Multiplus II grounding on a boat

DIY
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1

Hi,

I am installing a 24/5000 Multiplus 2 on my boat. I have three AC sources, Shore 1 (30amp), Shore 2 (50 amp 240/120) and the generator (70amp 120v only). All three feed a 3 way lockout switch (this switches/disconnects line and neutral). My plan is to take the output of this switch and route whichever of the three AC sources is selected to the AC In on the MP2. In regards the 240v on Shore 2, I will be disconnecting one of the two legs of that input and sticking to 120v only. I understand that the AC In and AC out must have isolated neutrals. My understanding is that this is to ensure current is not leaked into shore power via the netral, however the why is somewhat irrelevent as I can and will isolate the neutrals.

I have read the manual and am still confused on how to best ground it though. As far as I can tell from the diagram depicting the internal ground/neutral bonding relay, all grounds are bonded within the MP2. As such, it should not matter if I use the in ground, the out ground or the M8 case ground, the “primary ground connection” as it’s labeled in the manual. The grounds are not switched in my boat’s lockout switch and all go directly to the ground bus (via galvanic isolators for Shore) and thus I dont know how I could isolate the incoming grounds if it is required. I suppose I could find and direct all three to their own ground bus BEFORE the AC In on the MP2 but I just can’t decipher why I would have to. And again, since all grounds are on the same bus, do I really need to run three wires from the in, the out and the case “primary” ground to the same bus? Are the multiple grounding points on the MP2 just to allow for wiring simplicity when one does not have a ground bus?

I know that grounding can cause an issue with GFCI shore power so I want to make sure I do this right. Given the generator has (I assume) an internal ground/neutral bond, If I do have to find the input grounds, should I just isolate the shore grounds and not the gen ground? The lockout switch should take care of this when it disconnects the neutral, but I still worry.

any help is much appreciated.

2

OK, perhaps I have answered my own question in the Victron Wiring Unlimited book. The below image clearly shows both AC connections bypassing the MP2 and connecting directly to the central ground busbar.

So… I guess I can ignore the ground connections in the AC terminals and just use one very large cable from the case ground to my busbar because “The DC ground cabling should be able to carry a fault current at least equal to the DC fuse rating.” On the 24/5000 that means TWO 2/0 cables or one 4/0 if I skimp ever so slightly. Surprising but ok I guess.

anyone contradict this?

image
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Conventionally, fault current rating for a wire is twice the continuous current rating, so the wire is about 1/2 the size that you would use for a positive or negative conductor. This is because a fault current, though large does not flow for very long if the system design is good.

Do also remember to use an isolating transformer on the shore power connections.
Inside the Multi, ACin ground is connected to the case, the AC1out ground and the AC2out ground.
In inverter mode, Ac out neutrals are also grounded by a relay.