Instalation CPI externe RS450/100 et multiplus + Fornius

Bit of an odd one I've been mulling over for my cabin setup, so keen to hear from anyone who's navigated similar territory.

I'm running a Victron SmartSolar MPPT RS 450/100 alongside a Multiplus-II, and I've been going back and forth over how to properly configure the earthing and isolation arrangement when an external CPI (or equivalent monitoring relay) is brought into the mix. The RS 450/100 has integrated galvanic isolation, which is great, but it does complicate things when you're trying to satisfy an installer or, in some cases, a DNO or building control officer who wants to see documented earthing topology.

In the French thread that inspired this discussion, the installer had declared an IT earthing scheme on their compliance paperwork — which is broadly similar to what some of us run off-grid in the UK, where there's no TN-S or PME reference to bond to. That's actually quite relevant if you're running a fully islanded system with no grid connection at all, which several of us here do.

My specific question: does anyone have practical experience documenting the earthing arrangement for a Victron RS + Multiplus combo when submitting for building regs or any formal sign-off in the UK? I know most of us don't need CONSUEL (that's a French thing), but building control can still ask awkward questions if you're wiring a permanent structure.

Also curious whether anyone's added a Fronius inverter into a Victron ecosystem — the AC coupling side of things with frequency shifting adds another layer to the earthing conversation.

@TransitAdventure the RS 450/100 is a beast of a controller — I've got the smaller RS 450/200 running in my tiny house setup so fairly familiar with the ecosystem.

One thing worth flagging: the RS series communicates over VE.Can rather than VE.Direct, which changes how it integrates with a Cerbo GX (if that's in your chain). Make sure your CAN bus termination is sorted at both ends or you'll get intermittent comms dropouts — caught me out initially.

If Fronius is involved as a grid-tie inverter on the AC input side, look into Fronius Zero Feed-in via the frequency shift method — Victron and Fronius actually play together reasonably well once configured properly in VEConfigure.

What's your battery bank? That'll affect how the RS behaves under absorption/float handoff.

Had the RS 450/100 singing nicely with a Multiplus-II on my motorhome — the magic is making sure they're all on the same VE.Can network so the Multiplus can boss the MPPT about via DVCC (Distributed Voltage and Current Control).

Key bits:

• Enable DVCC in VenusOS (Cerbo GX or similar)
• Set SVS (Shared Voltage Sense) on
• Let the Multiplus be the master charger — the RS 450/100 will follow its charge curve obediently like a well-trained spaniel

The Fronius side is trickier — it talks via AC coupling, so Victron's frequency-shift power control handles that automatically when batteries are full, no manual fiddling needed.

Check your VRM portal shows all three devices chatting — if anything's silent, it's usually a dodgy RJ45 on the VE.Can daisy chain. Classic.

@MarineGeoff beat me to the VE.Bus point but nobody's mentioned the CAN bus vs VE.Can gotcha that trips up half the forum every other week — the RS 450/100 speaks VE.Can, not plain CAN, so make sure your GX device (Cerbo, Venus, whatever you're running) has the right port configured or you'll be staring at a blank screen wondering why your £800 controller has gone mute, which is exactly what happened to me in my static van last winter whilst everything froze solid.

Great points from @MarineGeoff and @ForestDaz already. One thing worth flagging specifically for the RS 450/100 is that it doesn't support VE.Direct — only VE.Can — which catches people out when they're used to the smaller MPPT range. Make sure your GX device (Cerbo, Venus, whatever you're running) has a spare VE.Can port rather than assuming VE.Direct will do the job.

Also worth double-checking your Fronius firmware is up to date before you start commissioning — older versions can be a bit temperamental when the Multiplus is orchestrating zero export or grid assist functions. Victron's compatibility list on their website is your friend here. What GX device are you planning to tie it all together with?