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24v GROUNDING HELP and overall critique

So, as regards grounding on an RV setup, with a multiplus inverter, where it can sometimes have a grid connection, which diagram below has the correct grounding configuration .... A, B or neither...please explain your conclusions(Grounding cables in green)

A
1 RV electrical layout.png

B
1 RV electrical layout vers 2.png
 
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I think it is important to realize that grounding is not a strict "right or wrong" choice. Depending on your application will determine how you need to ground or bond your system.
I agree with this, for me at least, grounding is a bear to really wrap my head around, and fully understand conceptually, particularly for unorthodox systems where there aren't necessarily established 'best practices' or clear code to follow.

And as you say its pretty situational and from what I've gathered thus far different approaches have trade-offs. Mobile systems are particularly confusing (for me at least) due to having a variety of charge sources, and a more complicated and less orthodox system than most homes, and having a chassis which can emulate some but not all of the functions of true earth-ground. Victron describes chassis-ground as being like a "Local Earth" ground.

The best way to understand ground is to think of it as a reference voltage of 0V for the system. Or as a sink for excess electrons. That is how I make sense of it all.
Understanding this definitely helped me make sense of it as well. This video was very helpful for me in understanding ground conceptually. It also taught me I hadn't truly conceptually grasped voltage either.

But a big point in my original comment is that the chassis is not a true earth ground. It is insulted from earth ground due to the tires of the vehicle, so excess charge accumulation will have nowhere to go (if that is your intentions with grounding). You can still use the chassis as a negative bus, but there are no benefits compared to using a tinned copper bus bar.
To clarify, is what you are envisioning/describing keeping the DC negative and the vehicle chassis totally separate/isolated from one another. Or are you describing using dedicated DC negative and ground conductors connected to a busbar which is then bonded to chassis and the negative battery terminal (such as in the image below):
victron-grounding-example-simplified.png


My still developing understanding is that either option is viable. But one advantage of the design that is pictured (DC system is grounded to chassis) is that by creating a bond between (1) DC negative (2) The Equipment grounding system (3) Chassis-Ground, you create a common reference and equalize potential between these three components. And while it may not be a 'current sink' to the same extent as true earth ground, I think it emulates this functionality on a localized level (by nature of being connected to the negative battery terminal, and being a massive hunk of metal). Current would not normally flow through the chassis, but any stray current would have a path back to source (and as @tazmann pointed out) a better chance of tripping a breaker if a fault occurred between DC positive and ground. So in short, I think either system can work safely, but I feel there are some advantages to a chassis-grounded system as pictured above. At least that is my current understanding (and I am not confident enough in my understanding to feel like it carries much weight, but I do feel I'm starting to grasp some of the complexities).


I suggest for anyone trying to wrap their head around grounding in a mobile context (with the Caveat that as Will mentioned, it is complicated and contextual), read or skim these excerpts on grounding from Samlex, Magnum, and Victron.

Short excerpt from the Samlex Evo Manual:
3.11 Grounding to earth or to other designated ground
For safety against electrical shock, the metal chassis of unit is required to be grounded to the Earth Ground or to the other designated ground. for example, in a mobile RV, the metal frame of the RV is normally designated as the Negative DC ground / RV Ground. Similarly, boat Ground is provided in the boats.
An Equipment Grounding Connector (5 in fig. 3.8) has been provided on the metal chassis for connecting to the appropriate ground through the Equipment Grounding Conductor (EGC). This Equipment Grounding Conductor (EGC), when appropriately bonded to Earth Ground, helps to prevent electric shock and allows over-current devices to operate properly when ground faults occur. The size of this conductor should be coordinated with the size of the over-current devices involved. Marine installations require the size of this conductor to be of the same size as the battery Negative wire.

Short excerpt from Victron's Wiring Unlimited:
7.5 Mobile installations
A mobile installation is an installation that operates independently from the grid. When it connects to AC power it usually connects to the grid at different locations and/or generators. For example, like boats, vehicles or mobile back-up power systems. In this chapter a boat installation is used, however, this information can be used for any mobile installation.
A mobile system does not have an earth stake. So, something else in its place is needed to create a central earth potential. All touchable metal parts of the boat or vehicle must be connected to each other to create a local earth. Examples of metal parts in a boat or vehicle are: chassis, hull, metal fluid pipes, railing, engine, power point earth contacts, lightning conductors and the earth plate (if present).
A mobile system typically connects to a variety of power sources it is sometimes not clear which of the leads in the shore power supply is connected to earth or if earth is connected at all. Also phase and neutral may have not been wired correctly. Connecting a supply like this to a mobile system can potentially create a short circuit to earth. Or earth is missing completely.It also matters if the mobile system connects to power or if it is disconnected from power and running autonomously.
 
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So, as regards grounding on an RV setup, with a multiplus inverter, where it can sometimes have a grid connection, which diagram below has the correct grounding configuration .... A, B or neither...please explain your conclusions(Grounding cables in green)

A
View attachment 26067

B
View attachment 26068

It is my understanding (take that for the little that its worth) that both A and B would not be wrong (with the possible exception of the PV grounding in option A), but that as a generalization option B would offer some advantages, and would be more inline with what some reputable inverter manufacturers recommend.
 
I like equipment enclosures and PV frames tied to vehicle chassis.
Whether or not battery negative should be tied to chassis is a separate question; check equipment manual for that.

Then there is the AC ground. Shore power probably ties ground to neutral, and it would tie to equipment enclosure. Shouldn't copper connection extend to chassis, so a short from hot to chassis gets back to neutral and ground without going through enclosure sheetmetal?

If shore power is backwards and hot ties to neutral ... better figure that out before plugging in and do something about it.

When not on shore power, does your inverter tie neutral to ground?

The idea is to make sure chassis will never get electrified and shock someone touching it. And that any GFCI detects and protects against leakage that could be going through someone.
 
I like equipment enclosures and PV frames tied to vehicle chassis.
Whether or not battery negative should be tied to chassis is a separate question
I'm struggling to follow the logic here, I don't see these as separate questions, I think they are very much related.
If you bond your equipment to chassis-ground but don't bond chassis-ground to the source (battery negative in this case) you have not given current an alternate path back to the source (which is one of--if not the--main purposes of equipment grounding). Ground (in the context of a fault) is not the destination, it is a convenient alternate return path. At least that is my limited understanding.


Then there is the AC ground. Shore power probably ties ground to neutral, and it would tie to equipment enclosure. Shouldn't copper connection extend to chassis, so a short from hot to chassis gets back to neutral and ground without going through enclosure sheetmetal?

When not on shore power, does your inverter tie neutral to ground?
Inverter/chargers designed for mobile or marine applications--specifically UL 458 inverters--internally switch the Neutral-Ground Bond depending on whether the inverter/charger is connected to shorepower or not.
  • When shorepower is connected the internal neutral ground bond is disconnected because the bond is made on the shorepower side. and should only be made at one point.
  • When shorepower is disconnected, the connection between neutral and ground is made internally.
 
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One advantage I perceive in using the chassis as a “local ground” for a vehicle-installed DC circuit, is that when your mobile rig is parked, the chassis can be literally earth-grounded, via a ground strap from the chassis to the earth, thus earth-grounding the DC circuit for the dual purposes of excess charge accumulation dispersion and also having an alternate return path for faults in the DC circuit.

Am I understanding this correctly?
 
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I'm struggling to follow the logic here, I don't see these as separate questions, I think they are very much related.
If you bond your equipment to chassis-ground but don't bond chassis-ground to the source (battery negative in this case) you have not given current an alternate path back to the source (which is one of--if not the--main purposes of equipment grounding). Ground (in the context of a fault) is not the destination, it is a convenient alternate return path. At least that is my limited understanding.
Probably battery negative to the chassis is the thing to do. For all I know, an inverter or charge controller could tie battery positive to chassis and leave negative at -12V, -24V, -48V whatever. Not likely, but I would check documentation.

For my Sunny Island, it appears battery can be grounded but isn't necessarily required. Either positive or negative grounding is possible. Taps for 48V battery positive and return (for signal relays) both have PTC fuses.

I said bonding frames to make sure they are same voltage as chassis, and current isn't flowing through a resistive screw to vehicle sheetmetal.
 
Very good conversation with input from great minds! The only thing I'd restate just to clarify what was said is:

Chassis-Return: using the chassis as a bus for the primary circuit should only be considered for low voltage applications, typically no higher than 24V, due to shock hazard. This is why we never consider it for 120V AC, and are unlikely to do it with PV circuits. The commonly discussed automobile case is 12V, which is clearly low voltage.

Chassis-Ground: connecting the chassis to earth ground is commonly a good idea independent of voltage. You are providing a path for static discharge and making it easier to install ground fault detection because you now have access to ground at various locations where you might want to install a ground-fault based breaker, a breaker that will open (disconnect) the primary circuit if current is detected between the ground and a primary conductor.
 
Good discussion. But to make progress in a system grounding discussion, there has to be the universal use of terminology. Terms have to be used by all involved in the same way and with the same meaning. I prefer the terminology I found in a Magnum inverter manual, where grounding their inverter properly is discussed. Here they are (both terms and grounding discussion):
"2.6 Grounding Inverters
The MSH-M Series inverter/charger uses both AC and DC electrical systems, therefore each electrical
system is required to be properly connected to a permanent, common “ground” reference. An
inverter that is properly grounded limits the risk of electrical shock, reduces radio frequency noise,
and minimizes excessive surge voltages induced by lightning. The installation must ensure there is
a well-defined, very low resistance path from the electrical system to the grounding system. The
low resistance path helps stabilize the electrical system voltage with respect to ground and carries
fault currents directly to ground, causing a fuse to blow or a circuit breaker to trip if the electrical
system malfunctions (i.e., short circuits). To understand how the conductors in the electrical circuit
will be connected to the system ground, review the following terms along with Figure 2-10:
• Grounded Conductor (GC): The wire in the electrical system that normally carries current (i.e.,
AC neutral and DC negative), and is intentionally connected or “bonded” to the ground system.
This wire, or the ends of this wire, should be colored white or gray.
• Equipment Grounding Conductor (EGC): A wire that does not normally carry current and is
used to connect the exposed metal parts of equipment—that might be accidentally energized—
to the grounding electrode or to the grounded conductor.
• Grounding Electrode Conductor (GEC): The wire that does not normally carry current and
connects the grounded conductor and/or the equipment grounding conductor to the common
ground reference, usually at the ground busbar in the service equipment.
• Grounding Electrode (GE): A conducting element that establishes an electrical connection for
a common ground reference. In vehicles, it is usually the chassis or frame; in vessels, it is the
largest metal item onboard, usually the main engine frame.
• System Bonding Jumper (SBJ): The connection or “bond” between the grounded conductor
in the electrical system (AC neutral/DC negative) and the equipment grounding conductor.
For proper grounding, each electrical system must connect all exposed metal parts of equipment
(via Equipment Grounding Conductors – EGC) and one of the current-carrying conductors
(Grounded Conductor – GC) together at a common point (ground busbar), usually by a System
Bonding Jumper (SBJ). The common point of each electrical system is then connected (via
Grounding Electrode Conductor – GEC) to the Grounding Electrode (GE). The connection to the
grounding electrode should be made at one and only one point in each electrical system (AC
and DC); otherwise, parallel paths will exist for the currents to flow. These parallel current paths
would represent a safety hazard and are not allowed in installations wired per the NEC/CEC.
Since the MSH-M Series uses both AC and DC power—to prevent parallel current paths—the AC
electrical system is isolated from the DC electrical system by an internal transformer.

1608330662581.png"

There is a wealth of information here, if you take the time to digest it, your understanding of mobile equipment grounding will be improved. Mine was when I first found this source, after reviewing and trying to understand it for some time.
 
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Very good conversation with input from great minds! The only thing I'd restate just to clarify what was said is:

Chassis-Return: using the chassis as a bus for the primary circuit should only be considered for low voltage applications, typically no higher than 24V, due to shock hazard. This is why we never consider it for 120V AC, and are unlikely to do it with PV circuits. The commonly discussed automobile case is 12V, which is clearly low voltage.
Agreed
Chassis-Ground: connecting the chassis to earth ground is commonly a good idea independent of voltage. You are providing a path for static discharge and making it easier to install ground fault detection because you now have access to ground at various locations where you might want to install a ground-fault based breaker, a breaker that will open (disconnect) the primary circuit if current is detected between the ground and a primary conductor.
I don't think we are on the same page here. I use and understand Chassis-Ground to mean Chassis-As-Ground (such as in a non-grid/non-earth connected vehicle based system), not to mean what I think you are referring to (connecting the chassis of a piece of equipment to ground via the grounding/bonding system). This I believe would be considered an EGC ("Equipment Ground Conductor").
 
OK, been reading through this, but let me see if I understand now.

My application will be the following.

1. a mobile installation with both DC and AC (camper, AC via inverter or shore power)
2. a 12V vehicle
3. a 24V house battery
4. a Victron MultiPlus inverter/charger

In my vehicle, there's a chassis-return on the 12V system. The 12V battery minus terminal is connected to the chassis.

The camper body - still a work in progress - is built with polyester sandwich panels, so no current flowing through the walls or anything.

The camper body will contain a 24V system with positive and negative busbars and the MultiPlus inverter for 230V AC, with or without shore power.

The MultiPlus has a specific connection 'ground' and it has an internal 'ground relay'.

This comes from the manual (more text below these pictures):
1629397379448.png
1629397524227.png

If I understand correctly, the following things should be done/will happen:
- connect the MultiPlus (and other Victron stuff) 'ground' to the negative busbar of the 24V system, this will 'ground' the casing for both DC and AC
- the camper body AC system should have a safety breaker, connected to the 3-wire AC output of the MultiPlus inverter
- the MultiPlus 'ground relay' will open up to shore power if AC is flowing, grounding the case to the shore power ground ("true earth ground?")

Questions:
1. Should I also connect the negative 24V DC busbar to the chassis (which acts as the 12V return), or is there no requirement? If so, why? If not, also please why? :)
2. is there no problem with the DC negative busbar connection to the casing of the MultiPlus when the ground relay is open with regard to shore power? From reading the pages in the manual (see above), these should all be open/closed for the appropriate situation, right?

Hope to hear from you.

With regards,

Martijn Tonies
 
Since I posted here last, I have discovered a good grounding resource. It is a difficult subject, many things to consider code wise.
Here it is. Then Mobile grounding section would apply, but you should read all grounding section types to fully understand the correct grounding requirements.
(PS, you need to select the download button, top right, to view and save the pdf file)
 
Since I posted here last, I have discovered a good grounding resource. It is a difficult subject, many things to consider code wise.
Here it is. Then Mobile grounding section would apply, but you should read all grounding section types to fully understand the correct grounding requirements.
(PS, you need to select the download button, top right, to view and save the pdf file)
That's a very useful document.

One thing I did not yet find, though, is an explicit reference to voltage difference between house and starter battery, eg 24V house and 12V starter.

But from checking the diagrams, like the one on page 13, it looks like both DC systems are 'grounded' to chassis, even with a chassis-return for the 12V system.

Can you confirm, @FilterGuy ?
 
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