Generator mit IT-Netz am Multiplus 2 im Inselbetrieb?

Been thinking about this one for a while actually, so curious if anyone else has run into it.

My setup is a Multiplus II 3000 paired with a Cerbo GX, SmartSolar MPPT, and a Fogstar 48V lithium bank — primarily for my garden office and emergency backup. Works a treat 99% of the time.

The question I keep coming back to: when you bring a generator into the mix on a pure off-grid island system, does the Multiplus care about the generator's earthing arrangement? Specifically, I'm wondering whether a generator with a floating neutral (sometimes called IT network in European discussions) causes any grief with the Multiplus's internal transfer switch and earth relay logic.

From what I've pieced together:

• The Multiplus has an internal relay that connects neutral to earth in inverter mode
• When AC-in is present (generator), it should open that relay and defer to the incoming source
• But if the generator is floating, there's potentially no proper earth reference at all

Has anyone actually tested this with, say, a Honda EU22i or similar inverter generator? Those tend to have floating outputs by default.

I've not hooked mine up yet — still using the Multiplus purely in inverter mode — but I want to get a backup generator sorted before winter and don't want to bodge the earthing.

Does Victron's documentation actually cover this clearly, or is it the usual "consult a qualified electrician" non-answer? Keen to hear from anyone who's actually wired this up properly in a UK off-grid context.

@GlenDoug — the thread title is in German, which is slightly puzzling given the forum, but I'll press on regardless.

The short answer is that the Multiplus II does not play nicely with IT networks (isolated terra, for anyone unfamiliar — where neither conductor is referenced to earth). It expects a TN-S or TN-C-S arrangement on the generator side.

On my narrowboat I discovered this the hard way when running a small Honda EU22i through the Multiplus II 5000. The earth fault detection kept tripping because the generator's floating neutral wasn't being properly bonded. The fix was a neutral-to-earth bond relay on the generator output, which the Cerbo GX can actually manage via an assistant if you configure it correctly.

Worth checking the Victron community docs on the "ground relay" assistant — it's specifically designed for this scenario.

@GlenDoug the thread title threw me as well, but I've been down a similar rabbit hole with my narrowboat setup — Multiplus II 3000, Cerbo, and a Fogstar 48V pack.

The short version from my experience: generators with IT earthing arrangements can cause the Multiplus's earth relay and RCD logic to behave unexpectedly in island mode. Victron's documentation touches on this but doesn't make it obvious.

What sorted it for me was setting ground relay to "off" in VEConfig and handling earthing externally and deliberately. Worth checking your ESS/inverter assistant settings carefully too — the defaults assume a more conventional earthing scheme.

Would genuinely recommend posting your full assistant configuration if you can. The details matter enormously here and it's easy to overlook one toggle that changes everything.

Good point from @SophieFisher and @WattAMess25 on the IT network question. Worth flagging that the Multiplus II's ground relay behaviour is critical here — in island mode it creates a TN-S style earthing arrangement internally, which can conflict with an IT-sourced generator.

A few things I'd want to clarify before going further:

• What generator are you running? Some smaller EU-market units ship with floating neutral by default
• Have you checked the ground relay assistant settings in VEConfigure?
• Is the Cerbo showing any ground fault warnings during generator handover?

In my static caravan setup I had earthing oddities when first integrating a generator — turned out the neutral-earth bond was doubled up. Victron's documentation on the "ground relay" assistant is worth a careful read before adjusting anything physical.

@BlownFuse raises a good point on the ground relay — that caught me out early on with my shepherd's hut build.

When I first wired my Multiplus II to a little Honda genny, the ground relay behaviour in island mode had me scratching my head for a good fortnight. The key thing I'd add is that VEConfigure's ground relay assistant is your friend here — you can explicitly control when that relay opens and closes rather than leaving it to default logic.

Also worth checking your generator's neutral-earth bonding arrangement before it meets the Multiplus. Mismatched bonding between the two is where things get properly messy, especially if you've got RCDs downstream.

The German title suggests @GlenDoug might have been cross-posting from the Victron community forum — same rabbit hole, different language.

Ran into exactly this with my van build last summer. The Multiplus II's ground relay behaviour had me scratching my head for a good few days when I switched between my Honda EU22i and shore power.

What eventually sorted it for me was digging into the ESS assistant settings and making sure the generator input was configured correctly in VE.Configure — the IT network isolation thing becomes a bit of a non-issue once the ground relay logic is handled properly by the assistant rather than fighting it manually.

@MuddyNomad's point about early confusion rings very true. I'd also suggest keeping an eye on the Cerbo's generator start/stop settings if you're running automated charging cycles — mine was behaving oddly until I tightened up the warm-up and cool-down timers.