Grounding question

[Dzl]

Mike Holt, stresses the importance of a single connection to earth-ground in an electrical system.

I am trying to understand how this logic might apply to a vehicle (and chassis-ground) where there is generally a chassis ground connection at the starter battery, and a chassis ground connection at the house battery.

It seems like the same principles would apply in either context, but I'm still pretty ignorant when it comes to grounding and bonding theory.

Should it be a best practice to only have one point where both batteries are bonded to chassis-ground?

 

[FilterGuy]

Should it be a best practice to only have one point where both batteries are bonded to chassis-ground?

Yes. That is the best practice, but it is often not the case on mobile systems.

 

[RandyP]

In autos/rv DC systems, some use the 'chassis' in place of the 'earth' and attach their battery '-' bus to it. There is no grounding electrode required like a ground rod driven into the earth as a termination point for earth ground on an AC system. A wire from battery '-' is just terminated to a clean unpainted spot the metallic chassis. This connection chassis to 'B -' also provides a return path for dc fault current if all metallic enclosures of DC equipment on the vehicle are electrically connected to the chassis and there is a ground fault (wire insulation abrasion at a metallic contact point, contacting conductor to metal).
And some use the chassis as a return current path for some of the circuits in the vehicle, running only one '+' wire to an electrical device and connecting the device '-' wire to the chassis at the nearest point possible. Such connections are subject to galvanic corrosion (bimetallic corrosion). Maintenance is required to keep the termination resistance low. I do not have a copy of RVIA-LV electrical standards yet, so I do not know if that code gives any direction for these practices.

 

[newbostonconst]

In a house the ground wire is attached to the neutral in only one place. This is the grounding you are talking about. In a car there technically isn't a ground. In a car the negative is wrongly called a ground but really isn't ground.

In a house it is very important because there are 3 wires and the voltage is high. In a car there is only 2 wires. The ground in a house is a safety wire because the voltage is so high. The ground is supposed to safely get rid of energy that is in the wrong place.

In a house if the neutral and the ground are connected in more then one place then power can flow on the ground wire. AC voltage on the ground wire can kill you....and this wire is normally not covered in an insulator.

 

[Dzl]

Yes. That is the best practice, but it is often not the case on mobile systems.

It seems like it could be difficult in many situations as the starter battery, and house battery bank can be far apart.

In autos/rv DC systems, some use the 'chassis' in place of the 'earth' and attach their battery '-' bus to it. There is no grounding electrode required like a ground rod driven into the earth as a termination point for earth ground on an AC system. A wire from battery '-' is just terminated to a clean unpainted spot the metallic chassis. This connection chassis to 'B -' also provides a return path for dc fault current if all metallic enclosures of DC equipment on the vehicle are electrically connected to the chassis and there is a ground fault (wire insulation abrasion at a metallic contact point, contacting conductor to metal).
And some use the chassis as a return current path for some of the circuits in the vehicle, running only one '+' wire to an electrical device and connecting the device '-' wire to the chassis at the nearest point possible. Such connections are subject to galvanic corrosion (bimetallic corrosion). Maintenance is required to keep the termination resistance low. I do not have a copy of RVIA-LV electrical standards yet, so I do not know if that code gives any direction for these practices.

Randy, this is a very nice high level overview/explanation of the typical RV/Motorhome/Etc electrical system, concise but detailed and clear. But the scenario you sketched out above is the foundation of my question. What I am trying to figure out is, in the picture you painted above (with one addition: B- of the house battery bank, and B- of the starter battery are both bonded to the chassis), should there be only one connection to chassis ground?

The question arises from two places. First, I have heard it recommended that all equipment ground wires be brought back to a single ground point (can't remember where I read that--maybe Samlex manual or victron manual), taking this logic one step further, it seems this logic would apply to bonding to chassis as well. Second, the video I linked to above stresses the importance of a single bond to earth-ground, it seems like the same logic explained in that video may apply to chassis grounded systems, but I"m not sure.

 

[Supervstech]

It seems like it could be difficult in many situations as the starter battery, and house battery bank can be far apart.



Randy, this is a very nice high level overview/explanation of the typical RV/Motorhome/Etc electrical system, concise but detailed and clear. But the scenario you sketched out above is the foundation of my question. What I am trying to figure out is, in the picture you painted above (with one addition: B- of the house battery bank, and B- of the starter battery are both bonded to the chassis), should there be only one connection to chassis ground?

The question arises from two places. First, I have heard it recommended that all equipment ground wires be brought back to a single ground point (can't remember where I read that--maybe Samlex manual or victron manual), taking this logic one step further, it seems this logic would apply to bonding to chassis as well. Second, the video I linked to above stresses the importance of a single bond to earth-ground, it seems like the same logic explained in that video may apply to chassis grounded systems, but I"m not sure.

Bonding and grounding and chassis grounding are related, but different. Bonding is a method to unify fault current pathways to metal wire ways... grounding is a fault current pathway connection. Chassis grounding is creating a negative pathway to battery devices. Vehicle electronic devices need chassis grounding at a common point, instrument clusters will have a post on a metal surface that is common to the pack of gauges. The vehicle computer, and all related computer controlled devices will have a post usually on the engine block, where all grounds converge for seamless baseline voltage.
it would make sense for victron to specify this as well, so voltage differences are minimized through the controllers. Steel is a TERRIBLE conductor, and having all ground reference in one spot keeps the control baseline as close to battery voltage as possible.

as for house and starting banks both being attached to chassis ground in one location, I think it depends on the reason for the chassis ground of the banks. Vehicle ground is needed for the vehicle devices. House ground is just for fault path. No current is flowing through it. Tie it to the chassis, as close to the house bank, and be sure any HOUSE loads, all ground to the same location.

 

[FilterGuy]

IMHO, you should never count on chassis as a return path. You should *always* wire the negative from the device back to the battery. (Chassis is never a reliable return path) Consequently, if all of your devices have a dedicated wire for negative, connecting the house battery to the chassis is optional for circuits operating below 50 volts (See note 2 below).
(48 volt battery systems operate at above 50 volts so it is considered mandatory for them). For 12 or 24 volt systems, I think it is good to always bond the house battery negative to the chassis in order to drain off any built up charge. That way the house battery circuit stays close to the same potential as chassis.

If you have no battery combiners or DC-DC chargers between the house battery and the starter battery, you can pretty much consider the house circuit completely separate from the starter circuit. In this case bonding house battery negative to chassis separate from the starter battery is probably fine....but I would still try to bond it at the same place as the starter battery.

If you do have a battery combiner or a DC-DC charger, things get more complicated. If any of these devices connect the house negative to starter negative, that becomes the one place your house negative is 'connected' to chassis and you should not do it in another place. If you have multiple devices that connect the two negatives, you have just created a ground loop (and there may be nothing you can do about it.) As I recall, when Victron released their DC-DC chargers, they released a version that kept the two negatives isolated.... presumably for this very reason. (@Justin Laureltec might be able to confirm)

Note 1: There is a different school of thought about bonding the house neg to chassis: If everything has a dedicated return line back to the house battery, don't bond house-negative to chassis. That way there is less likelihood of a short from the house battery positive to something connected to house battery negative. While I understand this logic, I still think it is better to bond DC negative to chassis.

Note 2: In the NEC, 'grounding' is optional for DC systems operating below 50 Volts and mandatory for systems operating above 50 Volts. The interesting thing is that it says either positive or negative can be 'grounded'.
In this case I believe the term grounding is equivalent to 'bonding to chassies' not necissarily 'bonding to earth'. The thinking is that 50V is high enough that if positve gets in contact with the chassis, you want the fuse to blow rather than leaving chassis 'hot' and dangerous.

Note 3: I hate the word 'grounding' because it means so many different things to different people. The term should be outlawed.....but I admit I catch myself using it in ambiguous ways

 

[Ian]

This came from a victron resource and this is how I plan on wiring my grounds. It does seem odd to me though running a ground from my 12v battery. From what I’m reading from some other people’s post it seems like i may be able to delete that ground? I am inclined to follow victron instructions though.

 

[RandyP]

.....................................
Second, the video I linked to above stresses the importance of a single bond to earth-ground, it seems like the same logic explained in that video may apply to chassis grounded systems, but I"m not sure.

Mike Holt is the man for AC system grounding up to 600v ac systems. But you should watch some more of his videos on Earth Grounding of AC electrical systems so you understand the reason for earth ground ac systems and the challenges involved and the effects of lightning strikes on the earth or faults to a ground rod if you contact then rod and the ground just 3' from the driven rod. DC chassis grounding will not have the same danger associated with ac grounding to earth during a lightning strike or high energy ac line to ground event, voltage gradient in the ground near the fault. Lightning protection and high voltage ground faults have different safety concerns than low voltage ac system earth grounding.
You are right, I did not answer your question because I do not know the standards for RV DC low voltage systems per RVIA-LV code.

 

[FilterGuy]

View attachment 10743
This came from a victron resource and this is how I plan on wiring my grounds. It does seem odd to me though running a ground from my 12v battery. From what I’m reading from some other people’s post it seems like i may be able to delete that ground? I am inclined to follow victron instructions though.

The Victron diagram looks correct as far as it goes. Notice that from any one device there is only one path to the 'ground' point.

However, things can start to get complicated in a couple of ways:

1) If the body of the device is tied to negative, it will create a ground loop. (I don't know of any Victron equipment that does this....and most decent equipment won't do this)

2) If the metal body of the device is mounted on a metal part of the vehicle chassis you already have a chassis bond. In this case, adding another wire to chassis is probably counter productive.

 

[Jims94vmx]

Been really looking at these threads. If using the Victron schematic shown above, where does the shunt fit in?
On the bus bar, before, after?

Example of my system so far:
2 BB in parallel with a positive going to a converter and another positive from batteries to inverter
Negative goes to shunt then a wire to converter and another negative wire from shunt to inverter.

Where would/should bus bar come into play?

Should I put another negative wire off battery to bus then ground? Then run grounds from scc and inverter to bus bar?

Note:the converter has a copper ground wire already to breaker/fuse center and chasis.

 

[Zil]

Mike Holt is talking about alternating current from the power supply mains, in a stick and brick.. He is not talking about a vehicle DC circuit. There is no comparison. Yes, critical or heavy ampere devices should have a dedicated negative return cable to the battery. Lessor devices such as fans, lights, and phone chargers not so necessary if the negative connections to the chassis are sound. In a vehicle it matters not how many negative battery to chassis connections there may be. Just insure the chassis connection is sound. Shunt question. The shunt must be the last thing at the battery negative. Every negative return must go through the shunt.

 

[HRTKD]

Been really looking at these threads. If using the Victron schematic shown above, where does the shunt fit in?
On the bus bar, before, after?

Example of my system so far:
2 BB in parallel with a positive going to a converter and another positive from batteries to inverter
Negative goes to shunt then a wire to converter and another negative wire from shunt to inverter.

Where would/should bus bar come into play?

Should I put another negative wire off battery to bus then ground? Then run grounds from scc and inverter to bus bar?

Note:the converter has a copper ground wire already to breaker/fuse center and chasis.

That Victron image doesn't use a shunt. Check out this image that does: https://www.victronenergy.com/upload/documents/Quattro-system-example-5KW-24V-120V-AC.pdf

The bus bars would be placed immediately after the shunt:
Battery (Neg) -> BMS -> Shunt -> Negative Bus Bar
Battery (Pos) -> Fuse -> Switch -> Positive Bus Bar

 

[Dzl]

edit: I'm currently questioning some of the bits I have written below. Bear this in mind when reading the below:

Glad you chimed in @Zil I always appreciate your insight on grounding and wiring. In this case I'm going to push back a little on some of your points. Or maybe more accurately, push you to explain/support some of them a bit more.

Mike Holt is talking about alternating current from the power supply mains, in a stick and brick.. He is not talking about a vehicle DC circuit. There is no comparison.

While this may be true, If you think it doesn't apply in a mobile or marine context (or DC), I think it would be helpful if you try to explain why the same concept/principle does not apply, or point to a resource that does.

My recollection is that the ABYC marine code wants (1) NO chassis-return, 'two wire' out and back wiring for all circuits (2) single connection between DC negative and the chassis. I'm a little more shakey on the second point, not 100% sure I interpreted it right, and not sure whether it is a recommendation/best practice to shoot for or a requirement. I don't know what the best practices or code is for mobile systems. I suppose cars use chassis-return and multiple grounding points, but they are low voltage, low current systems.

I'd appreciate if you could help me understand why the concepts explained by Mike Holt with regard to AC structure grounding, do not apply to DC grounding in a mobile context. This (grounding theory) is admittedly one of my weakest and shakiest areas of understanding, and I have found virtually no good high level resources specific to DC system grounding.

Yes, critical or heavy ampere devices should have a dedicated negative return cable to the battery. Lessor devices such as fans, lights, and phone chargers not so necessary if the negative connections to the chassis are sound.

I won't argue with this. I've never heard anyone argue that chassis-return won't work, just that its non-ideal and has the potential to cause headaches and issues down the road (and is less predictable). Here I am just repeating what I've heard, and as we should all know by now "I read on the internet" is not a valid source on its own ?

But. if we are talking about low power devices we are normally talking about relatively small and cheap wire, dedicated + and - conductors out and back would not cost too much more.

In a vehicle it matters not how many negative battery to chassis connections there may be. Just insure the chassis connection is sound.

Why would best practices be different in a vehicle w/ chassis-ground (and DC current) compared to a structure with w/ earth-ground and AC power?

I would like to understand this conceptually/logically. I have a lot to learn still.

 

[Jims94vmx]

That Victron image doesn't use a shunt. Check out this image that does: https://www.victronenergy.com/upload/documents/Quattro-system-example-5KW-24V-120V-AC.pdf

The bus bars would be placed immediately after the shunt:
Battery (Neg) -> BMS -> Shunt -> Negative Bus Bar
Battery (Pos) -> Fuse -> Switch -> Positive Bus Bar

I don't see a ground on the 100/50 or chasis ground off batteries.....in this schematic.

Keep in mind my inverter is 1500w

 

[Gazoo]

Why would best practices be different in a vehicle w/ chassis-ground (and DC current) compared to a structure with w/ earth-ground and AC power?

Isn't it because in a vehicle there is no path to ground due to the fact the vehicle sits on rubber tires?

But I am confused by something. Most of the inverters I have looked at that are greater than 1000 watts have a separate connection to go to DC ground. I am confused why the manufacturers even have this connection since the ground cable of the inverter goes to DC negative on the battery which is the same as DC ground. I would really appreciate an answer to this.

 

[Supervstech]

Isn't it because in a vehicle there is no path to ground due to the fact the vehicle sits on rubber tires?

But I am confused by something. Most of the inverters I have looked at that are greater than 1000 watts have a separate connection to go to DC ground. I am confused why the manufacturers even have this connection since the ground cable of the inverter goes to DC negative on the battery which is the same as DC ground. I would really appreciate an answer to this.

If there is a separate grounding point on the inverter, it is so it can be connected to the earth, or the chassis to bond surface charge from causing shocks.

 

[Dzl]

The below is my current understanding. It should be understood that I am absolutely not an expert and not fully confident in my understanding of this complex topic, but it should also be understood what I write is based on research I have done, not just conjecture or assumption. I put my understanding out there to hopefully help others learn, and equally important to improve and refine and correct my own understanding, with that in mind, if something isn't clear or if something seems wrong to you, bring it up, and if possible provide a reference or an explanation. I lean heavily on bits and pieces I have learned from Samlex, Magnum, Mike Holt, RSD Academy, the ABYC, and Victron. I am certain, I do not have a full and adequate understanding of grounding theory yet.

Isn't it because in a vehicle there is no path to ground due to the fact the vehicle sits on rubber tires?

Setting aside lightning,

My understanding is that chassis-ground (assuming isolated mobile system--no shorepower) is a conceptual stand-in for earth-ground in many important respects. In that it is (1) a massive common "0V" reference (2) a common low resistance return path for fault current.

Victron explains chassis ground as being conceptually like a "local earth ground"

The principle should be the same I think, or even more important with a less massive chassis (compared to the mass of the earth). My understanding is that their is nothing special about the physical earth as 'earth-ground' beyond that it is far and away the best option we have. But the concept, a massive "0V" common reference and universal common return path can be applied on smaller isolated scales and in other contexts.

Similar to how your house bank gets tied to chassis-ground in a vehicle, equalizing potential between B- and the vehicle Chassis, and creating a common fault path for current to flow back to the source (the battery), On a larger scale, the actual earth is serving the same purpose as the chassis, it works in the same way. The power grid and the original voltage source (power plant) are also tied to earth ground, in that respect the physical earth is a path back to the source.

So in the case of an isolated system like a vehicle (ignoring shorepower) where the voltage source is a battery (or alternator or PV panels or whatever), The chassis serves the same purposes as the earth in a grid tied system (common 0V reference, common fault current return path connected to the voltage source).

But I am confused by something. Most of the inverters I have looked at that are greater than 1000 watts have a separate connection to go to DC ground. I am confused why the manufacturers even have this connection since the ground cable of the inverter goes to DC negative on the battery which is the same as DC ground. I would really appreciate an answer to this.

For me it was useful to learn about the two distinct but connected grounding concepts (I'm not sure what your level of understanding here is, you may already understand this), "Equipment grounding" and "System Grounding."

System grounding is the concept of physically and electrically bonding one side of your electrical system to the designated grounding point (chassis or earth-ground rod usually). This is what "grounds" your electrical system. In a vehicle based system, this is commonly done off the negative busbar, or battery negative terminal.

Equipment grounding is the concept of bonding the exposed metal parts of your components back to the grounding busbar, negative busbar, or designated system grounding point. These Equipment Grounding Conductors (EGC) should not normally carry current, but they are a low resistance return path if there is a short that energizes the metal casing of a component. The important of this is threefold, (1) because its a low resistance path, it should hopefully allow enough current to flow that your breaker or fuse trips and de-energizes the faulty circuit and alert you to the problem, (2) its a much lower resistance return path than you might be, so you are unlikely to become the return path if you touch the wrong two things, (3) it decreases the chances of a fire for the same reasons as point 2.

The important takeaway is that the primary purpose of an EGC, is a low resistance fault path back to the source.

There is more complexity with inverter/chargers in a mobile/marine context if shorepower is involved. Explaining this is beyond the scope of this conversation, the term you want to research is "Neutral-Ground bonding" or "Neutral-Ground switching" and the broad idea is that AC neutral and ground should only be connected at one point in the system. Where that point is depends on the power source, since a mobile system will have different power sources (battery, shorepower/grid, generator, etc) the point at which Neutral and Ground should be bonded changes based on context. An inverter charger designed for mobile/marine applications (UL 458) will handle this internally. When in 'inverting mode' I believe neutral is bonded to the inverter chassis.

 

[Dzl]

I don't see a ground on the 100/50 or chassis ground off batteries.....in this schematic.

Keep in mind my inverter is 1500w

DC Negative to Chassis-Ground connection is there, it is made at the main negative busbar.

As for the smartsolar equipment ground conductor not being present, from what I gather Victron seems to consider it somewhat optional, in some schematics they show it, in others they ignore it, the manuals make reference it only in passing. Out of the top tier inverter manufacturers Victron is hands down the worst I have encountered at providing clear and direct grounding instructions, which is disappointing, most other top tier manufacturers devote whole sections of the manual to grounding.

 

[Samsonite801]

Victron forgot to put in their manual, that every time you stop to park your RV, you should pull out your 6ft copper ground rod and sledge hammer, pound the rod in, and ground your solar gear for safety/lightning strike, and when you leave use your slide-hammer to pull it back out (or it can get expensive to leave the rods there)...

You should also have an anti-static strap just for extra safety: