Galvanic Isolation on input side from shore power supply to yacht prior to auto transformer

Interesting topic this — been thinking about something similar for my motorhome setup but the yacht application raises the stakes considerably, especially with an aluminium hull where galvanic corrosion is a genuine concern rather than just a theoretical one.

For those running Victron Multiplus units off shore power in the UK, are you putting a galvanic isolator or a full isolation transformer on the input side before it hits the inverter/charger? I know these are quite different solutions —

• Galvanic isolator (diode-based): blocks DC galvanic currents but still passes AC faults back to the hull
• Isolation transformer: true galvanic break, floating secondary, proper protection — but heavy and expensive

My understanding is that for aluminium hulls specifically, a galvanic isolator alone isn't really sufficient — you want the full transformer approach before your Multiplus units even see the shore supply. The corrosion risk to an ali hull from stray DC is serious enough that cutting corners seems a false economy.

Has anyone here got a twin Multiplus arrangement on a boat and sorted this properly? Curious whether you're using a dedicated marine isolation transformer before the autotransformer stage, or whether the autotransformer itself is providing isolation (which I don't think it does — it's just a tapped winding, not a separate secondary).

Also wondering about the BS EN ISO 13297 compliance angle — is anyone actually specifying to that standard when commissioning, or is it mostly a self-build job?

TimGreen | Posts: 847

@WattLiz Spot on about the aluminium hull — that's where it gets really serious. Worth looking at the Victron Isolation Transformer if you haven't already; it handles the galvanic isolation and gives you a floating neutral on the output side, which sorts the autotransformer compatibility question in one go rather than stacking separate devices.

One thing often overlooked is that even with isolation sorted, you still want a galvanic isolator or zinc anodes as a belt-and-braces approach when connected to marina shore power — other boats on the same ring can still introduce stray currents through the water itself. The ISO 13297 standard is worth a read if you're doing this properly for a marine installation.

What shore power voltage are you working with — 230V single phase or are you on a 16A blue-plug setup?

A galvanic isolator fitted at the shore power inlet is basically cheap insurance compared to re-plating your hull — ask me how I know this after my mate's fibreglass cabin nearly became a science experiment with stray current corrosion! 🔌

For the yacht application specifically, an isolation transformer rather than just a galvanic isolator gives you full floating earth too, which is the proper belt-and-braces approach — Victron's isolation transformer is the go-to but doesn't come cheap. If budget's tight, at minimum get a proper galvanic isolator rated to your shore power amperage and don't bodge it with something undersized. Seawater and dodgy earthing are not friends you want introduced to each other.

EssexNomad | Posts: 312

@BatteryPaula nailed it — though if you're running an auto transformer anyway, a proper isolation transformer on the input side does the job and sorts your neutral-earth bonding headaches in one go, which on a saltwater-adjacent setup is worth every penny of the Victron price tag.

AlexJones | Posts: 43

Slightly different angle here — does the position of the galvanic isolator relative to the auto transformer actually matter for protection effectiveness? I'm wondering because on my narrowboat I've seen setups where the isolator sits after various bits of kit rather than right at the inlet, and I always assumed the closer to the shore power entry point the better?

Also curious whether anyone has experience with the Victron isolation transformer vs a standalone galvanic isolator for this kind of application — I know they're solving slightly different problems but on a steel narrowboat hull the full isolation transformer seems overkill, whereas an aluminium yacht hull sounds like a different conversation entirely. Are there specific certification requirements for the yacht install that wouldn't apply to inland waterways craft?

GoldenTrekker | Posts: 847

@AlexJones raises the critical point — position absolutely matters here. The isolator must sit between shore earth and vessel earth, before anything else in the circuit. Placing it downstream of the auto transformer defeats the purpose entirely, as the transformer's neutral-earth bonding can create a bypass path.

From a purely practical standpoint, I'd specify a BS EN 61140-compliant unit rated well above your maximum shore supply current — don't cheap out on the rating. Victron's isolation transformers handle this elegantly by providing full galvanic isolation rather than just blocking DC galvanic current, which is arguably the proper solution for an aluminium hull where the stakes are higher than my van setup.

Worth checking MGN 181 (Maritime & Coastguard Agency guidance) — specifically addresses shore power arrangements for leisure craft and clarifies the regulatory expectation around earth continuity versus galvanic protection.