Let's Get Grounded!

ScottG

WARNING: Lengthy post ahead. This discussion may elicit drooping eyelids and persistent yawning in the non-technically inclined...  :-)

I've recently been pondering 12VDC chassis grounding issues, both in a general sense and with respect to recent and past Alde issues such as aluminum corrosion and the persistent blowing of the glass fuse on the negative lead of some older 3010 units. Testing led to some interesting discoveries, which led to more questions. While I understand electrical circuitry and wiring, the subtleties of grounding DC systems are surprisingly complex and a bit above my pay grade. I've been discussing this on the side with a couple other members--and have learned a lot--but thought it would be interesting to throw this out to the larger forum in the hopes that some additional insights may surface. Read on if you are still interested...

First, a little general background. Newer T@Bs have two chassis grounds:

The 120VAC side is grounded by a bare copper wire that runs from the ground bus bar in the WFCO power center to a bonding screw on the trailer frame just below the camper floor:



The 12VDC side is grounded by a jumper wire that runs from the common negative terminal in the forward junction box to a bonding screw on the frame just adjacent to the junction box:



My 2015 does not have the 12VDC chassis ground described above. I'm not sure when exactly these were added, but they are present on 2019 models, and it appears nuCamp added them to comply with RVIA certification. While grounding the DC side to the chassis is generally not required for the proper operation of DC devices--such a system would be considered "floating" and is advocated by some--the lack of a DC chassis ground opens the door for some weird electrical gremlins like ground loops. This is where things start getting a little over my head, but also where I suspect the relationship to those Alde issues possibly resides.

Still interested? Even through my 12VDC system appeared to floating, I was finding continuity between the 12VDC negative bus and the frame.



As a result, I originally assumed the DC side was being grounded to the chassis via the AC-->DC converter in the power center, and that the AC and DC sides ultimately shared the same bare copper ground wire. A little peculiar, but what do I know, right?



Turns out this was not the case. A bit more testing revealed that the continuity between the 12VDC negative bus and the 120VAC ground wire was NOT occurring through the AC-->DC converter. This suggested the continuity was somehow occurring through the chassis, but this also seemed peculiar as I could identify no obvious chassis grounds on the DC side. 

So where/how was the 12VDC side getting grounded? That's where things got interesting. I'll spare the details of my testing for now, but suffice it to say I found continuity between the 12VDC negative bus and the chassis occurring via two unlikely pathways:

• The safety ground pin of the Alde's 120VAC power cord. This presumably occurs via a connection between the Alde's internal AC and DC circuitry, and ultimately through the 120VAC chassis ground.
• The floor of the camper. One of the two small wood screws securing the 12VDC negative bus to the plywood floor appears to be driven into an aluminum floor frame member which is apparently connected to the trailer frame.    

So, here are the (admittedly weak) conclusions I've drawn thus far:

• The bonding of the 12VDC negative bus to the chassis via the aluminum frame may or may not be intentional. If it's intentional, it's a pretty sketchy connection. If it's accidental, it may not apply to all T@Bs.
• Additional grounding of the DC side occurs through appliances that use both DC and AC, such as the Alde. 
• These indirect and possibly unintentional grounds create ground loops that result in different voltage potentials in the DC circuits.
• These stray voltage potentials create electrical gremlins that can be very hard to isolate and diagnose.

And finally, here are the questions I have:

• Is it coincidence that both nuCamp and Airstream have addressed Alde issues in part by prescribing new or rerouted connections on the 12VDC negative side? 
• Is poor DC chassis grounding contributing to blown fuses and corrosion in the Alde?  
• Could adding a simple chassis ground at the forward junction box (as nuCamp does on newer T@Bs per RVIA standards) mitigate these issues? 
• Does anyone out there have additional relevant thoughts or insights? 

I've omitted many of the mundane details in the interest of brevity, but feel free to ask for clarification or more info if interested.

MuttonChops

ScottG said:

• The floor of the camper. One of the two small wood screws securing the 12VDC negative bus to the plywood floor appears to be driven into an aluminum floor frame member which is apparently connected to the trailer frame.   
  

• The bonding of the 12VDC negative bus to the chassis via the aluminum frame may or may not be intentional. If it's intentional, it's a pretty sketchy connection. If it's accidental, it may not apply to all T@Bs.

• Is it coincidence that both nuCamp and Airstream have addressed Alde issues in part by prescribing new or rerouted connections on the 12VDC negative side? 
• Is poor DC chassis grounding contributing to blown fuses and corrosion in the Alde? 
  

Just Opinions follow, no real facts.

My RVIA Approved 2018 has a very solid DC Ground Bus Bar connection to something below the floor. I confirmed this connection point after shorting the battery directly to ground (old story can be found is an old post). A large screw goes deep into the floor structure . . . my assumption has been it contacts the chassis not NüCamp aluminum framing.

While shopping for my T@B I did notice every 320 had the DC Ground Bus in the same location.  Do not believe it is a random factory technician event but that NüCamp has a DC Bus Bar placement template to insure the chassis ground connection.

Additionally my 2018 does not have the Junction Box chassis ground you note as being on newer years.  However I have seen that ground connect on 2016 T@B wiring diagrams.  So why is it that the junction box ground comes & goes over different years; a great NüCamp question.

Alde 3010 ground fuse failures are an Alde design issue.  The simple fact that all 3020 Alde does not have the ground fuse (independent of country of use) shows Alde understood their error as soon as Airstream, NüCamp, and am sure several EU manufacturers highlighted the fuse failure issue - - - which IMHO caused Alde to first instruct the manufactures about the work around extra ground wire fix, while those manufacturers decided not to implement on the factory floor but did provide 'after sale fix' instructions as requested.  Meanwhile Alde was working on the 3020 model and designed out the fuse.

BrianZ

Very interesting, @ScottG!
I have a feeling that my multimeter is about to get some exercise.  (And our junction box an inspection, as I don't offhand know whether we have a DC ground to the frame near that box). 
I can say that we've never had any issue that I can recall with the Alde blowing a fuse, though we do have a kill switch that disables the Alde 12V power line when not in use.

MuttonChops

BrianZ said:
. . . never had any issue that I can recall with the Alde blowing a fuse,

Your model-year 2018 most likely has the Alde model 3020.
NüCamp started installing the model 3020 starting with December 2017 builds.
My December 2017 factory order required a 3020, NüCamp reply was not an issue that is all we install now.

Model 3020 changes included:
   +  dropped the ground fuse
   +  added High Altitude combustion fan mode

BrianZ

Nope, we have the 3010 model.  I think ours was built May, 2017.

ScottG

@MuttonChops, thanks for chiming in. I don't know why there is inconsistency in the use (or at least documentation) of the forward DC chassis bond. I just assumed nuCamp started adding them sometime between 2015 and 2019 and that was that. It would be good to hear from owners of different vintages whether they have this ground wire or not. 

I'm aware of the changes in the 3020 Aldes, but have always been curious about the fuse blowing experienced by some (but not all--including me) 3010 owners, and why  connecting the Alde directly back to the battery was supposed to fix it. After all, the Alde was already connected directly to the battery via the 12VDC negative bus and the forward junction box. I'm just guessin' here, but this smells like one of those ground loop gremlins I mentioned previously.

You may be right about the intentional placement of the DC bus bar. One of my screws is longer than the other, but even the short screw is long enough to make the connection (which is only about 1/2" deep). Regardless, IMHO a thin wood screw into and aluminum cross member still seems like a sketchy way to make a chassis ground connection.

There is no steel trailer frame member below that spot, and a really long screw would go right through the top of the gray tank.  :-0 

ScottG

@BrianZ, I was hoping this conversation would reel you in. Your Alde had some significant corrosion, and was by far the newest of the units I'm aware of to have that problem. I'll be curious to hear what you discover when you get in there with your meter.

The possible connection between the corrosion and the DC grounding is largely conjecture on my part. I understand (vaguely) that stray electrical potentials can promote galvanic corrosion in metals. I also know that Airstream in their 2018 Alde-related technical service bulletin not only prescribed a change in glycol, but also the addition of two isolated DC grounds. The airstream power center is quite different, but FWIW I've attached the TSB for your reference.

I have written both Truma/Alde and nuCamp soliciting advice on if and how I should modify the DC grounds on my 2015 to address the corrosion issue. I got a terse response from Truma (copied to nuCamp) telling me I had to talk to nuCamp since they installed the unit. nuCamp has not responded at all.

I'm not convinced that anyone knows what is going on here, and if they do, they are certainly not talking (or at least not talking to me). In the meantime, I'll continue my fishing expedition in the hopes that something somewhere will start making sense!  :-)

Bayliss

@ScottG, I believe I have separately shared with you the following two statements, which I previously received from Creed at nüCamp.  I also posted them back in November 2019 in connection with a long discussion about the WFCO converter and "grounding."  I am re-posting them here simply to add to this very interesting conversation.  I have inserted the bracketed numbers (i.e., [1}, [2] and [3]) in the 11-14-2019 statement so that I can reference the individual points in my comments below: 

INFORMATION PROVIDED BY CREED (11-08-2019):  “The 12V Side of the converter is grounded to the bus bar.  And the 120V side of the converter is grounded to the Chassis.  The way we ground is specifically by RVIA standards.  It would make sense to have a single ground point as the bus bar, but for some reason RVIA wanted it grounded differently.  So, unfortunately, I cannot give you a specific reason why it is so.”   

INFORMATION PROVIDED BY CREED (11-14-2019):  "Here is the response I got from foreman and quality control - - [1] The copper wire is only for the 120V side.  [2] The only ground to the chassis on the 320 is out of the junction box to the chassis. Small white wire.  [3] The bare copper wire from the converter to the frame is the main ground as all 12-volt assemblies run thru the converter."

@Bayliss comments:

The first part of the foreman's statement (i.e., that the copper wire is only for the 120V side) is consistent with what I originally surmised.  It seems obvious at first blush, but I too have long wondered if somewhere along the line, the AC and DC grounds are connected.  The second part of his statement (i.e., that the only ground to the chassis on the T@B 320 is out of the junction box to the chassis) appears to be inaccurate, because there are either two separate grounds (one for AC - - the bare copper wire between the converter and the trailer chassis; and one for DC - - at the chassis/frame bonding screw adjacent to the junction box, OR the DC ground somehow ties in with the AC ground (deliberately, or accidentally.)  The third part of his statement seems to contradict the first two, because he refers to the bare copper wire as the "MAIN" ground, and that all the 12-volt assemblies run through the converter.  The last statement tends to support what I had thought in the past, but I never opened up the the converter to take a look (however, I don't see a common connection in the photo you posted above.)

The second photo included at the beginning of this discussion thread is of my 2019 320 S junction box - -  I am a bit perplexed why the foreman refers to that ground wire  (red arrow) as a "small white wire" (red arrow.)  I wouldn't describe it as "small," because it is a bit more substantial than a small wire (maybe 10 gauge?)  BUT, maybe he meant to say "a short run white wire," since it only extends a short distance between the far right (#7) junction box terminal and the chassis bonding screw (yellow arrow.)]

Too bad we don't have those "in the know" at nüCamp handy to ask these questions in a one-on-one conversation.  That would be enlightening.

WilliamA

If I understand the Crux of the problem as you have described it, it's actually a description of two different problems. While they are probably (almost certainly) interrelated, they are not necessarily causing one or the other. Galvanic corrosion is the naturally occurring consequence of connecting an electrical circuit through a liquid, either by design or by accident. By design, such as the Aldi or your tow vehicles cooling system, there should be no electrical potential in the liquid. If the electrical circuit were "perfect" there would be no electrical potential in the fluid. But moving a fluid through a system generated electrical potential. It's just very small. In most cases, too small to measure. Add to that a couple of dissimilar metals, say, copper and aluminum or iron and you will have the potential for galvanic corrosion. In a non-perfect world, our fluid is constantly moving through the magnetic field created by pump motors, fans etc, polarizing the metallic particles in the fluid. These in turn, react with each other to make up a tiny electrical charge in the fluid. If you try to measure it, you'd find it's hardly there at all. But it's enough potential to create the anode/cathode relationship necessary to (slowly) dissolve one metal in favor of the other. Assuming things are normal, it would take years to cause any damage and regular maintenance easily mitigates the effects. 
The larger problem in vehicles and trailers is the ground system. The perfect d/c circuit originates at, and returns to, the battery via dedicated wire conductors. It doesn't depend upon connection to the vehicle frame to work, so there's no electrical potential in the frame, a.k.a. your "floating ground". Remember: the electrical potential in the ground circuit is exactly the same as the "hot" circuit it's grounding.
In "frame grounded" applications where all ground wires (lights, brakes etc) go to ground with no dedicated ground wire, the circuit is "exposed". That's to say, when you touch the frame, it's electrically charged. The frame essentially IS the ground wire. That "ground wire" is more than adequate to return voltage to the source battery, but all of the various circuit voltages are now fighting to get through the sketchy/dodgy rusted connection points to the battery. Dodgy connections add resistance to a circuit. That resistance drives up amperage (Ohm's Law: e=I*r) in order to do the same work. 
So, adding a frame ground as you describe to a floating grounded circuit should be redundant....except that there's a complicating factor: your tow vehicle. It's all Hoyle until you slam your ball hitch down onto an entire ADDITIONAL electrical circuit. The same rules apply to the tow vehicle system, but hook the two together and you double the electrical problems. I strongly suspect that if you never had your tow vehicle hooked up, you'd not need the short chassis ground wire to the frame. But I also suspect, electrically speaking, that this is what the chassis ground is for on the d/c side. As to why you'd fuse the ground (e.g. Aldi) for any device.... I'm stumped. Makes no sense to me.
The a/c side.... that's another story...

For those who REALLY want to get down into the weeds of electrical circuits, look up: "The Right hand Rule" of current flow. That's how non-connected circuits can be energized by energized circuits simply because the unrelated conducters are in proximity to each other. 
And......
Give me a call if you need some entertainment at your pool party.....

WilliamA

ScottG

@Bayliss, thanks for posting those notes. I agree some comments are ambiguous (even contradictory) but I suspect that results from hasty on communication rather than intentional obfuscation. It does make it difficult to get clear answers...

Opening the power center really doesn't tell one much about the chassis grounds, as any connection between the AC and DC sides would reside in the mysterious innards of the converter/charger itself. However, I can confirm that there is no continuity between the 12VDC negative from one side of the converter, and the bare copper 120VAC ground from the other. 

I can also confirm that that bare copper wire is bonded to the trailer frame. That's the whole point of having it--it prevents a potentially dangerous "hot skin" condition when attached to shore power.

The only connection between these two grounds appears to occur through the trailer frame. 

ScottG

@WilliamA, thank you for your clearly explained insights. As you implied, I suspect the questions regarding grounding may be a red herring with regard to the corrosion issue. I wish I had more insight on exactly what drove the Airstream TSB, and in particular why they prescribed modifying the DC grounds.

I can confirm that the T@B's 12VDC system does not rely on the trailer frame to complete the circuit, as all circuits are hard wired to the 12VDC negative bus, and the bus is hard wired back to the battery. This does seem to make chassis grounding redundant, but it seems that is a hotly debated topic among those smarter than I. (And your point about the tow vehicle is a good one.)

FWIW, here are some interesting discussions that touch on the finer points of DC grounding. Much of it is not relevant (or over my head) but the back and forth about the pros and cons of "grounding" the DC negative common offer a few clues. 

https://control.com/forums/threads/standards-the-negative-leg-in-dc-based-systems.5376/ 

https://control.com/technical-articles/grounding-dc-power-supplies/

This site appears to be a resource for people who geek out on this stuff and therefore (probably) know what they are talking about. At least it shines a little light on why nobody seems to able to offer a simple explanation of DC chassis grounding.  :-/ 

And I'll be reading up on the Right Hand Rule.  :-)

BrianZ

I inspected our 2018 and did not find any DC ground wire connection to the frame in the area of the junction box under the tongue.  Does anyone have a better photo of what that connection actually looks like, because the previously posted photo doesn't really show it or the white ground wire (nice arrow & label though ), but maybe it's wrapped & hidden under black electrical tape..

I don't have much to contribute yet as far as continuity testing goes, but I do have a question - Has anyone removed all wires from the DC ground bus bar & tested for continuity between the bar itself & the frame (or bare copper wire attached to the frame)?  My point is to confirm an actual ground connection to the frame via the bus bar screw.

@ScottG, I noticed that the placement of your DC ground bus bar looks different from ours.  See this photo where ours is behind the converter & close to the wheel well, whereas yours is at the near wall of the compartment, right of the converter..

So I wonder what the different bus bar placements means with regard to whether the screws may or may not be anchored into the frame.  I guess I can remove wires & test again.  I did find continuity between all of the following: bare copper AC ground wire, DC ground bus bar as shown, Alde AC plug ground pin, & Alde outer metal cabinet, for what that's worth.

I would guess that you're right, @ScottG, that stray ground voltages could promote corrosion in the Alde system, or why else would Airstream package the new ground wiring corrections with the Alde fluid update in their service bulletin?    Or was the issue of blown ground fuses seen in some T@Bs also an issue in the Airstreams too?

Still, the questions remain..  If we have no frame ground wire up front for RVIA compliance, shouldn't this issue have been addressed somehow by NuCamp with current owners, as Airstream has done with their Alde ground wiring corrections?  

PS:  @ScottG, Alde's service pump adapters for the glycol expansion tank look rather similar to yours! 

ScottG

Good points, @BrianZ --and interesting catch re: the bus bar placement. Based on its location, it looks like your bus bar could be screwed directly into the trailer frame--IMHO a far better approach than screwing it into an aluminum cross-member. Also interesting that your 2018 doesn't have the forward DC chassis ground.

Whether chassis grounding at the negative bus bar vs. the common negative stud in the forward junction box makes any difference is beyond me. It seems it shouldn't, but nuCamp statements imply the latter is preferred by RVIA. 

I have not removed all the wires from my 12VDC negative bus, but I did remove the one to the converter and confirmed the following. Note that these readings were taken WITH THE ALDE UNPLUGGED. If the Alde is plugged in you will get continuity through the Alde's AC power cord--a potentially confounding variable! 

• No continuity between bus bar and frame when bus bar is removed from floor. 
• Continuity between bus bar and frame when bus bar is screwed to floor with right-side screw (see photo above).
  
• Continuity between right-side screw hole and frame when bus bar is removed.

I also found continuity in all the places you reported.

P.S. I think @gregndeb (the original inventor of the glycol tank adapters) specifically modeled them after the Alde service pump. 

mona

Hmmmm. I stopped in and talked to Austin this week about scheduling maintenance, glycol exchange and whatever is necessary to do to address the corrosion issues. Very last thing he said was, “and we’ll ground it.” 

gulfarea

I still wish Alde would look into using oil instead of antifreeze in their system like oil filled electric home heaters do. Would sure eliminate a lot of problems. Art

MuttonChops

Additional observations:

One.  Difference in DC Ground Bus Bar location between trailers is greater than I had originally thought.

Two.  My Feb-2018 build is like @BrianZ in that there is no Junction Box 'chassis ground' connection.

Three.  When I directly shorted the Battery Positive Terminal to the tub metal frame (long ago accident) two sections of wire completely burned.
    -  Battery Negative Terminal to Junction box.
    -  Junction Box to DC Ground Bus Bar.
       The Bus Bar provided the chassis ground return.
       {black 8-guage wire in picture below is my replacement wire}

ScottG

mona said:

Hmmmm. I stopped in and talked to Austin this week about scheduling maintenance, glycol exchange and whatever is necessary to do to address the corrosion issues. Very last thing he said was, “and we’ll ground it.” 

Oh sheesh. Curiouser and curiouser. There's obviously a game plan here, why can't someone just tell us what it is?

@mona, if you can get any more specifics when you go for your service, we're all ears!

ScottG

gulfarea said:

I still wish Alde would look into using oil instead of antifreeze in their system like oil filled electric home heaters do. Would sure eliminate a lot of problems. Art

Interesting thought, though I'm not sure how well the oil would actually circulate through the convectors. My limited impression of those oil units is that the oil is heated directly but acts more like a heat sink than a transfer fluid. They do work well, though.  

Bayliss

@BrianZ, here are a couple photos showing the chassis bonding screw (yellow arrow), as well as the white ground wire (orange arrow) from the far right (ground connections) terminal in the junction box.  That wire runs through the black wire loom, which also contains the blue and red wires that lead to the emergency brake breakaway switch.   The two red wires that are outside and above the wire loom are the trailer brake wires.

FWIW, the 12V ground bus bar in my 2019 320 S (manufactured in July 2018) is installed in the same location as the photo of the Feb 2018 build posted by @MuttonChops - - see third photo, which I am only including because it shows the ground wire leading from the 12V bus bar to the 12V side of the WFCO converter.







 

mona

@ScottG I won’t be taking it to NuCamp til late fall. Will update then. 

ScottG

@MuttonChops, as with @BrianZ's, your bus bar also looks like it could be screwed into the trailer frame. The frame member under there runs front to back. In contrast, the aluminum floor supports (I assume) run side to side.

My evolving guess is that at some point nuCamp moved the bus so they could bond it more directly to the trailer frame, and at some point after that incorporated the ground wire at the forward junction box (and maybe discontinued the ground at the bus bar). 

@Bayliss, I wonder if yours is bonded to the frame, since you have the ground at the junction box?

MuttonChops

Gosh if only my T@B had the Junction Box chassis ground as seen in @Bayliss 's picture

Replacing my Ground Wire from the Junction Box to the DC Ground Bus Bar was a real pain when I burned out the Battery Ground to chassis connection.

fstop32

I'm getting a real charge following this thread! 
Actually, I am paying close attention since I moved my bus bar (not realizing it might have been grounding where it was, though I don't recall the screws being different).  
I'll be adding the chassis bonding screw to far right (ground connections) terminal in the junction box wire once the weather cools down here. Neither myself or the prev owner has had any problems with the Alde glass fuse blowing so I'll have to think about that project.  I will be adding the kill switch as well.

BrianZ

Thanks for the better photo, @Bayliss.
I guess it wouldn't be too difficult to add a chassis ground wire near the junction box, but I'm not sure we've seen any need for it yet.  Well, except maybe for the reason that @MuttonChops unfortunately experienced.

ScottG

While we await further information or insights on the DC side chassis ground, I'll toss out Part 2 of this topic for rumination and discussion. I avoided this in the OP to avoid going too far down the rabbit hole at once, but I suspect it is related.

There is also a question of "grounding" the Alde cabinet. The Airstream TSB I referenced above prescribes a new wire bonding the cabinet to the 12VDC negative bus. Also, IIRC, connecting the cabinet directly back to the battery was a fix floated by nuCamp back in one of the fuse-popping discussions. I'm wondering ifd this is what Austin was referring to when he told @mona he would "ground" her Alde.

Without getting mired in the minutiae (some of which I'm still wrapping my head around), under certain operating conditions I have found stray voltage potentials between the Alde cabinet and the 12VDC negative bus. These are greatest when the Alde is physically unplugged from its 120V receptacle (and therefore not grounded to the chassis via the AC side).

Again, I'm not sure what it means--or whether it relates to the known Alde issues--but I'm tossing it into the mix for consideration.     

tabiphile

This has become somewhat technical and maybe is beyond the comprehension level of the average mortal trailer dragger. IMHO, let an electrician check your shorts  

ScottG

Understood, @tabiphile. I did put a disclaimer up front...  :-)

Admittedly, this is largely an academic discussion for those of us who geek out on such things. Average trailer draggers will be not be held in contempt if they opt to cruise on by this one. (But if you have something to add, please do!)  

WilliamA

ScottG,
I'm still curious as to why the ground would be fused....
But to your point: Alde chassis grounding. Have you reviewed a schematic of the Alde system? If you have found a differential between the case and ground, that's a potential concern. Barring some Machiavellian ground resistance circuitry (for reference, see: "Lucas") there should be no differential there. 
Curious....
My interest (and this is hard to do) is piqued, and I don't even HAVE an Alde. Please continue to share...
WilliamA

ScottG

@WilliamA, I've no insight either as to why the original Alde design had a fuse on the 12VDC negative side. My vague recollection is that that was the fuse that tended to blow in those units that had persistent fuse-blowing problems.

I haven't reviewed the system schematic, but that's a good idea.

I measured the voltage potentials between the Alde cabinet and 12VDC negative bus at the request of a more electrically knowledgeable member, so I admittedly wasn't completely sure of what I was looking for, or why. The potentials (see below) were small but increased about five-fold if the Alde's AC plug was disconnected. (Note that whether or not the camper was actually plugged into shore power made no difference.)  

Alde running on gas, plugged in to its receptacle:

• OFF: 0.001 volts
• CIRCULATOR (NO HEAT): 0.010
• BURNER (NO CIRCULATOR): 0.017
• BURNER AND CIRCULATOR: 0.025

Alde running on gas, NOT plugged in to its receptacle:

• OFF: 0.005
• CIRCULATOR (NO HEAT): 0.049
• BURNER (NO CIRCULATOR): 0.090
• BURNER AND CIRCULATOR: 0.140

For giggles I ran a temporary wire from the cabinet to the 12VDC negative bus and found these potentials were reduced, and--more interestingly--the the "plugged" and "unplugged" measurements were nearly the same.

What this suggests to my simple mind is that the DC side of the Alde is not independently grounded to the chassis, and relies at least in part on the ground via the AC power cord. This doesn't seem right to me--and sounds like one of those potentially pesky ground loops I mentioned in the OP--but this is where the apparent nuances of DC "grounding" start to escape my current grasp on the subject.  :-)

I'm not a complete geek and as such will be focused on actual camping in the near future, but will pass along any more thoughts or information when and if available.

WilliamA

 
Keep thinking.... 

DanWeitzel

WilliamA said:

ScottG,
I'm still curious as to why the ground would be fused....
But to your point: Alde chassis grounding. Have you reviewed a schematic of the Alde system? If you have found a differential between the case and ground, that's a potential concern. Barring some Machiavellian ground resistance circuitry (for reference, see: "Lucas") there should be no differential there. 
Curious....
My interest (and this is hard to do) is piqued, and I don't even HAVE an Alde. Please continue to share...
WilliamA

Following this interesting thread.  Could the fused ground on the Alde be there to protect from reversed polarity?  Possibility a back feed from the tow vehicle energizing the T@B frame?