F

Flying Scotsman

Well Known Member
This might belong in the electrical forum...if so, my apologies and a moderator can move it, I guess.

Basically, looks like 3 options for the combination of battery/grounding strap/wire to ground block on the firewal:

1) Battery negative to ground block on the firewall and thence to engine via grounding strap
2) Battery to engine via grounding strap thence to firewall via something
3) Battery to engine via grounding strap, and Battery to ground block on firewell via big honkin' wire

I just installed option #3 (temporarily while I am doing electrical layouts, ordering wires, etc.). Is there any disadvantage to this? Is one option or another preferred, and if so, why?

TIA!

Steve
 
#1 is good because it has only one ground wire connection at the battery for convenience. But I added another (second) ground strap from the firewall to the motor case.

If the ground strap fails, the power for the starter will ground through your throttle and mixture cables.

You don't want that to happen!!
 
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How about Option 4: (what I did)

Battery to Engine
Battery to Firewall Grd blk
Firewall Grd Blk to Engine
 
Off the top of my head, Van's optional wiring instructions call for only:
1. ground engine case to firewall (where the breather tube is attached with the adel clamp).
2. ground battery to firewall ground stud (wherever you chose to put it).
 
Walt said:
How about Option 4: (what I did)

Battery to Engine
Battery to Firewall Grd blk
Firewall Grd Blk to Engine
Click to expand...

The reason I don't care for this option is you are dealing with installing two ground cables on the battery terminal. Extra work. You already have the ground cable from the battery to the firewall ground block. And from the firewall ground block to the engine case. Second strap can be a braided strap mounted out of the way.
 
Walt said:
How about Option 4: (what I did)

Battery to Engine
Battery to Firewall Grd blk
Firewall Grd Blk to Engine
Click to expand...

OK, let's add that as a fourth option, and I can see why (as a fail-safe in the event one of the ground straps to the engine fails, there is a backup).

That means there's a fifth :)

Battery to Firewall
Battery to Engine
Engine (different location than first strap) to Firewall/Motor Mount (different location than Grd blk)

I'm looking for pros/cons on these options, not just "Nuckolls said do it this way" or "Van's plans show this". Specifically, things like:

Number of fasteners on a single joint (e.g., battery terminal)
Possibility of EMI-type noise introduced into the system
Structural failure due to vibration
Etc.
 
Flying Scotsman said:
That means there's a fifth :)

Battery to Firewall
Battery to Engine
Engine (different location than first strap) to Firewall/Motor Mount (different location than Grd blk)

I'm looking for pros/cons on these options, not just "Nuckolls said do it this way" or "Van's plans show this". Specifically, things like:

Number of fasteners on a single joint (e.g., battery terminal)
Possibility of EMI-type noise introduced into the system
Structural failure due to vibration
Etc.
Click to expand...

Battery to firewall should be..... battery to firewall ground stud pass through. This one step grounds the airframe and creates a grounding stud on the back side of the firewall for panel grounding needs. The same grounding stud on the engine side can now have a ground strap that will ground the motor case. Existing hole at base of dip stick tube. This allows one ground cable connection at the battery. Now you only have one ground disruption during battery maintenance. Bad grounds cause most of your electrical and gauge problems. Each of these grounds should be installed coated with DE-OX. http://www.ilsco.com/ProductsDetail...=FrE65M9yM7sstRvASdKXh6hZXz2ikfy3H6bdLf9Gjks=

All you need now is another ground strap located out of the way. Failure of the motor ground, or not remembering to re connect it will melt your throttle and mixture cables when you hit the starter.

Try to keep all of your maintenance items as simple as possible. And less clutter helps during maintenance.
 
I ran a #2 from the right side of the engine to the firewall in a way that would allow it to be clamped properly and connected it via a bolt/nutplate on the firewall, having drilled out a rivet on one of the angle supports. Kept it away from a bunch of other stuff.

I connected the battery to the firewall "forest of tabs," which sits just above it.

I considered myself done at that point, as I recall. I believe this was what the Van's plans recommended
 
Flying Scotsman said:
1) Battery negative to ground block on the firewall and thence to engine via grounding strap
Click to expand...

As above, battery to ground block brass bolt with a #2 or #4 cable, well crimped ends or soldered ends. Braided ground strap from brass bolt to engine.

Good stuff:

http://www.bandc.biz/groundblock24-tab.aspx

http://www.bandc.biz/braidedbondingstrap.aspx

Bob Collins said:
I ran a #2 from the right side of the engine to the firewall in a way that would allow it to be clamped properly and connected it via a bolt/nutplate on the firewall, having drilled out a rivet on one of the angle supports. Kept it away from a bunch of other stuff. I connected the battery to the firewall "forest of tabs," which sits just above it.
Click to expand...

Bob, the ground cable from the engine is clamped to the firewall, not directly to the forest of tabs?
 
DanH said:
Bob, the ground cable from the engine is clamped to the firewall, not directly to the forest of tabs?
Click to expand...
No, as I mentioned above, Van's plans have you go from the the left side of the engine to a bolt used to secure the breather tube running down the left side of the firewall recess. Is that not good?
 
Optimum Connectivity and Shortest Path

If you've got two bolts coming through the firewall holding on the "Forest of Tabs" (FoT). It makes sense if possible to attach your Engine Ground Strap to one of these FoT bolts on the forward side of the firewall to optimise connectivity and minimise the path length to the battery. (Not quite as good as a direct Engine to Battery connection, but easier to fasten two terminals on the FoT bolt than at the battery).

I've run my Battery Ground Lead to an FoT bolt plus I have two Engine Ground Straps, one to each FoT bolt.
 
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alpinelakespilot2000 said:
No, as I mentioned above, Van's plans have you go from the the left side of the engine to a bolt used to secure the breather tube running down the left side of the firewall recess. Is that not good?
Click to expand...

Right. I installed the nutplate in that location as called for then went to attach the cable and the attach point the plans call out doesn't exist on my engine, so I moved it over to the other side (which I believe was noted in the 7A plans that moving it over was an option.
 
I think you want a central ground. The best way to do that, I feel, is to use the Forrest of Tabs grounding block with a big brass bolt. Run a strap from the battery to that bolt, which now becomes your central ground. All grounding can now be referred to that single point very easily.

For insurance against a broken main engine ground, I ran a smaller strap from the other FOT grounding bolt to another bolt on the engine.
 
I'm trying to sort out the sound advice here from the "I think." I'm not an electrical engineer. If I have -- as I do -- a ground cable attached to the firewall, a foot from the forest of tabs. And the forest of tabs is attached to the firewall. And a 2AWG connects the forest of tabs to the battery, what's happening -- or might happen -- in that one foot that makes it a potential problem? Like I said, this is consistent with Van's instructions and I'm not aware of any reported problems with this arrangement.
 
I'm honestly not trying to be dull or argumentative here :), but I've only seen one post that had a real reason for battery-to-FOT, then FOT-to-Engine: maintenance.

That's not a bad reason, of course, and certainly most of these will work fine. What I'm looking for is:

is there a known problem with one method over another? (Specifically, both FOT and Engine to battery vs. the daisy-chain style I mention above) Is there any EMI reason to prefer one over the other? Would one or the other be more "failure-prone"? (It seems that the daisy-chain method would have *three* potential places for a break or bad connection, which would make starting turn into a Bad Day, whereas going direct to the battery would only have two). And so on...

I'd read Nuckolls' article several times over the last few months, but I don't see a rationale in there, either, other than it's convenient and appears to work well. :)

This is probably overthinking the whole deal, but could we characterize at least those two options in terms of:

electrical
mechanical
maintenance
EMI

for pros and cons?
 
Steve,

My first priority for the Engine-to-Battery Negative connection is mechanical security. If the connection breaks then none of the other factors matter.

The FoT Bolt assembly is relatively robust. The PC680 Battery Negative Terminal less so. The Engine Ground Strap will transmit some vibration from the engine to the other end. The FoT Bolt is better suited to dealing with that and leaves the PC680 Battery Negative Terminal just to carry a (hopefully short) cable connection to another fixed point (the FoT Bolt).
 
Grounding (more why than how)

From the view of a mixed signals engineer the best low impedance, noise canceling item in my system is the battery so the closer I can get to direct runs to it the better.
It's all about current and impedance (resistance).
In the extreme, sharing the power and ground runs between a strobe unit and an audio panel would be a bad idea. The current impulses coupled with the impedance of the wire will induce noise via voltage changes (v = i*r). Most of these impulses can be filtered out by well designed electronics but the fly in the ointment is that the cost and complexity of our electronic devices were reduced by using ground as audio and digital signal returns (reference). This approach is usually a non-issue for more modern digital equipment but analog is a different story. Noise on audio grounds degrades the quality of the audio and lowers the accuracy of your analog engine sensors. Excessive noise on the grounds can even cause digital inputs to change state. Because ground is the reference for all other signals it needs more attention. * Your EFIS may run down to 6v but try moving the ground 'up' 2 volts and see what happens!
Reality Check:
There are several good ways to wire the grounds. Mine favors isolating the ground star point from the frame and using the battery post as the only common between engine, airframe, and EFIS/radios. This is a compromise for the engine sensors accuracy given the circuitous way the ground returns to the EFIS but it favors reducing what I fear most.
More reality:
I personally never though about losing my engine ground and frying my control cables. With that in mind I may place a second ground point/strap on my engine but tie both straps back to the battery.
 
Steve, Bob,

Here are the three most common single point ground schemes.

ndslh.jpg


Option one: The battery ground stud is isolated from engine vibration and clamps a single wire terminal, reliable and easy to maintain. The single point connection on the high amperage side clamps two terminal ends directly together with a 5/16" brass bolt. Direct contact and high clamp force (plus a bit of grease) results in low resistance over the long term.

Option two: The direct engine-to-battery connection offers slightly less resistance on the high amperage side. The compromise is the need for two connections at the battery stud. As the popular PC680 only has a 6mm screw for termination, you can't generate significant clamp force on multiple terminal ends. You also need to fixate the engine ground conductor to structure before going to the battery, for vibration isolation.

Option three: High amperage starting current is required to pass through a firewall connection to structure, then via structure to another connection at the single point ground, then to the battery via the usual connections. The result is additional resistance now and a higher probability of resistance growth with age.

I think #1 is the best choice overall. To use Steve's criteria, it is electrically and mechanically robust, easy to maintain, and practical regarding EMI prevention.

There's no significant disadvantage to #2 if installed corrrectly.

The additional connections, galvanic possibilities, and path length found in #3 are all downsides with no upside. I suspect it's on the plans due to "we've always done it that way" disease. It will work. It is not optimum. Mr. Ohm says (with credit to Mr. Nuckolls) at 200 amps cranking current you lose 0.2V at the starter for every additional milliohm.

BTW, use a braided ground strap between engine and your ground point, not a round wire. Run each end of it through an adel clamp attached to respective structure (the airframe on one end, the engine block on the other) before terminating. The high flexibility of braid combined with no vibratory load at the end terminals will virtually eliminate the possibility of breakage.
 
Alrighty, then. I'm still not seeing (at least in English) a compelling explanation for why the Van's method to the firewall might not be a good idea (I mean, sure, I could bolt it straight to the battery, but I'll bet the odds of a battery terminal screw unwinding what with the extra space and fewer threads is actually greater than an AN-3 bolt into a nutplate coming loose.:eek: But, sure. It might be a "we've always done it this way" scenario. But that also means it might not be. It might be that it's a proven an effective method for the RV. Is it the third-best solution? Maybe. But it's hard to evaluate without knowing the degree of difference in real life.

In my case, I have a central ground, the FOT at the firewall and everything grounds there EXCEPT, the strobe power supply (which is in the tail and grounds locally) and the engine case.

Nothing in the panel grounds locally. It all runs to the FOT. Is there likely to be noise in the audio under the current system? Is there likely to be a misreading of the analog instruments. From what I read, I guess there is, though it's not clear how much more likely it is to be the case. For all I know, it's the difference between the speed of neutrinos and the speed of light. BTW, I vaguely recall an old thread here where the backup ground strap is the steel braided fuel and oil pressure lines to the transducer manifold, though I can't remember how that ever turned out.

So from the original question on the post, it SOUNDS to me as the issue isn't really electrical problems per se as it is what happens IF the grounding from the engine to the firewall (or battery) breaks.

Which leads me to this question: (a) What's the likelihood of that happening? and (b) are there NTSB reports that indicate what happens when it does that I can read.

In many ways, this sounds like a "calculating risk" equation more than a significant scientific equation of the principles of electrons. Of course, from a "it might break" scenario, things are a little different in my case in that arena . I do preflights a lot differently than most pilots. I check everything. If it means pulling a cowl off to constantly check cables -- even the deadly round wire that I'm using for a ground strap (although less deadly than ones that has Van's crimped-on terminals) -- then that's why I do. Of course, I don't fly as often as a lot of people.

So, let's assume the ground wire between engine and firewall breaks. You're flying along on your one-mag-one-EI engine. What happens? (not rhetorical)
 
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Not really a flight safty issue

Bob Collins said:
Which leads me to this question: (a) What's the likelihood of that happening? and (b) are there NTSB reports that indicate what happens when it does that I can read.

In many ways, this sounds like a "calculating risk" equation more than scientific equation of the principles of electrons.:p
Click to expand...

Loss of engine case ground in flight (with mags) would most likely be incorrect engine sensor data but with electronic ignition it could get exciting real quick. Worst case loss on starting, possible extended damage due to starter current.
A belt and suspenders solution for sure but with little penalty in cost or weight.
 
Bob Collins said:
So, let's assume the ground wire between engine and firewall breaks. You're flying along on your one-mag-one-EI engine. What happens?
Click to expand...

The dilithium crystals will discharge. The ionizing radiation will cause your flesh to rot. You will become a zombie and crave brains, which will not be available in the airport vending machine. You'll be left brainless and grumpy.
 
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