedicated ground wires for my avionics directly to the engine, instead of grounding these through the forest of tabs.
Thoughts?
 
prkaye said:
I've had some minor issues and behaviours that suggest I might have a grounding problem (fluctuating sensor values, EFIS generating an audible hum on my radio). I have two forests of tabs on each side of the firewall, with brass bolts between them. I have braided cables from the negative of the battery to one of these bolts, and another braided cable from the bolt to the engine.
A local builder (of a composite aircraft) suggested I run dedicated ground wires for my avionics directly to the engine, instead of grounding these through the forest of tabs.
Thoughts?
Click to expand...

Yes, do this for your EMS. Good grounds are important here.
 
Phil,
We wouldn't generally recommend grounding the avionics to the engine. You can create a really good ground between the engine and battery, and then a good battery-electrical ground, and be successful. Running 20+ grounds through the firewall isn't really a robust solution, and the ground you care about is really the battery ground as the center of the ground world, not the engine.

The first thing to isolate is where your ground problem really is. If the sensor issues and headset noise are only there with the alternator spinning, and get worse with load, then it's engine to battery. If you have the issue even with the engine off, then it's somewhere else.

A jumper cable between things like the case of your intercom and various other grounds in the plane can do wonders in finding the ground issue.
 
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Thanks Dynon Support. The electrical noise I hear is audible when the Dynon EFIS is on, regardless of whether or not the engine is on.
A jumper cable between things like the case of your intercom and various other grounds in the plane can do wonders in finding the ground issue.
Click to expand...
Could you explain this diagnostic procedure? So I hook a jumper from the case of my intercom to my main ground forest of tabs... what am I looking for?
 
Ahh, if you have EFIS noise all the time, then your EFIS ground is probably iffy. The noise you hear is the EFIS drawing different amounts of power, which through a iffy ground causes varying voltage, which the intercom can "hear" because it assumes the ground is stable.

Chances are about 90% that if you touch a jumper cable to the EFIS case and the other end to the intercom case, the noise will go away.

Is this a non-metal instrument panel by chance?
 
dynonsupport said:
Ahh, if you have EFIS noise all the time, then your EFIS ground is probably iffy....
Is this a non-metal instrument panel by chance?
Click to expand...

It's a metal panel. So I should re-do the spade connection on the ground wire coming out of the D-180 wiring harness?
 
I would also make sure your panel is well grounded. Most of these instruments tie their case ground to the ground plane and problems here can cause noise. I believe Dynon recommends this in their installation instructions as well.

Good luck

Larry
 
I would also make sure your panel is well grounded. Most of these instruments tie their case ground
Click to expand...

Yes, last year in my attempts to fix some of this I ran a wire directly from the dynon case (through the screw at the back that secures it to the mounting tray) to my ground forest of tabs. But I didn't check the actual ground wire connection from the wiring harness. I guess that should have been the first thing I checked! I'll also run a dedicated ground wire from the case of my intercom back to my ground forest of tabs.
 
DO NOT run grounds to the engine!

If for some reason the main starter ground breaks or comes loose you WILL melt the small ground wires which in turn can cause all kinds of expensive problems.
I have seen this happen many times over the years mainly on certified airplanes with improper wiring done on them.
 
Ground

Norman CYYJ said:
DO NOT run grounds to the engine!

If for some reason the main starter ground breaks or comes loose you WILL melt the small ground wires which in turn can cause all kinds of expensive problems.
I have seen this happen many times over the years mainly on certified airplanes with improper wiring done on them.
Click to expand...

Norman
Not to hijack but I have a question it sounds like you can answer.
Would it be advisable to insulate the FOT from the air frame so the only path for equipment ground is through the ground lug to the battery?
Some FOTs are solid brass plates.
If it were making contact to air frame, any ground such as the starter could find a path through the FOT. Or am I over thinking?
Seems like the ground from the engine and air frame should terminate at the same battery ground lug on the FOT. Technically all should be at the same potential but, a loose or dirty connection results in a new path of least resistance and you don't want that path to be the equipment. If the FOT is insulated from air frame then a bad connection should cause a starter to fail. Just wondering. I'll be planning electrical soon.
 
wirejock said:
Norman
Not to hijack but I have a question it sounds like you can answer.
Would it be advisable to insulate the FOT from the air frame so the only path for equipment ground is through the ground lug to the battery?
Some FOTs are solid brass plates.
If it were making contact to air frame, any ground such as the starter could find a path through the FOT. Or am I over thinking?
Seems like the ground from the engine and air frame should terminate at the same battery ground lug on the FOT. Technically all should be at the same potential but, a loose or dirty connection results in a new path of least resistance and you don't want that path to be the equipment. If the FOT is insulated from air frame then a bad connection should cause a starter to fail. Just wondering. I'll be planning electrical soon.
Click to expand...

It is OK if the FOT is in the ground path / circuit. You just don't want a situation where the ONLY path the starter current can flow is through instruments or small wiring. The key is insuring that your ground strap from the engine has a solid connection to the battery through appropriately sized wiring. You want to think about the starter current flow if the strap were to break. You don't want any ground path from the engine to the battery through small wiring. Some of this cannot be avoided (typically sensors that have some resistance to slow down the current), but you should not add path's un-necessarily.

Larry
 
Grounding FOT

lr172 said:
It is OK if the FOT is in the ground path / circuit. You just don't want a situation where the ONLY path the starter current can flow is through instruments or small wiring. The key is insuring that your ground strap from the engine has a solid connection to the battery through appropriately sized wiring. You want to think about the starter current flow if the strap were to break. You don't want any ground path from the engine to the battery through small wiring. Some of this cannot be avoided (typically sensors that have some resistance to slow down the current), but you should not add path's un-necessarily.

Larry
Click to expand...

Hmmm.
That implies it's preferred to have a copper FOT secured to air frame with the engine and battery grounds on the same lug. If the engine ground fails, alternate path is through the plate to the ground lug. If more than one FOT is required, tie them together with a copper bus bar. Does that sound right?
The part I'm not clear on is how do you insure the equipment ground wire is the path of least resistance over the chassis to air frame path? Back to insulating the FOT. I'm stuck in a Mobious Loop. Maybe over thinking. :eek:
 
wirejock said:
Hmmm.
That implies it's preferred to have a copper FOT secured to air frame with the engine and battery grounds on the same lug. If the engine ground fails, alternate path is through the plate to the ground lug. If more than one FOT is required, tie them together with a copper bus bar. Does that sound right?
The part I'm not clear on is how do you insure the equipment ground wire is the path of least resistance over the chassis to air frame path? Back to insulating the FOT. I'm stuck in a Mobious Loop. Maybe over thinking. :eek:
Click to expand...

In my case, the copper bar (FOT) is bolted to the firewall and represents the primary airframe grounding point.. One of those bolts attaches both the engine ground strap and battery negative cable. The starter current would never flow to the frame as it can go directly to the battery. If the engine strap breaks, there is no other path from the engine to the airframe except a sensor or two and I believe that the resistors in these would fry before creating enough flow to melt the wire. A EE could probably better answer that one.

Larry
 
Phil - I think I know who it was that gave you the recommendation to ground your avionics to the engine. He and I have had this discussion several times, and through those discussions and some simple modelling on paper it has become clear that his approach is indeed the correct approach.

Just to be clear, he is not talking about grounding every piece of avionics through a separate wire to the engine case. He's talking about running a nice fat wire from the engine case to the primary avionics grounding point (your forest of tabs).

If you look at Grumman aircraft you will see they are wired this way. There's a very good reason to do this (as has been discussed above, giving consideration to where starter currents must flow). If you model it out on paper with each wire and each connection point represented as resistors, you'll see the logic in this approach.
 
Canadian_JOY said:
Phil - I think I know who it was that gave you the recommendation to ground your avionics to the engine. He and I have had this discussion several times, and through those discussions and some simple modelling on paper it has become clear that his approach is indeed the correct approach.

Just to be clear, he is not talking about grounding every piece of avionics through a separate wire to the engine case. He's talking about running a nice fat wire from the engine case to the primary avionics grounding point (your forest of tabs).

If you look at Grumman aircraft you will see they are wired this way. There's a very good reason to do this (as has been discussed above, giving consideration to where starter currents must flow). If you model it out on paper with each wire and each connection point represented as resistors, you'll see the logic in this approach.
Click to expand...

That's how mine is wired, except instead of a fat wire I have a braided grounding strap from B&C that runs from my engine to the firewall side of my FOT. Is it necessary? I don't really know, but I decided to go with a belt and suspenders approach following the AeroElectric wiring philosophy.

FWIW, when Dick Kohler (the A&P in a lot of the EAA How to Videos) looked at my setup, he stated he thought is wasn't necessary so go figure.

FP10122014A0004F.jpg
 
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Canadian_JOY said:
Phil - I think I know who it was that gave you the recommendation to ground your avionics to the engine. He and I have had this discussion several times, and through those discussions and some simple modelling on paper it has become clear that his approach is indeed the correct approach.

Just to be clear, he is not talking about grounding every piece of avionics through a separate wire to the engine case. He's talking about running a nice fat wire from the engine case to the primary avionics grounding point (your forest of tabs).

If you look at Grumman aircraft you will see they are wired this way. There's a very good reason to do this (as has been discussed above, giving consideration to where starter currents must flow). If you model it out on paper with each wire and each connection point represented as resistors, you'll see the logic in this approach.
Click to expand...


I would agree. Run all high current grounds (#2awg) to the large bolt on the FOT since that allows connections on both sides of the firewall for those with batteries in the back. All other grounds go the faston tabs.

The only exception would probably be audio grounds, which several vendors prefer the intercom to be the common ground for all audio circuits, then run a single ground to the FOT.
 
So, if memory serves, the way I have mine is:

- Forests of tabs on both sides of firewall (sandwiched) with big bolt between them.
- Braided ground cable from battery negative to the forest of tabs big bolt
- braided ground cable from battery negative to engine

So, you are suggesting that I should have a braided cable directly from the forest of tabs big bolt to the engine. Should I...

A) Move the braided cable that is currently battery->engine so that it is FOT->engine, or
B) Add another braded cable FOT->Engine.
?

If I do (A) then the path from battery to engine will not be direct, but be through the FOT. If I do (B), then I have two paths to the engine from the FOT... one direct, and one through the battery negative. Which is preferable?
 
I ran from battery to FOT to engine bolt. Then a braided cable from engine MOUNT to engine case. Simple, clean and gives a secondary path to the starter if the main ground ever fails. Engine monitor has it's own ground wire.
 
gasman said:
Then a braided cable from engine MOUNT to engine case. Simple, clean and gives a secondary path to the starter if the main ground ever fails.
Click to expand...

Isn't the engine mount electrically isolated from the engine via the rubber Lord mounts?
 
Talk to the plastic airplane guys; they've had decades to perfect "Holy Point" grounding. In short, you need an avionics/instrumentation Holy Point, an airframe Holy Point, and most often an audio Holy Point ground.

My first composite plane was built in 1981 and had ground loop issues, the second -> fifth composite planes were fine :D We wired my hangar mates RV-4 the same as the EZ's under the eyes of Alan Tolle.
 
deek said:
Talk to the plastic airplane guys; they've had decades to perfect "Holy Point" grounding. In short, you need an avionics/instrumentation Holy Point, an airframe Holy Point, and most often an audio Holy Point ground.
Click to expand...

I like that name... "Holy Point" :)

At work, we usually call these "Golden Nodes". LOL!
 
RV7A Flyer said:
I like that name... "Holy Point" :)

At work, we usually call these "Golden Nodes". LOL!
Click to expand...

That's what the North American Aviation Apollo guys called 'em when they were helping me straighten out grounding issues on bird #1
 
Phil

To answer your question -
I have an earth from both battery & engine to a bolt on firewall then another #10 wire on aft side (Instrument Panel side, using same bolt) to a FOT on sub panel (don't see the need for FOT on engine side) - this method has worked for me without any issues on all the 35 RV's I've wired so far:)

Jake J
 
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