Trying to understand grounding path

[48Rob]

In the attachments, I've drawn out three grounding scenarios to help me understand the ground path for a ground fault.

I want to connect a bonded-neutral inverter to a transfer switch, which will allow me to choose between grid, or inverter for 4 select circuits in my main AC sub panel.

In the first drawing, I have completely isolated the 4 circuits from the grid, except for the connected ground and they can only be run from the inverter. Is this truly "isolated" since the two systems share the ground? It is not my ideal setup, but a fallback in case I am unable to achieve the goal above.

In the second drawing, I am using a transfer switch that isolates all three wires. It also has its own ground rod, as it is a separately derived system.
I believe this shows that the system can safely handle a ground fault through the provided path, and that having a second ground rod 40' away from the AC side ground rod is okay because they are not connected to the same system. Am I mistaken?

In the third drawing, when I attempt to use a pass through ground in the transfer switch, I end up with neutral bonding in two separate places.

Do you see any errors in my understanding, and or an easier/simpler way to accomplish the goal?
An inverter with a floating neutral would make switching easy, and not require a separate system and ground rod, as I could pass through the ground and neutral, switching only the line, right?

Thank you.

 

[time2roll]

Does the inverter always have a neutral ground bond? Or bonded only when inverting?

Diagram 3 would be the most common set-up as the ground is not usually switched as shown in diagram 2.

Diagram 1 is fine as long as there are no shared grounds of the equipment powered by inverter vs from the main panels.

 

[48Rob]

Does the inverter always have a neutral ground bond? Or bonded only when inverting? - Yes, it is a basic inverter, always bonded.

Diagram 3 would be the most common set-up as the ground is not usually switched as shown in diagram 2. Great!

Diagram 1 is fine as long as there are no shared grounds of the equipment powered by inverter vs from the main panels.

I may not be understanding your statement? The only shared connection between the inverter system and the AC system is the ground from the AC panel to the DC distribution center, which feeds the inverter, as shown on the drawing.

Thank you.

 

[time2roll]

I may not be understanding your statement? The only shared connection between the inverter system and the AC system is the ground from the AC panel to the DC distribution center, which feeds the inverter, as shown on the drawing.

Thank you.

Yes in diagram 1 as long as the inverter loads have grounds isolated from the ground of the main panel loads all is good.

 

[48Rob]

Sorry if I am beating a dead horse here...but I want to be sure, and safe.

The inverter loads are isolated from the main AC ground in that they are not tied "directly" together, but because of the bonded ground/neutral in the inverter, and the inverter case being connected to the AC ground system through the DC distribution, it seems like a continuous path. I did not draw a green wire running through the inverter because there isn't one, but believed the result was the same as having one because of the internal inverter bond and the case ground attachment.
I'm sure you saw that in my drawing, but I want to clarify that even though they are connected in this way, is is indeed acceptable?

This was also my concern in drawing two, as it seems as though the two bonded neutrals would clash if not broken/divided into one per system at the transfer switch in the case of a ground fault?

Thank you for your patience.

 

[hwy17]

Are you working around a requirement that your inverter has to have a bonded neutral?

Imo it's a lot easier when the inverter is unbonded. Serious stationary inverters like Schneiders don't even come with any bonding ability.

 

[48Rob]

I was trying to use the inverter I already have.
I am also looking for a different inverter but am restricted by the fact that I have a 12 volt system and am trying to stay at or below $1200 for an inverter isn't making the chore easy.

I have a 2000 watt now, but should go up to a 3000 watt. The total draw of all combined DC loads and AC loads in the system is 14 Amps (AC out of inverter). 140ish amps DC. I have 880 Amp hour battery capacity. with each of the 4 batteries having a 150 Amp BMS. Extremely unlikely that all combined loads would ever be pulling at the same time.

I see a lot of all in one units, but they are auto switching, with grid as default and inverter as secondary, plus they have a charger that wants to charge my battery bank. I already have a 120 charger, and solar charging. My wish is to run about 50% of my minor loads (2 circuits out of 4 from the AC panel) on the inverter powered by solar all the time, and the other 2 as critical loads during a power outage. The ability to manually run the AC loads from the grid needs to be my choice.

I am new to this and may not understand the switching options of the inverters out there. Any education you might share is appreciated!

Thanks