Inverter earth ground - I’m confused

[Glenn]

This is probably an easy one for most of you…… I have never owned an inverter.

I picked up an 1100W pure sine inverter. It came with a small earth ground cable which I’m guessing is the same as case ground.

I’ve read multiple threads, but find myself still confused. So the easy part- the DC Pos and Neg go to their respective inverter battery posts.

But there is a small earth ground wire it says to hook to chassis. The videos I’ve watched and blogs I’ve read say hook up the DC ground and the AC inverter earth ground to the chassis.

I’m having a hard time getting my mind wrapped around having those shared neutrals oh the chassis especially with the truck having an ECU and other sensitive electronics.

But every source seems to say the same thing, unless I’m misunderstanding. I am simply wanting to make a short pigtail that plugs into my outlet on the inverter and hard wired to a duplex outlet on the camping box to make things look stream lined. No service box or panel for AC, just the inverter and two outlets.

The manual from the manufacture is pretty much useless.

Advice on the grounding issue is very much appreciated! I’ve included pictures of the inverter and circled in red the “earth ground” in question

Thanks everybody

Post Edit: I see in the picture their identifying it as Ground Terminal (red circle) and not earth ground as I had read elsewhere.

 

[Glenn]

Probably going to answer my own question but I found another thread and just talked to one of our EE electricians here at work.

On the latest thread I read it was stated “ with a digital volt meter set to Ohms, check the DC negative post (incoming on inverter) and the AC ground and they are probably already connected”

If this is correct then the connection is already there and the earth ground on the inverter is just another “safety” by hooking it to the chassis.

 

[Jon R]

That wire is not serving as a neutral return - it’s a safety ground. One of its purposes is to ensure that, if the AC hot wire shorts to the likely conductive things in the vicinity, you get a large fault current that is safely routed and quickly blows the circuit protection device to cut off the power rather than having that nearby conductive object stay hot and be a shock hazard. On a vehicle you want to connect it to the chassis.

While cars and trucks use the body as a common return for small loads with only a positive wire routed to those loads, that’s not a good practice in an RV. Your ac and dc loads should have a dedicated neutral or negative wire to carry the current. You then want to tie the neutral and negative to the safety ground at one location only to prevent “ground loops” or unintended circulation of current.

One potentially problematic situation is when you plug into shore power and then have both your inverter safety ground and ac neutral and the camp site source neutral and ground all connected to the chassis. I haven’t thought through the possible failure effects for that arrangement but my initial reaction is not to like it.

Vic has experience with inverter installations, and hopefully he can offer more information.

 

[Dadocut]

This is a great topic. Lots of people working on electricals get grounding issues wrong. My goto source for RV grounding are several tutorials on the DIY Solar Forum. See https://diysolarforum.com/resources/grounding-made-simpler-part-1-ac-houshold-grounding.157/ to download part one of a four part tutorial.

Glenn: Your comments about needing a "NG Plug" (a plug w/ a neutral - ground connection installed on the inverter output) is important. Many inverters don't bond the neutral to the ground. Some do, some do only when the AC input is lost. Unfortunately inverter manufacturers don't typically specify what they do with the inverter neutral output. Most manufactures do however attach the input grounding connector (ie the green wire) to the inverter case. THe DIY solar forum has a document listing the behaviour of certain inverters (see Grounding details for specific make/model of inverters )

Connecting the 120v to shore power affects this also. IF shore power is connected, you don't want the NG Plug connecting shore power neutral to ground. WIth shore power the NG connection is instead made in the distribution panel of the RV park and you don;t want a second NG connection in your truck. I solved this problen by using a multipole transfer switch that connects my plugs to either my inverter or to shore power. THe transfer switch has a center off position. THe transfer switch has multiple poles and I switched both the hot and nuetral. (note the green "grounding connector" is not switched. IT's ok to attach the RV parks grounding connector to the green wire in your RV's 120v electrical.)

Hope this makes sense, it's a broad topic.

 

[Glenn]

Thank you for the Links to grounding.

 

[Dadocut]

Glen, part 4 of the grounding tutorial covers mobile installations like our campers. It's worth reading. It includes detailed schematics that show exactly how to hook up the inverter.

Long story short: you'll need the NG plug if your inverter doesn't ground the neutral internally.

 

[Glenn]

Dadocut, I reviewed Letter 4 of the mobile systems. Above is a partial drawing from the chapter on inverters.

In the red circle I drew, is the “if not bonded inside of the inverter, it must be done externally” as noted in the caption.

While it doesn’t show a direct wire from the AC ground bus to chassis I believe there would be one “and only one” similar to that shown to the left of the diagram, on the DC ground bus. Is this correct?

Then if I understand correctly the purpose of the AC ground bus would be to bring all AC grounds to one bus bar and then only one bond between the AC bus bar and chassis so as not to create ground loops. ?

So essentially my starter batteries’ Negative (diesel engine) along with the engine, frame, body, etc. are all grounded to Chassis in the engine compartment.

The chassis acts as one big ground, though I have a dedicated 6AWG Negative cable (along with the 6AWG Pos.) going from starter battery to the Victron 12-12-30 non isolated smart charger.

My Negatives for my DC loads for the camping box only, in the bed of the truck, all go to the shunt first on the (load /charger side - including my inverter) and then on the battery side of the shunt (Victron BMV - 712 Model) back to my 280A LiFePo4 house battery.

Finally, the AC ground bus would be connected to chassis (as safety ground) somewhere underneath the truck bed to the chassis frame

Does that sound right? Thanks!

 

[Glenn]

On my way to work…..I stopped at shop first:


Post Edit: Removed the post detail and pictures….. I was in a hurry this morning and just realized from the picture that I need to check the AC neutral to case ground……I was on the AC outlet ground to case I’ll check when I get home tonight and repost. Sorry for any confusion if thread was previously viewed.

 

[Glenn]

I was beat down tired, up too late reading up on the inverter wiring.
Unfortunately, it turns out the AC Neutral and Ground is not bonded.

So, for my simple setup of the inverter going to two outlets (no metal), No shorepower, everything attached to wood, could somebody draw up how I externally bond the AC neutral to the inverter case ground?

Later in another thread I read: do not attach the AC neutral to the chassis as it could become live with AC current.

I still have the green inverter ground that has to go to the truck frame, and from what I have read that should be sized to the inverter DC input cables to the inverter by going one size smaller in wire size. I’m running 2 awg to the inverter so if this is true then the inverter ground should be 4 awg.

This whole thing is making my head hurt, I need the Tylenol

 

[Dadocut]

Glen, you're on the right track and are understanding the issues. Thanks for pointing out you are not connecting to shore power and are only using the outputs directly from the inverter. THat keeps it simper.

Like you stated, the truck battery, frame, the neg of the 12v in the camper, the neutral on the inverter output, and the AC ground are all tied together. Note your 12v-12v non-isolated charger is already connecting the truck battery neg to your 12v neg in the camper. Furthermore the AC ground is most likely already connected to the inverter chassis ground (internal to the inverter).

At some place you should attach the inverter cabinet ground to your 12v neg bus bar. The wire making that connection should be a size smaller than what you used to drive the inverter. I suspect most installations use a much smaller ground.

I wouldn't bother to use a special wire to connect the truck frame to your 12v neg bus bar. It's already connected through the heavy guage wire from the battery to your 12v-12v charger.

The inverter output AC neutral-ground bonding can be done with an NG Plug. This is simply a standard plug internally wired such that the neutral plug is attached to the ground plug. Just plug it into your inverter output and it takes care of bonding the neutral to ground. Many people would skip this step but it's safer to have the neutral & ground attached.

That should do it. I don't think you need an additional bonding of the truck frame to your negative bus bar. You mention ground loops. They're an issue for RFI (radio frequency interference). Often times the safety rules are in conflict with RFI best practices. I would suggest you not worry about ground loops unless you have a radio interference problem. Unless you're a ham radio operator or running a nice hifi off the inverter I bet you won't have any ground loop issues.

hopefully I addreseds each of your questions.

 

[Glenn]

Glen, you're on the right track and are understanding the issues. Thanks for pointing out you are not connecting to shore power and are only using the outputs directly from the inverter. THat keeps it simper.

Like you stated, the truck battery, frame, the neg of the 12v in the camper, the neutral on the inverter output, and the AC ground are all tied together. Note your 12v-12v non-isolated charger is already connecting the truck battery neg to your 12v neg in the camper. Furthermore the AC ground is most likely already connected to the inverter chassis ground (internal to the inverter).

At some place you should attach the inverter cabinet ground to your 12v neg bus bar. The wire making that connection should be a size smaller than what you used to drive the inverter. I suspect most installations use a much smaller ground.

I wouldn't bother to use a special wire to connect the truck frame to your 12v neg bus bar. It's already connected through the heavy guage wire from the battery to your 12v-12v charger.

The inverter output AC neutral-ground bonding can be done with an NG Plug. This is simply a standard plug internally wired such that the neutral plug is attached to the ground plug. Just plug it into your inverter output and it takes care of bonding the neutral to ground. Many people would skip this step but it's safer to have the neutral & ground attached.

That should do it. I don't think you need an additional bonding of the truck frame to your negative bus bar. You mention ground loops. They're an issue for RFI (radio frequency interference). Often times the safety rules are in conflict with RFI best practices. I would suggest you not worry about ground loops unless you have a radio interference problem. Unless you're a ham radio operator or running a nice hifi off the inverter I bet you won't have any ground loop issues.

hopefully I addreseds each of your questions.

First, thank you very much for your help.

QUOTE:
“Furthermore the AC ground is most likely already connected to the inverter chassis ground (internal to the inverter).”

Yes. I checked this yesterday

I also checked and there are two AC 115A outlets on the inverter. With the DVM it confirms both neutrals are tied together and both power feeds are tied together.

It is a small inverter. I only need one AC 115A outlet. But, since there are already two I can (on your suggestion of NG plug) plug this:

Into one of the outlets (which bonds them both) and then run power from the other outlet out two my other two receptacles on the camping box.

I say receptacle(s) on camping box as in two, not to both be used at the same time, (unless very small amp loads) but for convenience of where to plug in at.

I also understand not needing the additional ground cable to frame and can run inverter ground to negative bus bar as I am using the non isolated dc to dc charger.

My last question: On my Inverter AC output, would it be a good idea to run the outlet power out to a GFI receptacle first, and to protect the second outlet (on line side) or isn’t a GFI needed with the NG bonding plug at the inverter?

You’ve been most patient, thank you for your time - Glenn