Multiple Chassis Grounds OK?

[Sinjins]

Hello All. I have a question about having multiple chassis grounding points...

The situation background:
I have converted an ambulance (Ford F-350 Super Duty chassis) into a camper. The rear box is entirely (and robustly) built with aluminum. The ambulance came with a complex 12V DC distribution circuit board/panel in the rear box with each DC circuit separately fused and relayed (fuses and relays mounted on a circuit board). Next to that existing rear DC distribution panel/circuit board is a beefy grounding block where all of the existing rear box DC circuits are connected to for their individual grounds. That grounding block is then connected with a beefy cable to the steel truck chassis underneath the rear box. The rear distribution panel is connected to the two 12V starter batteries under the hood of the vehicle via beefy DC wire as well (with a master battery disconnect switch/solenoid in between to turn off the whole system).
FYI, Ambulances out-of-the-box are meant to have their engines running (or connected to shore power) in order to run the rear box and related emergency equipment. As I am planning on boondocking/remote camping, relying on an engine running or shore power to run equipment is a definite no-can-do. So I invested heavily and have installed (more like mounted at this point) a separate power system on the opposite side of the rear box compared to the existing DC distribution panel in order to provide power to the rear box in a more autonomous fashion (no requirement to have engine running or shore power). I have installed a 700W rooftop solar array, an appropriately sized MPPT charge controller, an-in-one Inverter/Charger/Transfer Switch (i.e. a 3000W Victron MultiPlus II 2x120V) along with a Victron Lynx Distributor (a fancy efficiently-designed fused positive and negative bus bar device), and a Sterling 60A DC-DC charger, all powered by a 300Ah LiFEPO4 battery bank, and am now currently wiring it all up.

The grounding question:
I have a question about grounding to make sure I haven't done (or plan to do) anything incorrectly in that regard. As the new Victron-based system is physically located on the opposite side of the rear box compared to the pre-existing 12V DC distribution panel (and its associated grounding block mentioned in the first paragraph), I have installed its own 2/0 chassis ground wire from the negative bus bar of the Victron Lynx Distributor to that side of the truck chassis, rather than run a big ground wire from that aforementioned bus bar through/across the interior of the rear box to the drivers side in order to somehow connect to the pre-existing grounding block in the existing cabinet (also not really any room to connect to that grounding block given number of ground wires already connected to it). So now as a result there are 2 separate chassis ground wires connected to 2 separate grounding points associated to the systems in the rear box, one on the drivers side chassis frame rail and the other on the passenger side chassis frame rail. First, is that ok? Second part of the question is: If I now run the necessary DC power wire(s) across to the pre-existing DC distribution panel from the Victron Lynx distributor, effectively replacing the wire running from the truck's starter batteries so that the pre-existing rear DC circuits are powered from the LiFEPO4 battery bank instead of the starter batteries, but leave the two existing separate chassis grounds in place, is that a problem or incorrect at all?

 

[FilterGuy]

There are 3 different aspects to this.

1) Functionality:
If you connect to two different pieces of metal, can you assure there is a good electrical path between them? Are there connections between parts of the chassis that are not welded and might have paint or oxidation that can create a resistive path? However, if you can assure a good connection to the chassis and insure that the path through the cassis is solid it can work fine.

2) Safety: In general, if you have a solid path through the chassis, there is no concern with safety. (but if there are resistive points in the path they can get hot)

3) Radio Frequency Interference.
Inverters tend to create a lot of radio frequency noise and ground loops can become antennas for broadcasting that noise.
(If from one point on the ground you can trace two different paths to another point.... you have a ground loop). You should avoid ground loops as much as is practically possible.... but sometimes it is impossible to avoid all of them.... particularly in a mobile install.

 

[Zil]

One way to avoid radio interference on DC circuits is to have the positive and negative cables separated, not run close together. Or done as twisted pairs, not always easy with heavy cables.
As Filterguy stated, Your negative "grounds" depend on the quality of the connections as well as the integrity of the metal chassis.
I always prefer dedicated negative cable returns for critical loads. Devices such as inverters, chargers, and safety equipment.
I bet the ambulance has an alternator that can do a job on charging. I would think about having a Battery to Battery charge controller.

 

[Sinjins]

Hello @Zil . Yes as mentioned in my post, I have a Sterling 60A DC-DC charger that I intend to use to connect the rear LiFEPO4 battery bank to the starter batteries. There are two alternators under the hood of this rig. What do you mean by "dedicated negative cable returns", exactly?

 

[Sinjins]

Thanks, @FilterGuy. Very informative answer. I'll have to look into how I can test the connectivity of the path through the chassis metal between the two grounding points. However, before creating the new second grounding point, I did make sure I got down to bare metal and also used some electrical conductive grease (used to prevent corrosion on battery terminals) where I exposed the bare metal and attached the stainless steel grounding bolt/lock washer/washer to the frame.

 

[John Frum]

What is the bonding lug made out of?
What is it connecting to?

Where are talking about the multiplus case ground and not the negative current carrying conductor, confirm?

 

[time2roll]

Multiple connections to the frame is fine. Same as multiple connections to a ground bus. Most items should have a bonding connector to the frame.
There is truly only one ground and that is the ground connector to the pedestal supplying utility power. Otherwise the frame is a bit of a floating ground and is perfectly normal.
Just keep the 120v neutral and ground separate. These should only be bonded from an internal connection with on-board power source such as inverter or generator.

 

[Sinjins]

What is the bonding lug made out of?
What is it connecting to?

@smoothJoey I used a wire wheel to get down to bare metal on the main driver's side frame rail of the chassis, then coated bare metal with electrical conductive grease (normally used on battery terminals to prevent corrosion), then installed 2/0 x 5/16" cable lug (tin plated copper lug) to the frame via a sturdy M8 (I think) stainless steel bolt/lock washer/washer. Is that the info you are after?

 

[Sinjins]

@smoothJoey You asked "Where are talking about the multiplus case ground and not the negative current carrying conductor, confirm?"

So there are three different 2/0 cables connecting the MultiPlus II Inverter to the Lynx Distributor. One positive (to a positive bus bar terminal on Lynx Distributor), one negative (to a negative bus bar terminal on the Lynx Distributor), and then a third one which is the case ground of the inverter which I connected to the recommended center lug of the negative bus bar of the Lynx Distributor. Finally, the Lynx Distributor is connected to the chassis also via a 2/0 cable from the load side of the smart shunt (which sits between the battery bank and the Lynx Distributor on the negative side). Hopefully that answers that question for you?

 

[John Frum]

@smoothJoey I used a wire wheel to get down to bare metal on the main driver's side frame rail of the chassis, then coated bare metal with electrical conductive grease (normally used on battery terminals to prevent corrosion), then installed 2/0 x 5/16" cable lug (tin plated copper lug) to the frame via a sturdy M8 (I think) stainless steel bolt/lock washer/washer. Is that the info you are after?

Yes.
Well stated.

 

[John Frum]

@smoothJoey You asked "Where are talking about the multiplus case ground and not the negative current carrying conductor, confirm?"

So there are three different 2/0 cables connecting the MultiPlus II Inverter to the Lynx Distributor. One positive (to a positive bus bar terminal on Lynx Distributor), one negative (to a negative bus bar terminal on the Lynx Distributor), and then a third one which is the case ground of the inverter which I connected to the recommended center lug of the negative bus bar of the Lynx Distributor. Finally, the Lynx Distributor is connected to the chassis also via a 2/0 cable from the load side of the smart shunt (which sits between the battery bank and the Lynx Distributor on the negative side). Hopefully that answers that question for you?

I'm not myself today so if I say something silly feel free to mock me.
I think you said the new system will be connected and supply power to the old system, confirm?
What AWG positive and negative wire did you run between the 2 systems?

Assuming the new system and old system are interconnected, how bout just making a jumper from the multiplus grounding lug to the multiplus negative stud?
That should take care of grounding the multiplus.
Make sure the jumper is thick enough to handle the current allowing the over-current protection to trip.

The negative wire between the old and new system busbars will keep them at equi-potential.
Since the old system is grounded the new system will be too.
If you do this, I think any ground loops would have to be a legacy issue.

Am I missing something?

 

[John Frum]

@FilterGuy do you know why we don't jumper the case grounding lug on a device to its dc negative lug?

 

[Sinjins]

I'm not myself today so if I say something silly feel free to mock me.
I think you said the new system will be connected and supply power to the old system, confirm?
What AWG positive and negative wire did you run between the 2 systems?

Assuming the new system and old system are interconnected, how bout just making a jumper from the multiplus grounding lug to the multiplus negative stud?
That should take care of grounding the multiplus.
Make sure the jumper is thick enough to handle the current allowing the over-current protection to trip.

The negative wire between the old and new system busbars will keep them at equi-potential.
Since the old system is grounded the new system will be too.
If you do this, I think any ground loops would have to be a legacy issue.

Am I missing something?

Hi @smoothJoey . No worries. I'll attempt to answer your questions as you posed them:

I think you said the new system will be connected and supply power to the old system, confirm?
Answer: Correct.

What AWG positive and negative wire did you run between the 2 systems?
Answer: I have not connected the new DC system to the old DC system yet, but I plan to. I'm not sure of the gauge of the positive and negative wires I'll need to do that yet. However, my current guess is "at least" #2 AWG. When I disconnect the old feed to the rear box from the starter batteries, I'll check what gauge those wires are, and at least match that from the new system. Or at least that is my current plan unless told otherwise. Maybe upsize one gauge?

Assuming the new system and old system are interconnected, how bout just making a jumper from the multiplus grounding lug to the multiplus negative stud?
That should take care of grounding the multiplus.
Make sure the jumper is thick enough to handle the current allowing the over-current protection to trip.
Response: I wasn't savvy enough to think of that, and was following a diagram of a similar system from explorist.life. Hence, I already grounded the MultiPlus II case to the Lynx Distributor negative bus bar and then grounded that to the chassis (i.e. the 2nd chassis ground described earlier in this thread).

The negative wire between the old and new system busbars will keep them at equi-potential.
Since the old system is grounded the new system will be too.
If you do this, I think any ground loops would have to be a legacy issue.
Response: Thats interesting. This whole grounding vs. negative is a new concept to me, and I still had/have unknowns about how I was going to replace the power source from the old system with the power from the new system, so I instead took the "how to eat an elephant approach". One bite at a time. My first bite was to mount the components of the new system and install the new battery bank. Second bite was to connect the new battery bank to the new Lynx Distributor (via 400 A fuse and battery disconnect on positive side and then negative via the new smart shunt). Then connected the new MultiPlus II to the Lynx Distributor and the Lynx Distributor to the chassis. The third bite was to connect the shore power (AC in) to the MultiPlus II and AC out to the new 30A AC distribution panel. The fourth bite was/is to get the solar modules connected to the solar charger and then to the Lynx Distributor (I am doing this step today). The 5th step (and maybe most difficult/complicated step to me at least) is to figure out how best to replace the DC feed powering old system with the DC power from the new system, at the same time making sure grounding is appropriate - hence my original question about having two chassis grounds. The 6th and final step is going to be how to best connect my new 60A Sterling DC-DC charger between the Lynx Distributor and the starter batteries (positive wire would be fused). Do I try to utilize the old DC feed wires coming to the rear box from the starter batteries on the other side of the box? Or do I just run brand new wires directly to the starter batteries from the Lynx Distributor? I suppose that answer depends on two factors: A: existing wire gauge (is it enough for the 60A DC-DC charger) and B: how easy is it to connect to/leverage the old existing wires feeding the other side of the box.

Thoughts? Is this approach that I've taken make sense for a novice, or is it going to bite me sometime soon?

 

[12VoltInstalls]

I’m not a fan of using a chassis or frame as the ground for a system that could potentially be in the hundreds of amps. In fact don’t like it anytime.

I like (-)negs to a stud or bus then a homerun to the bat

 

[John Frum]

Hi @smoothJoey . No worries. I'll attempt to answer your questions as you posed them:

I think you said the new system will be connected and supply power to the old system, confirm?
Answer: Correct.

What AWG positive and negative wire did you run between the 2 systems?
Answer: I have not connected the new DC system to the old DC system yet, but I plan to. I'm not sure of the gauge of the positive and negative wires I'll need to do that yet. However, my current guess is "at least" #2 AWG. When I disconnect the old feed to the rear box from the starter batteries, I'll check what gauge those wires are, and at least match that from the new system. Or at least that is my current plan unless told otherwise. Maybe upsize one gauge?

Assuming the new system and old system are interconnected, how bout just making a jumper from the multiplus grounding lug to the multiplus negative stud?
That should take care of grounding the multiplus.
Make sure the jumper is thick enough to handle the current allowing the over-current protection to trip.
Response: I wasn't savvy enough to think of that, and was following a diagram of a similar system from explorist.life. Hence, I already grounded the MultiPlus II case to the Lynx Distributor negative bus bar and then grounded that to the chassis (i.e. the 2nd chassis ground described earlier in this thread).

The negative wire between the old and new system busbars will keep them at equi-potential.
Since the old system is grounded the new system will be too.
If you do this, I think any ground loops would have to be a legacy issue.
Response: Thats interesting. This whole grounding vs. negative is a new concept to me, and I still had/have unknowns about how I was going to replace the power source from the old system with the power from the new system, so I instead took the "how to eat an elephant approach". One bite at a time. My first bite was to mount the components of the new system and install the new battery bank. Second bite was to connect the new battery bank to the new Lynx Distributor (via 400 A fuse and battery disconnect on positive side and then negative via the new smart shunt). Then connected the new MultiPlus II to the Lynx Distributor and the Lynx Distributor to the chassis. The third bite was to connect the shore power (AC in) to the MultiPlus II and AC out to the new 30A AC distribution panel. The fourth bite was/is to get the solar modules connected to the solar charger and then to the Lynx Distributor (I am doing this step today). The 5th step (and maybe most difficult/complicated step to me at least) is to figure out how best to replace the DC feed powering old system with the DC power from the new system, at the same time making sure grounding is appropriate - hence my original question about having two chassis grounds. The 6th and final step is going to be how to best connect my new 60A Sterling DC-DC charger between the Lynx Distributor and the starter batteries (positive wire would be fused). Do I try to utilize the old DC feed wires coming to the rear box from the starter batteries on the other side of the box?

Depends on how much current will flow and how much much distance related voltage drop there is.

Or do I just run brand new wires directly to the starter batteries from the Lynx Distributor?

If the legacy stuff is good enough then I suggest to go with it.

I suppose that answer depends on two factors: A: existing wire gauge (is it enough for the 60A DC-DC charger) and B: how easy is it to connect to/leverage the old existing wires feeding the other side of the box.

Yes ease of implementation is also a consideration.

Thoughts? Is this approach that I've taken make sense for a novice, or is it going to bite me sometime soon?

If the old system is bonded to the chassis and the new system is connected to the old system I would not bond the lynx busbars to the chassis.
Its an unnecessary ground loop.
But since you have already done it maybe just leave it alone unless you get RFI or some other issue.

Is the dc2dc charger isolating or non-isolating?

They could also be floating if you remove the legacy

non-isolating dc2dc chargers typically have a common negative.
Isolating ones have negative in and negative out.

 

[Sinjins]

Depends on how much current will flow and how much much distance related voltage drop there is.

If the legacy stuff is good enough then I suggest to go with it.

Yes ease of implementation is also a consideration.

If the old system is bonded to the chassis and the new system is connected to the old system I would not bond the lynx busbars to the chassis.
Its an unnecessary ground loop.
But since you have already done it maybe just leave it alone unless you get RFI or some other issue.

Is the dc2dc charger isolating or non-isolating?

They could also be floating if you remove the legacy

non-isolating dc2dc chargers typically have a common negative.
Isolating ones have negative in and negative out.

Ok, I'll leave both chassis grounds when I connect the two systems, and if I run into any RFI problems I can maybe remove the new chassis ground I placed from the Lynx Distributor.

Is the dc2dc charger isolating or non-isolating?
Answer: I'm not entirely sure, to be honest. It is a Sterling BB1260. Here is an excerpt from the manual of that DC-DC charger:

 

[John Frum]

Ok, I'll leave both chassis grounds when I connect the two systems, and if I run into any RFI problems I can maybe remove the new chassis ground I placed from the Lynx Distributor.

Is the dc2dc charger isolating or non-isolating?
Answer: I'm not entirely sure, to be honest. It is a Sterling BB1260. Here is an excerpt from the manual of that DC-DC charger:

It has a common negative so its non-isolating.
It's common negative can be connected to either the old or new negative busbar.
Since the starter battery bank negative is bonded to the frame then you can use the chassis as the return path.
Steel is much less conductive than copper but there is so much more of it that it is worth doing?
Where is the starter battery bank and how is the old system connected to it?

 

[John Frum]

The other thing is that since both busbars are bonded to the chassis you don't need a negative interconnect between them.
Just interconnect the positive busbars.
Each positive wire should be fused at close as possible to the charge source.
I would connect the dc_2dc charger to the new busbars is possible.