How many grounding blocks?

OK?I'm well into wiring my RV-8, and have a 40-tab "forest of tabs", aka FOT on my avionics bulkhead, (aft side of luggage compt.) and I have a small FOT on a longeron/rib in the belly, near my right wing root, where I plan to use for my wing-mounted equipment; lights, pitot heat, and rear-mounted transponder. This FOT will be convenient for my inter phone jacks, and PTT switches in the joysticks.
My question is; should I have another grounding point, other than the engine case, for any of my comm/interphone?
I've seen guidance that all grounds should terminate at the same place?
then, I've read that I need to isolate "some" of the grounds from others to avoid audio interference?
Could someone help me with this? Electronic ignition? Interphone? Radio? Strobes? Transponder?
I'm using Vertical Power Pro, and have a Dynon Skyview with ADS-B, EMS, single Comm with dual inter phones.
Thanks,

The problem with more than one grounding point is that you may have a difference in potential in between the two. This can cause what is known as a ground loop, not the tail dragger kind the electrical kind. It can give interference and be very hard to identify. I used one ground point on the firewall withe a through penetration giving me tabs on both sides of the firewall and from ther going directly to battery ground. The through penetration is two 5/16 fine thread all thread which allows connections of battery ground and other larger wires such as starter ground, a small forest of tabs also bolt through both all threads on both sides of firewall, allowing smaller grounds to be attached. Two locations can be done if they are properly connected together with the proper size wire and grounded to the battery. The term isolated ground refers to running a wire for that device directly to the ground point, not using frame or chassis ground.

Bird

As Bird explained, the idea is that there is one but only one path all the way back to the battery's negative terminal. Your second forest of tabs should be insulated from the airframe, and then either connected to the battery's negative terminal, or to the first forrest of tabs, but not both.

In practice this can be very hard to accomplish. For example, if you ground your intercom to FOT #2, and, say, a com to FOT #1, you just created a loop - because both of these boxes have metal cases which are also tied to their grounds, and, unless you have insulated them somehow, their grounds are now connected together via the metal racks and attach hardware. For this reason most people try to use a single ground buss bar or FOT as close as possible to the avionics rack.

Attaching your phone jack ground to FOT #2 while your intercom or audio panel ground is attached to FOT #1 is exactly what you want to avoid. Insulate the jacks from the airframe, and bring their grounds back to the audio panel ground.

Ahhh…
So if I insulate FOT #2 from the airframe, and use it to gather several ground wires, and connect it to FOT #1 with a large gauge wire, then I should be OK..or am I risking too much in an attempt to save running a few more ground wires all the way back to FOT #1?
And Bob, you used a term that is sorta confusing…"audio panel ground". This implies that you SHOULD use a separate ground post for audio…True?

AAflyer said:

Ahhh?
So if I insulate FOT #2 from the airframe, and use it to gather several ground wires, and connect it to FOT #1 with a large gauge wire, then I should be OK..or am I risking too much in an attempt to save running a few more ground wires all the way back to FOT #1?
And Bob, you used a term that is sorta confusing?"audio panel ground". This implies that you SHOULD use a separate ground post for audio?True?

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This stuff is a black art. For DC wires I think your #2fot, running only to fot #1, is probably okay. But for ac (especially RF) signal lines you want the 'hot' signal line and the ground return wire to run side by side (twisted together is even better) as much as possible. e.g., the ground return from your phone jack should run side by side with the audio 'high' wire all the way back to the audio panel. Any electric field induced voltage on one wire is cancelled by an equal pick up on the other. And any pick up due to ac magnetic fields is minimized due to the area between the wires being nearly zero. (another reason to avoid loops).
For the audio panel, I meant that all audio ground wires should have a single connection to the ground connection at the audio panel, and that panel should have a single connection to fot #1. Bringing all audio grounds to fot #1 is almost as good, as long as the fot is close to the audio panel.

Thanks for your responses, Bob. I was never very good at 'art', much less black art. I, like so many others, am just trying to do things once, instead of trial and error.
I think I'll pull all my ground wires back to the giant FOT. One size fits all.

Sorry to stray a little OT, but what's the consensus on grounds for steady-draw lights, like the landing light or an interior dome light? Any problems with a local ground here?

Doug, it is done a lot. I did not do my plane like that. If you are using local grounds for led lights or strobe system, motors(fuel pumps) etc. it is possible to run into problems with electrical noise and interference with headsets, intercoms and electronics.


Bird

rightrudder said:

Sorry to stray a little OT, but what's the consensus on grounds for steady-draw lights, like the landing light or an interior dome light? Any problems with a local ground here?

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I have back-to-back forest of tabs on mt firewall, one on the cold side and one on the hot side. My engine and battery grounds are bolted through both, effectively making them one common ground.

My landing and position lights are grounded to the outboard ribs with a platenut.

Initially my stobe (old style discharge type strobes) were grounded on the firewall but their discharges could be heard in my headsets. I added a ground right next to the strobe pack and grounded both the shielding and the power pack negative there and the sound went away.

If you are using LED strobes, your mileage may vary.

Thanks. I like the simplicity/straightforwardness of back-to-back FOTs. I'm using LED strobe/position lights, which will be shielded, but will probably go with local ground for landing light. Question: Shield gets grounded at FOT, and negative wire on strobes is grounded at the same FOT?

I read somewhere that is was possible to use the shield as the ground wire itself--dubious advice from a noise perspective?

rightrudder said:

Thanks. I like the simplicity/straightforwardness of back-to-back FOTs. I'm using LED strobe/position lights, which will be shielded, but will probably go with local ground for landing light. Question: Shield gets grounded at FOT, and negative wire on strobes is grounded at the same FOT?

I read somewhere that is was possible to use the shield as the ground wire itself--dubious advice from a noise perspective?

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If you are going to use a standard incandescent light local ground may be fine, but high voltage lighting, or LED lighting there are power electronics and that can be an issue. I ran grounds for everything but the ELT back to the FOT. When the ELT is needed I don't care about ground loops and harmonics.

I can not imagine you not moving to more modern, long lasting lighting.

I have all of my grounds going to the same forest of tabs, except for my LED landing lights and an LED baggage light. No issues with noise or interference so far!

rightrudder said:

... Question: Shield gets grounded at FOT, and negative wire on strobes is grounded at the same FOT?
...

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Here is what worked for me, YMMV.

I clipped the shield at the light and continued it all the way to the same grounding platenut I used for the power pack.

I was warned not to ground it at both ends as it will create a "loop". So, I added connectors for the shield wire at the wing root and connected it there.

Again, this is with "traditional" strobes. I didn't think those new fangled LED strobes required shielding but if you have it there, I would probably treat them the same as I did with the strobe pack.

Thanks, guys. I have LEDs for everything but I don't want to take any chances, so bring on the shielding!! I've got a terminal block under the control-stick covers to connect all three strobes, so one of the terminal strips will carry the shield ground (which will ultimately be grounded at the firewall FOT).

So my question is: Should I separate both the "shield ground" and the "normal ground" and run both all the way to the firewall? (Two separate ground connections all the way through?) I certainly will ground only the firewall end of the shield.

Just trying to get this through my thick head!

On an RV-8, the firewall is separated from the panel by the forward baggage compartment, so what are builders doing with the FOT grounding block? If you use the thru bolt style, the hot side of the FOT is on the engine side of the firewall, and the cold side ends up in the baggage compartment, and not the more convenient location behind the panel. Do people just use a single sided FOT on the firewall, and run a fat jumper to an insulated secondary FOT behind the panel?

maus92 said:

On an RV-8, the firewall is separated from the panel by the forward baggage compartment, so what are builders doing with the FOT grounding block? If you use the thru bolt style, the hot side of the FOT is on the engine side of the firewall, and the cold side ends up in the baggage compartment, and not the more convenient location behind the panel. Do people just use a single sided FOT on the firewall, and run a fat jumper to an insulated secondary FOT behind the panel?

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That's what I did. On the firewall I just used a single bolt and ran a wire to the FOT on the baggage compartment wall. Used shoulder washers and a piece of plastic cut to the shape of the FOT and mounted on the instrument side of the baggage compartment wall.

Bridging the baggage compartment...

OK, now I'm a little unsure.
I currently have my FOT in my -8 on the instrument bay side of the baggage compartment bulkhead?conveniently located for all my avionics. I thought I was "good to go", as it is securely grounded to this bulkhead with two screws with tooth washers.
Now I'm reading above, that I need to 'insulate' this FOT, and add a wire to another grounding point in the engine compartment??
Why?

AAflyer said:

OK, now I'm a little unsure.
I currently have my FOT in my -8 on the instrument bay side of the baggage compartment bulkhead?conveniently located for all my avionics. I thought I was "good to go", as it is securely grounded to this bulkhead with two screws with tooth washers.
Now I'm reading above, that I need to 'insulate' this FOT, and add a wire to another grounding point in the engine compartment??
Why?

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If you mount the panel FOT to metal, that sets up the potential for a ground loop if you already have a firewall grounding point - which is what you are trying to avoid with FOT.

I am running a grounding strap from a rear crankcase bolt to a brass bolt installed on the firewall that also penetrates a framing member. Also attached to the bolt is the battery negative cable, and a cable that runs back to the panel FOT - which is insulated from the aircraft body/metal as described earlier. Electrically, the firewall brass bolt becomes the single point ground, at least in theory.

Being an electronics newbie, I don't mean to argue, I just want to learn as well.

I was just going through this last night, and was reviewing docs on the AeroLEDs site. They explicitly call for 3 conductor 20awg shielded wire, and the nav/strobe doc states "to avoid interference, connect ground to the airframe close to the light". It also says to ground the Sheilds at both ends.

And lastly, that wire leaves no room for a home run ground, nor is it large enough to do that. For the strobe (the biggest pulse generator), 20awg at 3 amps is about 1 volt drop in 18', so running a ground wire would mean that needs to become a much bigger wire, out and back for the strobe, because that makes it a 36' circuit.

Trust me, I like the smaller wire in the wing, or else I am out of grommet space.

Can anyone help me understand why AeroLEDs recommends what they do?

One last question: I am putting a plug at the wing root, to allow me to get as much done as possible, before final assembly. What is the best connector type and method to carry the shielding through that connector? My setup is nearly identical to the author of this thread: in one wing AP servo, landing, and nav/strobe, and in the other heated AOA pitot, and matching lights.

I wish there was a permanent thread like "Show us your panel", but instead called "Show us your wiring", with people also reporting pictures, issues, or lack thereof.

As a "for what it's worth", I was planning on putting my two sided FOT on the firewall, far left (when looking forward), just under the baggage shelf. That puts the wiring out of the storage area, but still on the firewall, and keeps it out of sight while flying. Someone please tell me the IO-360 starter is on the left...

cdeerinck said:

Being an electronics newbie, I don't mean to argue, I just want to learn as well.

I was just going through this last night, and was reviewing docs on the AeroLEDs site. They explicitly call for 3 conductor 20awg shielded wire, and the nav/strobe doc states "to avoid interference, connect ground to the airframe close to the light". It also says to ground the Sheilds at both ends.

And lastly, that wire leaves no room for a home run ground, nor is it large enough to do that. For the strobe (the biggest pulse generator), 20awg at 3 amps is about 1 volt drop in 18', so running a ground wire would mean that needs to become a much bigger wire, out and back for the strobe, because that makes it a 36' circuit.

Trust me, I like the smaller wire in the wing, or else I am out of grommet space.

Can anyone help me understand why AeroLEDs recommends what they do?

One last question: I am putting a plug at the wing root, to allow me to get as much done as possible, before final assembly. What is the best connector type and method to carry the shielding through that connector? My setup is nearly identical to the author of this thread: in one wing AP servo, landing, and nav/strobe, and in the other heated AOA pitot, and matching lights.

I wish there was a permanent thread like "Show us your panel", but instead called "Show us your wiring", with people also reporting pictures, issues, or lack thereof.

As a "for what it's worth", I was planning on putting my two sided FOT on the firewall, far left (when looking forward), just under the baggage shelf. That puts the wiring out of the storage area, but still on the firewall, and keeps it out of sight while flying. Someone please tell me the IO-360 starter is on the left...

Click to expand...

AeroLEDs use a "switched" internal circuit design, which can cause noise in audio circuits if not properly shielded - hence their very specific wiring instructions (Whelen Orion 600 LEDs use an unswitched design that does not require the use of shielded wire, although I did anyway.) Grounding a shielded wire on both ends is generally a no-no - particularly with avionics audio equipment and jacks - but should not pose a problem with LEDs. Grounding the far end nearest the light to the airframe is OK. As far as the end nearest the switch / panel, you *could* extend the shield conductor to the FOT with an unshielded wire if you want, but keep it as short as possible - 20AWG would be fine. Alternatively, some people wire their switches with a common ground buss wire/bar for all the switches, using a single larger diameter wire run from the buss to the FOT. In this case, you could connect the LED shield to the switch common buss, and not have to extend a wire to the FOT.

As far as the wing roots break, I used a Molex / nylon connector. Just remember to keep the exposed wires at the connector as short as possible, and use solder sleeves and a pigtail for the shield conductor. Steinair has a YouTube video showing how to use them.

I don't have a need for the engine side FOT, so I don't have the double sided FOT. As for placing the panel side FOT on the firewall left side under the baggage shelf, that could be done. Just remember that you will have many ground wires running to the FOT, so have a way to organize and secure those wires so they don't interfere with the rudder pedals and brake lines.

I don't know about the starter on the IO-360, but on the O-360, the starter is on the left side facing forward, with the alternator on the right.