Isolation Transformer
Queen of Hearts came without a galvanic isolator. Rather than adding one, a Victron Energy 2000W Isolation Transformer was installed in 2024. A galvanic isolator blocks DC leakage currents on the shore power safety ground, but leaves the AC system electrically connected to shore. An isolation transformer goes further: it couples input to output magnetically, with no direct electrical connection between them. The boat's AC system and DC negative are completely isolated from shore power, from the dock, and from every other boat on the pier.
It lives on the starboard hull just forward of the aft cabin, inline between the shore power inlet and the Multiplus.
High Voltage at a New Marina
At the first marina, no issues. After moving to a second marina, the Multiplus stopped accepting shore power and ran on batteries instead. The Victron app showed the disconnect reason: high AC voltage, 142V on the input.
Multiplus refusing shore power. Grid disconnect reason: high AC voltage at 142V.
The marina was supplying roughly 132V at the dock. Higher dock voltages are increasingly common at marinas. The isolation transformer has a built-in ~5% boost, which helps when a marina is running low but works against you when they are already running high. 132V through a 5% boost becomes 141-142V, above the range the Multiplus would accept.
The Fix
Inside the isolation transformer, 8 wires connect the toroid to the control board. Their arrangement determines whether the unit boosts or bucks the incoming voltage. Swapping them flips the configuration from +5% to -5%.
Inside the unit before the swap. Original wiring in boost configuration.
Michael on the Victron Energy community forum had already worked through this exact problem, identified the wire arrangement, and documented the specific swap needed to go from boost to buck. 8 cables swapped and shore power was accepted again.
After the 8-cable swap. Buck instead of boost.
The full breakdown of the wire swap is in the Victron Energy community forum post. Thank you, Michael.