24v GROUNDING HELP and overall critique

[Dzl]

I see why you chopped up the post for responding, but ya omitted some info I used, and missed a step.
As I think I said in the post, he INVERTER chassis needs grounding if provisions are present, but the BATTERY needn’t be unless loads are passed through the chassis, especially if the voltage is over 24V...

I believe the below may be the piece sentence I overlooked? if so I'm unclear what you are referring to by the part in green, I think it may be a typo:

Vehicle batteries get “grounded” to chassis because wire is expensive, and the chassis is there anyway. On an inverter bank battery, unless you use the chassis for device current path, I don’t see the need for grounding it... and it could create a dangerous shock hazard with voltages above 24V...

If what you are saying is its not necessary to ground the battery bank (to chassis-ground), but the inverter and other equipment case ground points should be wired to the negative busbar or some other alternate path back to B- then I agree, it is not necessary. But I do think that it is common practice to do so, and I don't see how it is less safe than a floating system. Could you explain?

 

[Gazoo]

However in this scenario, the green wire in the appliance cord, offers a convenient low resistance path back to source, with thousands of times lower resistance than the human body and an actual path back to source. So I don't understand why this situation would be inherently more risky, even if a short occured.

30ft of 14awg extension cord = 0.075 Ohms
The human body is between 500 and 100,000 Ohms

I understand the green wire offers a path back to the source. I also understand the difference between the resistance of an electrical cord and the human body. I was just trying to envision in my mind why Supervstech made the comment and I admittedly guessed. I could very well be wrong.

@Vinzent P made it clear he intends to run power tools outside of the vehicle. I am sure others do the same. It's best to nip this in the bud once and for all so everyone is safe no matter what the solutiion.

 

[Dzl]

I understand the green wire offers a path back to the source. I also understand the difference between the resistance of an electrical cord and the human body. I was just trying to envision in my mind why Supervstech made the comment and I admittedly guessed. I could very well be wrong.

I know, I'm not looking for disagreement, just trying to think this through, explain my understanding, and hopefully learn from the discussion.

@Vinzent P made it clear he intends to run power tools outside of the vehicle. I am sure others do the same. It's best to nip this in the bud once and for all so everyone is safe no matter what the solutiion.

Yes, I plan to as well, I imagine its fairly common for people to do so, so I'm interested in getting to the bottom of this as well.

 

[John Frum]

Which in the case of an AC appliance being powered off a battery bank through an inverter would be the battery negative right?
I believe the situation would be the same if it were the SCC supplying the power, just substitute SCC negative for battery negative.

It would be the inverter alternating current polar opposite.

 

[Zil]

I'm not sure this is actually true. Are you? ....
As to using the chassis as DC negative, i've heard (and this is hearsay not first hand info) that even modern cars have moved away from the practice of chassis-return for the electronics.

Yes, newer vehicles have so much plastic the chassis can be a bad conductor. Also all the various sensors are considered critical devices and I always have recommended they have a return wire and not rely on common negatives.
As to codes. i always refer to ABYC marine standards of safety. Again we are talking about a 24 volt direct current system. I can see no reason the negative can not be connected to the chassis. Or why the chassis can't be a common negative reference.
I need to say this again. If there is no connection to mains ac power supply, there is absolutely no need for a "earth" grounding. If from mains ac the green or bare ground must be at the main panel entry only.

 

[Zil]

Yes.
But this is only needed if you are powering devices outside the vehicle with the inverter.

No. It is not needed.

 

[Dzl]

I think @Supervstech had a reason for recommending the vehicle needs to be grounded to earth if using appliances outside the vehicle. I would like some clarification on this so we can all understand the reason.

@Vinzent P made it clear he intends to run power tools outside of the vehicle. I am sure others do the same. It's best to nip this in the bud once and for all so everyone is safe no matter what the solutiion.

Still hoping @Supervstech can circle back and clarify/explain/support his understanding on this. (post #20)

 

[Supervstech]

Still hoping @Supervstech can circle back and clarify/explain/support his understanding on this. (post #20)

It is NEC code,
But the reason is fault grounding. Voltage variations crop up between earthed systems and ungrounded items. If you are using an inverter that doesn’t have a floating ground, and it is connected to the chassis of a vehicle, AND something goes wrong, on devices outside the vehicle, you can be energized, and be electrocuted if you touch the vehicle...

 

[Will Prowse]

Lots of great points covered in this thread. And yes, it can be confusing. It really depends on the purpose of grounding for your specified application.

These are my applications for DC negative grounding or bonding:

• Dissipate charge accumulation. If charge were to build on a systems conductors, lightning will be attracted to it. Grounding your solar array will allow a path to ground, for dissipation. This will allow earth and your system to have the same potential.
• Save money on negative DC conductors. This is why mass manufactured vehicles use the chassis for DC grounding. They have a large conductor that can serve as a negative bus, so they use it. But the chassis IS NOT a true earth ground, and cannot be treated as such!
• Organization and efficiency. Having all of your negatives on a single large bus bar makes troubleshooting easier and ensures that every component is grounded nicely. Stray currents can be found at one point.
• Reference 0V. DC negative conductors will experience a certain degree of voltage drop when current flows, and some devices are sensitive to this. Having a reference voltage connection point will minimize this problem. They will sense a similar potential when bonded. Sometimes these are non-current carrying, and used for voltage sensing, such as battery chargers and voltage sensitive equipment.
• Improve the performance of some radios and reduce noise in the circuit. It is common to have marine radios bonded to the negative bus bar for this reason.

My applications for AC grounding:

• AC fault return path. This is the most common reason that people ground. If there is a fault in an AC circuit, you do not want to energize the case of a component and cause a shock to occur. So instead, the case is grounded so that if a fault occurs, the OCPD will be triggered. This is what the NEC usually refers to when grounding an AC circuit for home/commercial/grid tie wiring.

There is also galvanic corrosion and stray current corrosion in marine application that will refer to grounding the AC system when docked, and is mentioned in the ABYC, but beyond the scope of this forum. You can create a galvanic cell from dissimilar metals in contact with an electrolyte such as ocean water, then inadvertently bonded with the shore power connection. So in this instance, proper grounding and a galvanic isolator is warranted.

When I personally ground my systems:

• If it is a mobile system in a vehicle, I avoid using the chassis for grounding. I prefer running my own copper conductors instead.
• In small solar systems, I usually do not ground anything. The negative conductors will serve as a ground if properly sized. No real reason to run non-current carrying grounds if everything is close to each other and nothing is dependent on voltage fluctuations.
• If I am building a large system with components that are voltage sensitive (radios/battery chargers etc), I will ground everything to negative bus bar.
• If I am working with AC, then typical NEC is to be followed.

So it really depends on your application and why you want to ground something.

 

[tazmann]

It is NEC code,
But the reason is fault grounding. Voltage variations crop up between earthed systems and ungrounded items. If you are using an inverter that doesn’t have a floating ground, and it is connected to the chassis of a vehicle, AND something goes wrong, on devices outside the vehicle, you can be energized, and be electrocuted if you touch the vehicle...

would a GFI receptacle take care of that ?

 

[Supervstech]

would a GFI receptacle take care of that ?

Maybe. GFCI/RCD is great to prevent shocks live to earth.

often however, a shock hazard can crop up BETWEEN grounds.

 

[Zil]

It is NEC code,
But the reason is fault grounding. Voltage variations crop up between earthed systems and ungrounded items. If you are using an inverter that doesn’t have a floating ground, and it is connected to the chassis of a vehicle, AND something goes wrong, on devices outside the vehicle, you can be energized, and be electrocuted if you touch the vehicle...

We started talking about a vehicle with a battery powered inverter for 120Vac. So. No. The inverter has no, that is no, reference to any earth ground. If you held the hot side of the ac wires and touched the inverter case you could get a shock. But unless there is a wire to earth, that would be your "earth stake", there is no way the vehicle inverter will reference the earth ground. The circuit completes back to the inverter where it originates. I am talking about an inverter. Not one of those Converters. The converter plugged into the grid must have a ground, usually green, wire back to the place the circuit originates. Not the battery or the inverter. If you drive a stake into the ground for your battery powered inverter you could create the problem you are trying to avoid.

 

[Hedges]

Maybe. GFCI/RCD is great to prevent shocks live to earth.

often however, a shock hazard can crop up BETWEEN grounds.

Better make that an all-pole GFCI.
Typical home GFCI only open the hot, and rely on neutral being at earth potential, not hazardous. On a vehicle and with whatever inverter, you can't be sure which wires are a hazard. Especially since some inverters deliver +/-60VAC on each wire.

I'm not immediately finding such GFCI for mounting in box or panel, but those in portable cords would (should) be all-pole.

Discontinued - Portable In-Line 15A GFCI Outlet Adapter, Manual Reset | GFCI and AFCI Outlets | Outlets and Receptacles | Wiring Devices

Portable Ground Fault Circuit Interrupter, 2' Cord 15amp 125volt, Manual Reset

legrand.us

 

[PictureMeFree]

When I personally ground my systems:

• If it is a mobile system in a vehicle, I avoid using the chassis for grounding. I prefer running my own copper conductors instead.

Preface: I am a humble newb, and just trying to learn.
My Question: in the type of circuit Will Prowse describes here, if exposed positive and negative wires ever simultaneously contacted the vehicle chassis, would that cause the chassis to become electrified? Thanks in advance for your knowledge and teaching.

it seems most contributors to this thread had recommended “grounding” a mobile solar powered DC circuit to vehicle chassis, via a negative bus connected to vehicle chassis, but then, Will says he doesn’t ground to chassis, without explaining why (maybe because the reason is elementary, and I just don’t know it), and no one addressed that difference of opinion, or why it might be preferable not to ground the negative bus to vehicle chassis.

 

[Gazoo]

Will says he doesn’t ground to chassis, without explaining why

I didn't get that either but then there are so many comments regarding grounding sometimes I get lost. I would appreciate a comment and explanation from @Will Prowse regarding this.

 

[tazmann]

Maybe I can shed some light on why you should ground to chassis, without a ground to chassis if you get a short IE positive Dc or the Hot wire on ac to chassis it will not blow a fuse because it has not made a path to ground, what it does do is energize your chassis and given the right situation you could walk up and touch vehicle and get shocked and if it is the hot wire from ac it would be the same effect as sticking finger in light socket

 

[Dzl]

I may be misunderstanding and hope he will clarify,
But I believe the confusion is due to the terminology. Here, I think Will may be using the term "grounding" in the conventional automotive sense of the word where 'ground' referred to both grounding as well as DC negative / the return current path.

I think all he is saying is that he prefers dedicated positive and negative wiring, and does not like to use chassis-return (i.e. doesn't use the chassis as DC negative).

This is a separate issue than the topic of using Chassis-ground as the mobile/vehicle based stand-in for Earth-ground.

Alternatively, he may be referring to a floating system, where the equipment ground connections (for instance the inverter grounding lug) are brought back to a central bus (DC neg, or a dedicated ground bus which is bonded to DC negative but isolated from the Chassis).

--

I propose we try to move away from the confusing and often misleading old automotive terminology and stick to unambiguous terms like:

Chassis-Return: Using the vehicle chassis, frame, or body in place of dedicated DC Negative wiring, as is/was common with vehicles.
Chassis-Ground: Using the vehicle chassis as a common ground reference, similar to how Earth-Ground is used with fixed structures.

 

[Will Prowse]

Yeah, Dzl summed it up nicely. I believe the terminology here is throwing people off.

Using the chassis as a negative DC bus, in my opinion, has no benefit beside cost saving on conductor (unless someone would like to prove me wrong here. My opinion pertains to vehicles with insulated wheels, and not marine application). This does not apply to the AC circuits!

A downside of using the chassis as a negative bus or ground (as reference voltage) is that the connection points need to be inspected. If a connection is dirty or corrosion occurs, you will need to clean it and apply dielectric grease. And there are more terminations required, which further increases complexity and potential failure points. Anyone who has had intermittent electrical issues in an older vehicle due to a bad ground connection can understand.

I would rather run a continuous length of copper between my components, with copper lugs, than to use the chassis as path to ground. And I would rather use a dedicated tinned copper bus bar, than to use the chassis.

A potential application that I could justify chassis return for a mobile system is if I am doing a long run with high current and cost is prohibitive. Then I will run a single conductor, and use the chassis as return path for ground.

But a big point in my original comment is that the chassis is not a true earth ground. It is insulated 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.

 

[Will Prowse]

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 like to separate DC and AC circuits when it comes to grounding. As mentioned in my original post, there are many reasons why you should ground, and what the benefits are. AC fault is the most common reason that people apply a "ground" conductor to a system. And I think people try to apply that logic to DC systems. But they are very different. And as Dzl mentioned, the terminology is throwing people off.

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.

 

[Supervstech]

I may be misunderstanding and hope he will clarify,
But I believe the confusion is due to the terminology. Here, I think Will may be using the term "grounding" in the conventional automotive sense of the word where 'ground' referred to both grounding as well as DC negative / the return current path.

I think all he is saying is that he prefers dedicated positive and negative wiring, and does not like to use chassis-return (i.e. doesn't use the chassis as DC negative).

This is a separate issue than the topic of using Chassis-ground as the mobile/vehicle based stand-in for Earth-ground.

Alternatively, he may be referring to a floating system, where the equipment ground connections (for instance the inverter grounding lug) are brought back to a central bus (DC neg, or a dedicated ground bus which is bonded to DC negative but isolated from the Chassis).

--

I propose we try to move away from the confusing and often misleading old automotive terminology and stick to unambiguous terms like:

Chassis-Return: Using the vehicle chassis, frame, or body in place of dedicated DC Negative wiring, as is/was common with vehicles.
Chassis-Ground: Using the vehicle chassis as a common ground reference, similar to how Earth-Ground is used with fixed structures.

Yep... terminology is difficult.
As an electrician, I use BONDING to refer to safety wiring between components,
Chassis return path is good to me.
And grounding as a method to provide fault current a return to pole neutral through earth.