Verständnisfrage MultiPlus-II und Notstromumschaltung / Inselbetrieb und die Allpolige Trennung

Right, this one's been rattling around in my head for a while and I reckon it's worth a proper discussion.

I've been running a Victron MultiPlus-II for about 18 months now in my tiny house setup, and the whole grid/off-grid switching behaviour still occasionally catches me out. Specifically around the question of all-pole isolation when the unit goes into island mode.

The MultiPlus-II does disconnect the grid input cleanly, but I've had a few head-scratching moments wondering whether my downstream consumer circuits are being isolated in the way my DNO and building regs would actually want. When I'm running purely off my Fogstar Drift LiFePO4 bank and the grid drops, everything seems fine — inverter kicks in, kettle stays on, wife stays happy. ✅

But here's my actual concern: is the neutral being properly broken during the transfer? In the UK we're mostly TN-C-S (PME) earthing, which throws a spanner in the works compared to continental setups. There are specific G99/G100 and BS7671 implications here that I don't think get discussed enough.

I've read through the Victron manual more times than is healthy, and I think the answer depends on your specific wiring configuration and whether you've got an external transfer switch doing the heavy lifting.

Has anyone here gone through this properly with a DNO or a registered electrician? Particularly curious whether anyone's using an external contactor arrangement rather than relying solely on the MultiPlus's internal relay.

I suspect there are a few of us running systems that are technically functional but wouldn't pass a proper inspection — no judgement, been there myself! Would love to hear how others have approached this.

Great timing on this thread @Titch - the allpolige Trennung (all-pole isolation) question is one that trips up a lot of people with the MultiPlus-II, particularly in the UK where our regs differ slightly from the German/Austrian setups most of the documentation seems aimed at.

The key thing worth understanding is that the MultiPlus-II does not provide true all-pole isolation from the grid by default in its transfer switch configuration. The neutral is typically bonded through, which matters enormously if you're looking at compliant islanding under G98/G99 or working toward MCS certification.

Depending on your intended use case - emergency backup versus proper off-grid island mode - the configuration in VEConfigure changes significantly.

What's your current ESS or non-ESS setup, and are you on a TN-C-S (PME) supply? That'll change the answer considerably.