Chassis as ground

[Bpike]

My conversion plan consists of a 12v DC system (lighting and water pump) and a 110v AC for fridge, AC, hot water heater, microwave, induction hot plate, and a charger for the batteries. I am initially thinking a separate pair of batteries for the 12v system to keep the bus chassis system separate from the conversion system. Is this foolish, should I just tap into the bus batteries for the 12v, and add a shore charger to it, and be done? Also if I do go separate 12v system, can I use the chassis for ground to keep from running ground wires, any danger here with both 12v systems using a common chassis ground?

[Danjo]

I am not an electrician. This is only based on my continuing research and understanding of vehicle-based AC systems.

As I understand, you shouldn’t ground AC to the chassis. You would do a “floating” ground where all the AC equipment is isolated from the chassis and ground is carried by an equipment grounding conductor. If you have a shore power connection then the floating ground would connect to the shore power ground at the pedestal.

[Bpike]

No intent to ground the 110V to chassis, that would all be through a conductor back to the breaker panel. My question is on the 12V system.

[dzl_]

Originally Posted by Bpike

My conversion plan consists of a 12v DC system (lighting and water pump) and a 110v AC for fridge, AC, hot water heater, microwave, induction hot plate, and a charger for the batteries. I am initially thinking a separate pair of batteries for the 12v system to keep the bus chassis system separate from the conversion system. Is this foolish, should I just tap into the bus batteries for the 12v, and add a shore charger to it, and be done? Also if I do go separate 12v system, can I use the chassis for ground to keep from running ground wires, any danger here with both 12v systems using a common chassis ground?

First lets clarify some terms. When talking about ground and grounding DC systems in vehicles the terminology is a mess. Can you define what you mean by ground.



Many people, particularly those with experience/history working on cars, erroneously refer to DC negative (the return conductor) as "ground or the ground wire." In vehicles (parficularly older vehicles often the returned conductor and ground are the same thing--the vehicle chassis, but conceptually these are two different and not to be confused things "chassis-return" (using the vehicle chassis as the negative current path, and "chassis-ground" (using the vehicles chassis as your common ground reference and fault path--similar to earth ground in a building)



DC has two current carrying conductors, positive (usually red) and negative (usually black, sometimes yellow in marine systems), additionally there is sometimes a green, bare, or green and yellow non-current-carrying conductor grounding conductor.



So the answer to your specific questions will depend on what you mean by ground


It is generally felt that you should not use the chassis as the negative return path (DC negative) unless you have a specific and well thought through reason to so, even cars have moved away from this in recent years I have heard.



If you are talking about both systems sharing chassis ground as the ground reference, than yes, in most situations this is okay and normal. You could even have systems of different voltages sharing the same chassis-ground reference.

[ol trunt]

Bpike,

"-if I do go separate 12v system, can I use the chassis for ground to keep from running ground wires, any danger here with both 12v systems using a common chassis ground?"

Yes you can and no there isn't.
Jack

DZL, the only time I'm seeing non chassis based negative grounding on a DC system it is in relation to multiplex wiring where a "ground wire" can, under specific conditions serve as the positive wire for a second application eg. low and high beam headlamps.

[Danjo]

Originally Posted by ol trunt

Bpike,

"-if I do go separate 12v system, can I use the chassis for ground to keep from running ground wires, any danger here with both 12v systems using a common chassis ground?"

Yes you can and no there isn't.
Jack

DZL, the only time I'm seeing non chassis based negative grounding on a DC system it is in relation to multiplex wiring where a "ground wire" can, under specific conditions serve as the positive wire for a second application eg. low and high beam headlamps.

It’s really interesting that on my Thomas almost every circuit has a negative wire also run for that circuit while the Chevy cutaway uses the chassis as a return path and the after market (I assume) installations like the wheelchair lift also
Use the chassis.

Seems like the chassis would make an acceptable ground. Especially if fastened to the same frame rail or other contiguous piece of steel.

Since Thomas started it, I decided to continue the practice of running a separate ground for each circuit. Is it better? I don’t know.

[dzl_]

Originally Posted by ol trunt

non chassis based negative grounding on a DC system

I'm struggling to parse this term. Can clarify what you mean.



[non chassis based] [negative grounding] on a [DC system]

Taken on its own, I would interpret the above to mean:

Grounding the negative side of a DC system to something other than the vehicle chassis.


But based on context, I think I am not correctly understanding your meaning.

[john61ct]

Did not read the thread.

Ground as a term should refer to proper Earthing, copper rod into wet subsoil.

Obviously not relevant to mobile contexts.

REFERENCE and COMMON voltage level are the proper terms

Chassis as in electronics device metal casings, vehicle frame, engine block, are the closest we get, all DC circuits' NEGATIVE RETURNS tied together so they share a common zero-Volts reference.

The AC circuits are not the same, for those consult with a professional electrician for safety.

My strategy is, the ONLY connection from my vehicle or trailer to mains/grid AC shore power is via my DC charger.

All AC load devices are powered via inverters fed from the House bank.

[john61ct]

DC systems have positive (power feed side) and negative (return side).

The latter in all cases should IMO be "home runs" back to the power source (e.g. House bank).

But they should also all have a **reference zero voltage** in COMMON, via connections at multiple locations to the vehicle's body/frame and engine blocks, just as the alternator and starter do with the Starter battery and DC engine electrics.

Even if you have a set of 24V or 48V DC circuits, their Negative Returns all should share the same Common Reference as your 12V circuits.

None of the above has anything to do with "grounding" in the sense of a connection to true Earth.

[dzl_]

Originally Posted by john61ct

Did not read the thread.

Ground as a term should refer to proper Earthing, copper rod into wet subsoil.

Obviously not relevant to mobile contexts.

REFERENCE and COMMON voltage level are the proper terms

Chassis-Ground is a proper and commonly used term as well (as differentiated from Earth-Ground, or Earth)

[john61ct]

Sure, that makes it clear.

I like Reference and Common because it reminds that the actual potential level as an absolute is arbitrary

Whereas Ground implies a fixed standard

it really is the **difference** in potential that matters, and where there are different levels in a system, they need to be brought together.

[dzl_]

Originally Posted by john61ct

DC systems have positive (power feed side) and negative (return side).

The latter in all cases should IMO be "home runs" back to the power source (e.g. House bank).

But they should also all have a **reference zero voltage** in COMMON

We are in agreement here and this is what I tried to articulate above. I think much of the confusion comes from people misunderstanding/misusing the concept of ground.


In a DC system, the current carrying conductors are positive and negative, 'ground' and grounding conductors should not carry current in normal operation. The only exception to this is when the chassis is used as both 'chassis-ground' (chassis acts as common reference and fault path) and chassis-return (chassis as negative conductor), but as discussed below, this is not the best practice.



Ideally dedicated positive and negative (return) wiring should be used. Generally the negative is bonded to chassis, usually at the negative busbar or battery terminal, and ideally at just one place in the system.


The Marine Code goes so far as to mandate dedicated positive and negative wiring.




I am not sure as to whether applicable RV codes permit the house side of the system to use chassis negative return or not, regardless of whether it is permitted you will find many that advise against it.

[dzl_]

Originally Posted by john61ct

Sure, that makes it clear.

I like Reference and Common because it reminds that the actual potential level as an absolute is arbitrary

it really is the **difference** in potential that matters, and where there are different levels in a system, they need to be brought together.

I agree, 'common, and reference' do illustrate an important conceptual component of what ground is/does.


Actually I would say learning that point, and really absorbing it was one of the most things that helped a complicated concept feel a bit more intuitive to me. Its difficult to understand the nature of ground if you don't understand the nature of voltage.



This video does a good job of explaining this point, particularly if you also watch his video on voltage:
https://youtu.be/KXjSSvIrKUw

[john61ct]

Exactly.

Which is why I try to avoid that G-word in a mobile context

since its connotations for most imply the idea of true Earth

which is irrelevant here

[Danjo]

I’ve been getting marine grade DC hardware and while at West Marine I’ve seen that some devices have a terminal for negative and a separate equipment grounding conductor.

Just an observation that’s a little applicable to this conversation.

[john61ct]

I never use chassis as negative return, for anything

expensive
important, or
high current

Round trip wiring to the power source for negative return is the way to go for long term reliability and safety.

Especially with chassis composed of ferrous metals, over time connections can easily become high-resistance, actual current path unpredictable.

That is also why in a larger / complex situation, there should be **multiple** Chassis Reference connections equalizing the level potential throughout all DC systems.

The "single Ground point" guideline is for high-voltage AC systems, and maybe also preventing galvanic corrosion in boats

both situations requiring a professional IMO.

[Bpike]

Yes, I should know better than to call this a “ground”. Guess what I meant was using the chassis as negative. Like strap the negative post of the battery to the chassis, and then only run a single positive wire from the fuse block to the 12v device, then the other side of the 12v electrical device to the nearest piece of steel.

[dzl_]

Originally Posted by Danjo

I’ve been getting marine grade DC hardware and while at West Marine I’ve seen that some devices have a terminal for negative and a separate equipment grounding conductor.

Just an observation that’s a little applicable to this conversation.

Possibly very applicable! Well--maybe--OP still hasn't clarified specifically what he is asking about so personally I'm not 100% sure I properly understand the question.



Many devices--most notably marine/mobile inverters--but also other devices have a designed case ground lug intended to tie in with the equipment grounding system. I think this example schematic from Victron does a good job of illustrating how they intend a simple system to be wired (note: dedicated Positive, Negative, and equipment ground conductors):




One place my understanding (and I can't say I have a full understanding) differs from John is that I recall the best practice for small/medium mobile or marine systems is that the connection between negative and ground should occur at one point (typically off the battery negative, or negative busbar, as in the image above). I can't say I understand all the reasons for this, but I believe it is in part to minimize ground loops and undesirable current paths. I don't think anything calamitous will occur if you don't follow this guidance, but I believe it is a best practice. I will try to see if I can find a reference for this.

[dzl_]

Back to your original question (which for me at least still needs some clarification):

Originally Posted by Bpike

I am initially thinking a separate pair of batteries for the 12v system to keep the bus chassis system separate from the conversion system. Is this foolish

This makes sense and is a very standard way to do things (separate house and starting battery banks)

Originally Posted by Bpike

should I just tap into the bus batteries for the 12v

No, at least not unless you have a pressing need to do so.

Also if I do go separate 12v system, can I use the chassis for ground to keep from running ground wires, any danger here with both 12v systems using a common chassis ground?

This is the portion that still needs clarifying. Are you referring to:
A: the current return path (back to the battery negative) i.e. 'chassis-return'
B: the common ground reference and fault return path, i.e. 'chassis-ground'

[flattracker]

I will add a couple cents to the discussion about grounding electrical on Crowns. Both of my Crowns used the frame/chassis as the negative/ground connection. The engine is connected electrically to the frame as negative. The alternator and starter also rely on the ground/chassis as negative. Examination of the control box wiring of the 6.5kw generator finds that the neutral is connected to the terminal that is connected to the metal control box housing, which in turn connects through a ground strap to the mounting cradle that secures the generator set to bus chassis. When a switch is made from generator power to outside (shore) power only the connections for the "hot" wires is switched. This means that the green ground IS connected to the chassis of the bus. On the old Crown I used only 120 VAC but it too was connected with ground connected to the chassis. I never had issues with the old Crown electrical. When connected to external power the bus will connect to whatever earth ground is supplied through that source.