Grounding- no idea what they're talking about!

[Madcodger]

Here's what I did for my off-grid "shed" install, and it works for me. But I am NOT an electrician, and advise you contact one. I invite others who know much about this topic to comment. @timselectric @Adam De Lay @sunshine_eggo all come to mind. Anyway, here goes, if my experience helps. My system is a Victron Multiplus 24V, 3000 KVA, installed in a shed as a sort of "off grid learning exercise" but with access to nearby AC power.

The Multiplus ground connection (on the lower right of the Multiplus frame as I recall) is connected to a ground rod just outside the off grid building, driven 8' in the ground. The Multiplus is the only connection to that ground rod, and the ground rod is the only connection to that grounding point on the Multiplus, as this building operates as a stand-alone unit, away from the house and garage, without being connected to the house or garage in any way, MOST of the time.

I do have a single, switchable, AC line running out to that "off grid" building from a nearby garage. It is NOT energized unless I specifically turn it on, back in the garage, and connect it to the Multiplus generator input via an RV outlet on the outside of the off grid building. This means that the AC line is essentially just an extension cord functioning as "shore power" if I plug it in and energize it. That AC line is, of course, connected to the AC panel of the garage when it is plugged in back at the garage, just as the rest of the circuits in the garage are, so it is therefore connected to the neutral-ground bond in the AC panel of the garage. However, I've noticed that I can leave this AC line plugged into the RV outlet of the off grid building, and into its outlet in the garage, and that this seems to affect the Multiplus only when the AC line is plugged in AND ENERGIZED, because otherwise the Multiplus ground relay appears to ignore it, and stays connected to the grounding rod. Everything operates just fine "off grid", whether or not I have that AC line plugged into the RV outlet, as long as I don't energize that AC line.

When I want "shore power" (usually after several days of very bad solar production), I energize that AC line. As soon as I do, I hear what sounds like a good-sized relay activate inside the Multiplus, which I presume is the relay switching from the grounding rod, to the ground for "shore power". The building operates perfectly, as if it were a ship connected to power at a dock. When my batteries are sufficiently recharged I can either de-energize that AC line and go back to off grid, or stay connected to "shore power", but when I do de-energize the line, I hear that relay in the Multiplus again, presumably as it disconnects from the shore power ground and reconnects to the grounding rod.

So, that's my experience and understanding. As I understand it, you never want the ground on the Multiplus to be connected to both its own ground (the grounding rod, in my case) and another ground (e.g., "shore power"), but if you connect through the generator input, the Multiplus will handle that. You want the Multiplus to be able to switch between the two, which it will do using its own relay, based on whether or not it is connected to "shore power". Again I invite others to comment.

 

[Adam De Lay]

All the ground connections on the MP are connected together. The relay you hear clicking is the relay which closes the L and N for AC In. There’s also another relay which creates a g/n bond when you run off batteries. That relay opens when AC In is connected.

You don’t want both g/n bonds connected at the same time. The grounds themselves are always gonna be connected.

 

[timselectric]

Because of the (sometimes) AC input from the garage. You need the ground conductor from the garage to the shed.
By adding an auxiliary ground rod at the shed. It must also be connected to the grounding system provided by the garage.
Auxiliary ground rods are not recommended. Due to ground loop issues. And possible equipment damage from a gradient pulse, created by a nearby lightning strike.

 

[SenjkoLee]

Geez.....Will does it again!!!

Very helpful....so far

This guy does it too! He explained it perfectly!!!

 

[Madcodger]

Because of the (sometimes) AC input from the garage. You need the ground conductor from the garage to the shed.
By adding an auxiliary ground rod at the shed. It must also be connected to the grounding system provided by the garage.
Auxiliary ground rods are not recommended. Due to ground loop issues. And possible equipment damage from a gradient pulse, created by a nearby lightning strike.

Thanks @timselectric and @Adam De Lay . Based on this, I think I'm in good shape. When the AC is connected, there is definitely a ground connection from the shed to the garage, so that's covered. That's accomplished via a cord that has an RV connector on each end, and I can just leave it disconnected during summer. The ground rod is only "auxillary" when that AC line is in place. Otherwise, it's the only ground connection as the shed runs solely on battery power when the AC line is disconnected. So, the only time that both the grounding rod at the shed and the connection to AC ground back in the garage coexist on the Multiplus is when I have that AC line is connected, and that is only needed during winter, when lightning doesn't happen here with any frequency.

I suppose the only way to create a truly perfect system while leaving the AC line plugged in but not energized would be to have a relay that opens the connection to the grounding rod anytime the AC line is plugged in, even if the H/N on that line are not energized. If I were in a southern location I'd consider that. But given that the only time I need AC plugged in is winter, and I'm in Maine, probably not necessary given that the "auxiliary" rod is not prohibited by code, even though it is not advised for the reasons stated.

Thanks again - really appreciate all the good advice and info each of you provide on the forum.