Chassis as ground

[john61ct]

Just to clarify, my posts wrt common reference (mobile grounding) do not take into account

AC circuits (use a pro)
preventing galvanic corrosion
lightning protection
EMI interference (e.g. with radio gear)

Also I am not implying there **should** be multiple connections from negative return busses to chassis common

But when there are widely separated multiple power sources and battery banks, different voltage DC systems maybe poorly conductive body work

the point is to **prevent** ground loops, differences in potential between the various References by ensuring solid Common connections.

The Victron example above illustrates this idea, although the multiple "chassis grounds" appear to not be the same Common as the power circuit Negative Return, in fact that would only happen in a **stationary** context, or in a very rare "positive ground" or "floating earth" setup

in a normal mobile system they are (should be) the exact same Common Reference, and cohld indeed all be tied together if e.g. the mounting surface were non-conductive.

And finally, again, the chassis ground should not be used for power current flow, negative return, each load should have a pair of same-gauge wiring for positive and negative, back to the power source.

[flattracker]

One last clarification about the grounds in my Crown. Since I am wiring the bus like one would wire their house, there will be ground wires that connect in the electrical panel and connect to all outlets in the bus. The ground from the outside power cable will be connected in the electrical panel. The ground from the inverter will connect to the ground in the electrical panel. The neutral wire happens to connect to the electrical panel just like in a house. This means all of the grounds in the electrical circuits end up in the electrical panel, and all of the neutral wires end up in the electrical panel also, just like a house. There will not be current flow through the chassis for any AC circuits. There will be current flow through the chassis for DC circuits for the Crown's original wiring. The inverter will not get any power through the chassis as both a negative and positive wire will connect the batteries. The solar array will have positive and negative wires connecting to the charge controller. The second alternator does use the chassis for current flow. At the battery bank in the trunk there will be a negative connection to chassis ground for current flow to charge the batteries from the second alternator. I need to evaluated where the negative connection will go physically. Since the wire will carry 250 amps, it need to be possibly 0000 gauge possibly. The least possible connections to pass through is the best so that any corrosion issues are minimized. Most of the DC circuits in the bus will be for LED lighting and all but one or two circuits will be protected by 5 amp circuit breakers. Where feasible maybe run a negative busline for multiple DC circuits to avoid chassis current flow.

[roach711]

"Ground" is a different concept in AC and DC circuits.

DC circuits have a hot and a ground conductor while AC circuits have hot, neutral conductors and a ground wire. This is confusing to beginners because the return wire in a DC circuit is called a ground wire but in an AC circuit the return is the neutral. The ground wire in AC circuits is only a safety feature and will only carry current when there is a short in the circuit. The fact that DC grounds carry current and AC grounds don't explains why you can (and should) ground your AC panel to the body.

Without an AC ground to the bus body the body can become a 120 volt conductor if a loose AC hot wire were to come in contact with the steel body (search on "RV hot skin"). The ground wire enables the AC breaker to sense the short and trip the breaker. With no AC ground connection a short won't trip the breaker and anyone outside touching the bus could get a nasty shock if they were decently grounded.

Definitely ground your AC breaker panel to the bus body.

[ol trunt]

Once again roach you've said it best and in the fewest words
Jack

[flattracker]

Originally Posted by roach711

"Ground" is a different concept in AC and DC circuits.

DC circuits have a hot and a ground conductor while AC circuits have hot, neutral conductors and a ground wire. This is confusing to beginners because the return wire in a DC circuit is called a ground wire but in an AC circuit the return is the neutral. The ground wire in AC circuits is only a safety feature and will only carry current when there is a short in the circuit. The fact that DC grounds carry current and AC grounds don't explains why you can (and should) ground your AC panel to the body.

Without an AC ground to the bus body the body can become a 120 volt conductor if a loose AC hot wire were to come in contact with the steel body (search on "RV hot skin"). The ground wire enables the AC breaker to sense the short and trip the breaker. With no AC ground connection a short won't trip the breaker and anyone outside touching the bus could get a nasty shock if they were decently grounded.

Definitely ground your AC breaker panel to the bus body.

I agree with Roach

[dzl_]

Originally Posted by roach711

"Ground" is a different concept in AC and DC circuits.

DC circuits have a hot and a ground conductor while AC circuits have hot, neutral conductors and a ground wire. This is confusing to beginners because the return wire in a DC circuit is called a ground wire but in an AC circuit the return is the neutral. The ground wire in AC circuits is only a safety feature and will only carry current when there is a short in the circuit. The fact that DC grounds carry current and AC grounds don't explains why you can (and should) ground your AC panel to the body.

Without an AC ground to the bus body the body can become a 120 volt conductor if a loose AC hot wire were to come in contact with the steel body (search on "RV hot skin"). The ground wire enables the AC breaker to sense the short and trip the breaker. With no AC ground connection a short won't trip the breaker and anyone outside touching the bus could get a nasty shock if they were decently grounded.

Definitely ground your AC breaker panel to the bus body.

With respect, I think you are misunderstanding and misusing some of the terminology and concepts in a way that is adding to the confusion.

Specifically I think your understanding of the term DC 'ground wire' has led to erroneous conclusions about the DC grounding system.


Both AC and DC circuits have two current carrying conductors. Generally referred to as Hot and Neutral in a single phase AC system and Positive (+) and Negative (-) in a DC system.

The terms are unimportant so long as you differentiate the concepts. It is incorrect to say that the return wire in a DC circuit is called the 'ground wire' unless the return path flows through a common ground (such as the vehicle chassis).

Both AC and DC systems can be grounded or ungrounded. It is nearly universal to ground AC systems, with DC, grounding is commonly employed but it is more situational. In both cases, the grounding conductor is not normally a current carrying conductor.

The exception to this (and the source of your confusion I think) is situations like vehicle wiring or some integrated electronics where traditionally the grounding conductor and the negative conductor have been one and the same and both purposes are served by the same conductor. This is only the case in certain applications. It is important to understand that in these situations one conductor is filling both roles.

Confusing the terms confuses the concepts. The negative conductor in a vehicles electrical system came to be referred to as a ground wire because it is a ground wire in that it bonds the negative side of the circuit to common ground. In a system that does not use chassis-return (or a common return path) it is inaccurate and misleading to refer to DC negative as ground. They are fundamentally different concepts.

Technically the terms are:
"grounded conductor" = AC Neutral or DC negative (in a negative ground system), these current carrying conductors are bonded to ground and thus at the same potential / reference voltage as ground.

"grounding conductor" = The grounding system conductors (green, green-yellow, or bare copper) that bond the 'grounded' conductor to ground, or that connect the metal cases of components to ground or the grounding system ('equipment grounding conductors')

In practice is more common we use the terms Neutral or Negative wire for the grounded conductor and 'ground wire'

Further, a DC system does often need a safety grounding system for many of the same reasons an AC system does (safety, fault current path, etc). Discrete DC grounding systems are required and recommended in marine electrical systems, PV systems, RV house systems, etc. If you look at any reputable inverter designed for mobile systems (for example), you will notice the DC side has three terminals Positive, Negative, and a grounding lug for an equipment grounding conductor. And if you read the documentation you will see instructions are given for DC side grounding.

This is not the only component that is often grounded on the DC side, the National Electrical Code requires PV arrays to be grounded, battery banks are frequently bonded to ground with a dedicated grounding conductor and many manufacturers recommend or make provisions to ground solar charge controllers.

The long and short of it is whether AC or DC, the negative current return path and the path to ground are different concepts with different purposes, sometimes they overlap but frequently they don't using the term 'ground wire' confuses these concepts and is not applicable outside of the limited areas where ground and negative are the same thing. Skoolies and RV's straddle this line, on the vehicle side, ground=negative, but on the house side (AC and DC) ground and negative are not the same thing. If you look at any of house side components in your system or solar components you will see the terminals are marked + and - or positive and negative, and on at least a few of them you will see a separate connection for ground. Using the terms interchangeably leads to a lot of confusion, particularly for newbies but even for experienced DIYers. One thing that I agree with you about is the DC side can be different and more confusing than the AC side--one of the reasons I think proper terms are important.

I wish I could convey this in a more clear and concise manner, unfortunately that is not my strong suit. I have consulted the NEC code, the ABYC and USCG code, as well as documentation from a small handful of electrical component and wire manufacturers and am fairly sure none ever conflate or call the grounded current carrying conductor (negative or neutral) a 'ground wire' or anything similar. All of the resources I mentioned can be found here

[john61ct]

And I hate to harp on it, but it is also confusing to use that "ground" term, without distinguishing between the two very separate functions

common reference zero volts

and true Earth ground.

There are plenty of AC systems in a mobile context that do not (often cannot) get earth-grounded.

In fact, only the grid-connected examples "almost always do".

Inverter-driven AC circuits feeding House bank stored energy directly to AC loads

and portable gensets

are two examples that rarely connect to Earth.

[roach711]

Originally Posted by dzl_

With respect, I think you are misunderstanding and misusing some of the terminology and concepts in a way that is adding to the confusion.

Specifically I think your understanding of the term DC 'ground wire' has led to erroneous conclusions about the DC grounding system.

I do understand that the DC "ground" is more properly called a negative or neutral but I basically never hear the word neutral used when describing a DC circuit. There are two licensed electricians in our family and neither uses "neutral" and rarely uses "negative" when describing a DC circuit. Both normally call the DC return wire a ground.

Many DIY'ers understand both 12 volt DC auto electricity and 120 volt AC house wiring without any confusion as to which wire does what. It's when the two systems are combined in the same vehicle and attached to the same steel body that the "ground" confusion raises it's ugly head. Inaccurate popularly used terminology on the DC side definitely exacerbates the problem. No argument there.

Your post is technically correct but inexperienced people are always going to be reluctant to attach two very different electrical systems to the same steel bus body. That just seems wrong until they gain a clear understanding of the interaction of the two.

[dzl_]

Originally Posted by john61ct

And I hate to harp on it, but it is also confusing to use that "ground" term, without distinguishing between the two very separate functions

common reference zero volts

and true Earth ground.

There are plenty of AC systems in a mobile context that do not (often cannot) be earth-grounded.

In fact, only the grid-connected examples "almost always do".

Inverter-driven AC circuits feeding House bank stored energy directly to AC loads

and portable gensets

are two examples that rarely connect to Earth.

I agree with you in part.



There are important differences between chassis-ground and true earth-ground, so it is important to distinguish between the two. Considering that the term ground can also refer to equipment grounding, it is a very vague term without context.


This is why I try to hyphenate the terms (but often forget to) Chassis-ground or Earth-ground when appropriate, and use "ground" as the overarching broad term when it applies. There are differences between chassis-grounded (or local ground) and true earth-grounded systems but they share much in common on a conceptual level, and as a conceptual term 'ground' and 'grounded' can refer to both.



Current does not want to 'go to ground' whether ground is true earth or a vehicle chassis, current flows to its opposite, ground is just a way to get there. A chassis-grounded mobile system has no connection to earth-ground but it emulates much of the same functionality on a local scale. Like earth ground, it is a alternate current path and a common 0V reference.


My understanding is that a mobile off-grid system (i.e. no shorepower and isolated from earth-ground), can still be properly and safely 'grounded' without being connected to earth-ground. A connection to earth-ground only becomes necessary when the power source is earth-grounded. A chassis-grounded system will accomplish the same primary functions (and some of the secondary functions) as an earth-grounded system (1) act as an alternate return path to the power source in case of a fault (2) bond one side of the circuit to a big body of conductive mass (in this case the chassis of the vehicle, in other cases the earth) equalizing potential and creating a shared common 0V reference.


I'm far from an expert when it comes to grounding but I have spend some time trying to understand the logic, and feel slowly I am starting to get there.

[john61ct]

100% spot on afaict.

I still prefer only using "ground" for true Earth.

"Negative Return" vs "Common Reference" are what I use for the other two functions.

Originally Posted by dzl_

A connection to earth-ground only becomes necessary when the power source is earth-grounded.

Necessary is too strong, there are many cases where a professional will install a set of circuits as floating, but that are a special case.

And again I want to emphasize, soon as we are talking AC circuits or especially connecting to grid, DIY errors can destroy stuff, start fires, be fatal.

Get a pro for that, to at least advise & test / inspect.

Although the specific super simple strategy I outlined in my first post above, should be fine.

Originally Posted by john61ct

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.

[dzl_]

Originally Posted by roach711

I do understand that the DC "ground" is more properly called a negative or neutral but I basically never hear the word neutral used when describing a DC circuit.

I agree, If I accidentally implied Neutral was ever used in reference to a DC circuit it was not my intent. I'm not always as clear as I could be. I only use the terms Neutral or Hot in the context of AC.

Any place I wrote Neutral / Negative it should be interpreted as (AC Neutral or DC negative).

DC: Positive, Negative, and Ground(ing)
AC (single phase): Hot Neutral Ground(ing)

These are the terms I use, and the most commonly used terms in my experience, but they are not necessarily the only proper terms or the actual technical terms (there are definitely more correct and precise technical terms but I don't want to overcomplicate things).

I don't care too much which terms are used so long as they match the concept they relate to. I will admit I get a bee in my bonnet about misuse of the term 'ground wire' as it is so frequently misinterpreted and misunderstood, which greats a snowball of misunderstanding.

Both normally call the DC return wire a ground

Maybe in the context of cars or small electronics/ciruit boards or some other application where it applies? Or could they have called it the GroundED conductor which is a technical term that is not the same as a ground wire.

National Electric Code states (at least for branch circuits) DC wiring should be identified by black insulation, or have 'NEG' 'negative' or '-' printed on it.

In any case, calling a wire that does not ground or relate to the grounding system a 'ground wire' can only lead to confusion, particularly considering there are actual ground wires used in DC systems.

Many DIY'ers understand both 12 volt DC auto electricity and 120 volt AC house wiring without any confusion as to which wire does what. It's when the two systems are combined in the same vehicle and attached to the same steel body that the "ground" confusion raises it's ugly head. Inaccurate popularly used terminology on the DC side definitely exacerbates the problem. No argument there.

We agree on this, Vehicle based 'house systems' are like the worst of all worlds (in terms of trying to understand them and use proper terms) One foot in the automotive world, one foot in the RV world, one foot in the residential world, and a few toes in the marine world, multiple voltages, multiple charge sources, AC & DC. It makes your head spin.

Your post is technically correct but inexperienced people are always going to be reluctant to attach two very different electrical systems to the same steel bus body. That just seems wrong until they gain a clear understanding of the interaction of the two.

I agree, it felt 'wrong' to me at first to--for a long time. It wasn't until I started to understand the nature of voltage (and the point John is making about a "0V" common reference) that I started to understand how it could be okay to connect two systems (even of different voltages) to the same designated ground.

[dzl_]

Originally Posted by 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.

Sorry, I missed this comment.


In answer to your original question, I don't think there is any danger in using the vehicle chassis in place of dedicated negative wires. My understanding is that it can be non-optimal in several ways. I have never considered doing it so haven't researched in depth and can't articulate the specific issues, but I have seen the question asked and answered a handful of times and the answers tend to center on: 'you could but it would be better to have proper copper conductors round-trip--specifically with high current devices or sensitive electronics'.

[roach711]

Originally Posted by dzl_

In answer to your original question, I don't think there is any danger in using the vehicle chassis in place of dedicated negative wires. My understanding is that it can be non-optimal in several ways.

No danger in using the body as the negative in a DC circuit, it's been done safely probably as long as there have been batteries in autos and many of the buses being converted here use this system. The main issue with using a chassis as neutral is corrosion at the body-to-chassis and body-to-engine grounding straps (there's that word again). If the strap connection goes south the entire body becomes electrically isolated and all of the body-connected negatives lose their connection to the battery. Corroded connections are a common problem on this forum since most of us are converting old, rusty buses.

There are advantages to the body-as-negative system too. Firstly, the round trip wiring cost is essentially cut in half since there is usually only a short negative wire to attach to the body. The vehicle manufacturers like that a lot. Second, DC circuits are prone to large voltage drops over long wire runs. Since the bus body is a huge conductor you can potentially use a smaller gauge wire to feed a device. Running dedicated negatives on a long circuit run could require a larger wire gauge to properly deal with the voltage drop.

[dzl_]

Originally Posted by roach711

No danger in using the body as the negative in a DC circuit, it's been done safely probably as long as there have been batteries in autos and many of the buses being converted here use this system. The main issue with using a chassis as N̶e̶u̶t̶r̶a̶l̶ negative is corrosion at the body-to-chassis and body-to-engine grounding straps (there's that word again). If the strap connection goes south the entire body becomes electrically isolated and all of the body-connected negatives lose their connection to the battery. Corroded connections are a common problem on this forum since most of us are converting old, rusty buses.

There are advantages to the body-as-negative system too. Firstly, the round trip wiring cost is essentially cut in half since there is usually only a short negative wire to attach to the body. The vehicle manufacturers like that a lot. Second, DC circuits are prone to large voltage drops over long wire runs. Since the bus body is a huge conductor you can potentially use a smaller gauge wire to feed a device. Running dedicated negatives on a long circuit run could require a larger wire gauge to properly deal with the voltage drop.

This seems like a good and balanced overview of the pros and cons.

Beyond what has already been said, I would only add chassis-as-negative return can make troubleshooting more difficult and less predictable.

It is also worth noting, the marine electrical code, which I lean heavily on does not permit chassis-return (chassis-as-negative). Dedicated positive and negative conductors must be used,

[ol trunt]

I ain't got no boat but I'd guess all their electrical has totally floating connections

[john61ct]

As is true for any mobile rig.

There are **lots** of scenarios where using a chassis for negative return could cause fires even explosions and certainly shock fatalities at high power, high voltage.

In corrosive conditions, little mass to start with, split frames loose bolts cracked welds, anywhere resistance of connections between the metal parts increases over time.

Little cheap gadgets you don't care about, go for it.

For me, nothing important, expensive, or high current, use a round trip pair every time.

[BriteLeaf]

Originally Posted by dzl_

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.

You just HAD to confuse the issue, didn’t you? LOL 😂
Seriously though, I ran a #2? Welding wire connected from the negative side of my 24 volt (2x12 volt in series, then all of the 24 volt batteries in parallel) battery bank straight to the frame under the bus 🚌

[dzl_]

Originally Posted by BriteLeaf

You just HAD to confuse the issue, didn’t you? LOL 😂

If there is anything I'm good at, its taking a confusing issue and further confusing it

Seriously though, I ran a #2? Welding wire connected from the negative side of my 24 volt (2x12 volt in series, then all of the 24 volt batteries in parallel) battery bank straight to the frame under the bus 🚌

This is your chassis-ground connection, or also your negative current path for your inverter and other loads?

[BriteLeaf]

Originally Posted by dzl_

If there is anything I'm good at, its taking a confusing issue and further confusing it








This is your chassis-ground connection, or also your negative current path for your inverter and other loads?

I just did that because all of my circuits were floating ground until I actually ground the batteries to the chassis

[wireguy]

OK, let's look at a very plausible scenario. You are running an alternating current system in your bus with a "floating" ground. You haven't grounded your AC system to the chassis, and thus the skin, of your bus.

It rained last night, the earth is wet. An ungrounded conductor in your AC system has gotten disattached from it's insulated connecting point in some piece of equipment or another. Coffee pot, microwave, whatever. As is completely normal and happens all the time, that ungrounded "hot" current has found a path to the metal skin of your bus. A little boy or girl in bare feet walks over to your bus and touches the skin. Now the ungrounded current that is seeking to get to ground, or earth, which IS grounded at every transformer whether overhead or underground, leaves the skin of your bus, flows through the body of the child to earth, and electrocutes them, because some EXPERT told you not to ground your alternating current system to the chassis.
Treat your bus exactly the same as you would treat a steel building, which REQUIRES your AC system to be grounded to the frame and skin. Ground your AC system to the frame and skin at multiple places if you are so inclined. Now when the ungrounded conductor manages to come in contact with your frame or skin, the ungrounded current that is pushing at 120 volts of electrical pressure to get to ground, or earth, will instantly flow through the comparatively low resistance current path of your steel infrastructure, into the copper conductors you grounded it to your AC grounding system with, into your service panel ground bus, back through the equipment ground wire in your shore power conductors, where it will flow FAR more current than the circuit breaker your shore power is rated to trip out at, and trip that breaker and MAKE SAFE (by shutting off) your entire bus AC system. This will happen near instantly on any kind of dead short. Not so much with a high resistance short which is why we are so careful to build our AC system well and use quality components.

Yes you can use the bus frame for the ground on both your AC and your 12 volt systems. These two systems do not recognize one another and any current flowing in your 12 volt system won't get crossed up with the current in your AC system, and vice versa. This assumes you are keeping those two systems isolated from one another except for the shared grounding on your bus frame.

I am running exactly this system in my '77 Gillig and it works just fine. Steel frames, especially in salt country aren't the best conductors out there so yea, ground your chassis to to the negative side of your 12 volt system at multiple points if your 12 volt stuff isn't behaving properly. I ran a #10 copper conductor from my 12 volt negative to the rear of my chassis (batteries are up front) just to make sure. You can also just run a #10 copper conductor to wherever your 12 volt stuff is and connect to it wherever needed, and not use the frame at all, which would be a good idea if your frame is corroded.

All of the 12 volt commentary is not necessarily correct if you are running solid state electronics like an inverter. In that case refer to the manufacturers instructions. Solid state is a law unto itself, but by and large the system I have outlined will work.