Off Grid Earthing at Power in

Been puzzling over this one for a while and could do with some guidance from those who've already sorted it.

My setup is a static caravan with a Victron MultiPlus-II as the inverter/charger, Fogstar Drift lithium batteries, and a separate consumer unit feeding several circuits. I've got both off-grid solar running day-to-day and occasional shore power hook-up for top-up charging.

The question I keep coming back to is around earthing at the DC/AC boundary — specifically whether the neutral should be bonded to earth on the inverter's AC output side, or whether it's better to let the system float.

From what I understand:

• A floating system can make fault detection harder, since a single fault to earth won't necessarily trip an RCD
• Bonding neutral-to-earth at the inverter output creates a proper reference, allowing RCDs downstream to function as intended
• But if you're ever switching between off-grid and shore power, having two neutral-earth bonds simultaneously is apparently a problem

My MultiPlus has an internal transfer switch, so when shore power is present it passes through — does that mean I should only bond at the inverter output when running off-grid, and rely on the DNO supply's own earth reference when on hook-up?

Has anyone used an external relay or the Victron assistant to switch the bond in and out automatically depending on which source is active? I'd rather not have this as a manual step I might forget.

Also wondering if there are any specific UK regs considerations (BS 7671 / 18th Edition) that affect how this should be handled differently to, say, US-based advice which seems to dominate most of the online resources.

@Nessa the earthing question on a MultiPlus-II tripped me up too when I did my van build.

The key thing Victron's documentation doesn't shout loudly enough: the MultiPlus has an internal transfer switch that handles the neutral-earth bond automatically when you're running on inverter. When shore/generator power is connected, that bond breaks.

So for a static caravan install:

• Don't create your own neutral-earth bond at the caravan chassis
• Let the MultiPlus manage it via the internal relay
• Your chassis earth still needs connecting properly to the system earth bar

The gotcha is if you're also running an RCBO board — test it actually trips correctly in both inverter and passthrough modes. Mine behaved differently depending on source, which was alarming until I understood why.

What Fogstar battery are you running? That can affect how you configure the system earth depending on whether it's got a BMS with isolation.

Great topic @Nessa - the static caravan context is worth flagging specifically because it changes things slightly from a van build.

With a static caravan you're essentially treating it as a permanent dwelling for earthing purposes. If you're ever connecting to mains hookup, the MultiPlus-II's internal transfer switch handles the neutral-earth bonding automatically on inverter mode, then hands it back to the incoming earth when grid-tied. So you shouldn't be creating a duplicate bond yourself at the consumer unit.

The bit people often miss with statics is whether your caravan plot has a proper TN-S or TN-C-S supply arrangement - worth confirming with the site owner before assuming anything about the incoming earth quality.

What's your hookup situation - are you purely off-grid or occasionally connecting to site power? That'll determine exactly where your bonding needs to live.

What @SussexVanLifer is alluding to is the MultiPlus-II's internal transfer switch behaviour — when grid or shore power is present, it passes through the earth from that incoming supply. When you're running on inverter alone, the unit creates its own neutral-earth bond internally (assuming you've set the relevant assistant correctly in VE.Configure).

Done my cabin build with near-identical kit and spent a weekend chasing an RCD that kept tripping before realising my incoming earth rod and the inverter's switched earthing were fighting each other.

@Foggy80 is right that static caravans add complexity — if yours sits on a permanent plot with a DNO supply nearby, you'll want to confirm whether you're on TN-S or TN-C-S, as that affects how your earth electrode should be handled.

Worth grabbing the Victron MultiPlus earthing whitepaper — it's dry but genuinely answers most of this.

On a narrowboat we call the MultiPlus-II's earthing behaviour "the thing that'll fail your BSS inspection if you don't read the manual properly" — and yes, static caravan regs are equally unforgiving.

The bit nobody's mentioned yet: when you're floating on shore power, the MultiPlus-II's neutral-earth bond is internal and switches with the transfer relay. If you've got an external N-E bond on your incoming supply and the unit creates its own, you've got a parallel path that'll confuse your RCD something rotten.

Check what your site's supply actually is — TN-S, TN-C-S, or TT — because TT sites (common on caravan parks) mean you're relying entirely on your earth electrode, and suddenly your Victron earth relay settings become very relevant. Get a socket tester with RCD function on site supply before you assume anything.