Loads connected to AC in during external anti-islanding disconnection

Been wrestling with this exact head-scratcher on my narrowboat conversion — running a Victron Multiplus-II grid-parallel setup and the G99 compliance rabbit hole goes deep.

So when your external anti-islanding relay kicks in and boots you off the grid, what actually happens to any loads you've got sitting on the AC-in side of the inverter-charger? They're not on AC-out, they're not protected by the Multiplus's internal transfer switch logic — they just... fall off a cliff, yeah?

My specific concern:

• Battery charger for the engine bank sitting on AC-in
• A small immersion element wired pre-inverter for grid-priority hot water

When the G99 disconnect fires, do those loads simply die until reconnection, or is there some funkiness where the Multiplus momentarily tries to backfeed and causes grief with the isolation relay?

I think the answer is "they just lose power, deal with it" but I've seen conflicting stuff about the Multiplus's behaviour in that transition window — particularly around the UPS function setting and whether it influences anything upstream of AC-in.

Anyone running a proper G99-compliant grid-parallel setup in the UK actually tested this in the real world, or are we all just reading the Victron CCGX manuals and hoping for the best? 😅

Fogstar/Victron combo owners especially welcome — curious if DNO response time affects how long that disconnection window actually lasts in practice.

@Stu1991 classic G99 rabbit hole — I've lost weekends to this one.

Key thing people miss: during anti-islanding disconnection, the Multiplus-II's AC-in relay opens, but anything wired directly to AC-in (before the inverter) loses supply entirely. Only loads on AC-out-1 get the seamless UPS-style switchover.

On my narrowboat setup I learned this the hard way — my 240V bilge pump was wired to AC-in. During a DNO test disconnect, silence. Not ideal.

Practical fix: anything critical goes on AC-out-1. Non-critical shore-dependent stuff (battery charger for tools, etc.) can stay on AC-in.

Worth checking your ESS Assistant configuration too — specifically the "prevent feeding back to grid" and "keep batteries charged" thresholds interact oddly with disconnection events. Victron's Venus OS logs are your friend for diagnosing what actually happened during a trip.

@Stu1991 this caught me out badly during a storm last winter — mains dropped, anti-islanding kicked in as it should, but I'd assumed my critical loads would seamlessly transfer to inverter mode.

What actually happens is there's a brief but real interruption during that disconnect event. The Multiplus-II needs to "see" the loss before switching, so anything sensitive on AC-in side gets a nasty hiccup.

My fix was moving everything genuinely critical — router, medical kit chargers, NAS — onto AC-out 1, which maintains supply through the inverter regardless. AC-in loads are now only things that can handle a blip.

For emergency backup purposes specifically, this distinction between AC-in and AC-out 1 is the most important thing to understand about the whole topology. Victron's documentation calls it out but buries it rather.

@Stu1991 narrowboat makes this trickier than most — you're potentially moving between marinas with different supply qualities, which trips anti-islanding more readily than a static install.

Here's the bit that bit me on my motorhome build before I knew better: loads on the AC-in side stay connected to whatever's upstream during disconnection — they don't magically transfer to inverter output. If you haven't split your critical loads onto AC-out-1, they're dark when G99 pulls the pin.

Victron's PowerControl assistant doesn't solve this — it's a separate wiring decision entirely.

On a narrowboat I'd strongly consider treating AC-out-1 as your only domestic ring and relegating shore power purely to charging. Cleaner logically, G99 stops being a drama, and your Multiplus handles the transition invisibly.

DrawPoint the critical loads, wire accordingly, job done.

@Stu1991 welcome to the forum — G99 is a proper rabbit hole and you've picked a genuinely tricky edge case to start with.

Quick question before I weigh in: when the anti-islanding disconnection occurs, are your AC loads hardwired to the Multiplus-II AC-in specifically, or are some of them on the AC-out-1/AC-out-2 outputs?

The behaviour differs significantly depending on which output rail they're sitting on. AC-out-1 on the Multiplus-II will drop with mains during a disconnection event unless you've configured UPS-mode behaviour correctly in VEConfigure.

What firmware version are you running? There were some relevant changes in recent releases affecting transfer switch timing that might be relevant to your narrowboat setup specifically.

@Stu1991 this is something I've been puzzling over with my static caravan setup too — slightly different scenario to yours but same fundamental question.

When anti-islanding disconnects, does the Multiplus-II immediately switch to inverter mode to keep your AC loads alive, or is there a crossover gap where everything just drops? I've been trying to understand whether the transfer switch relay timing inside the unit matters here, because even a 20ms gap could cause sensitive equipment to reset.

Has anyone actually measured this with an oscilloscope? I'd love to know what the real-world transfer time looks like during a G99-triggered disconnect specifically, as opposed to a normal mains failure — wondering if the firmware handles them differently under VE.Bus?

@SolarRachel sounds like your storm experience might be relevant here — did your loads actually drop or did the inverter catch them seamlessly?