Galvanic isolator — how a diode bridge blocks galvanic corrosion on shore power without giving up the earthing function

The guide on marinas and berths (§709) briefly mentions two solutions against galvanic corrosion via the shared earth conductor of a shore power connection: the galvanic isolator and the onboard isolation transformer. This article goes deeper into the first solution — how a galvanic isolator actually works, and why its design must carefully serve two, at first sight conflicting, purposes at the same time.

The problem, briefly recapped

A vessel connected to shore power shares an electrical path with other vessels in the same marina, via the earth conductor of the shore power cable and the surrounding seawater. A small galvanic voltage difference between underwater parts of different vessels (different metal types, sacrificial anodes in different states) drives a small direct current through this shared path, accelerating the corrosion of sacrificial anodes and underwater parts — a process entirely separate from the normal AC safety function of that same earth conductor.

Working principle: a diode bridge that only blocks low voltage

A galvanic isolator is placed in series with the earth conductor of the shore power supply and consists, at its core, of two (or two pairs, oppositely poled for both current directions) series-connected semiconductor diodes. A diode only conducts once the voltage across it exceeds the forward voltage — for a silicon diode typically around 0.6–0.7 V. With two diodes in series, a blocking threshold of typically about 1.0 to 1.2 V results:

The typical galvanic voltage between underwater parts of different vessels (typically on the order of a few tenths of a volt, well below 1 V) stays below this threshold, so the diode bridge does not conduct and the galvanic current path is interrupted.
A real earth fault — for example a phase breaking down to the chassis of an appliance on board — drives a much higher voltage across the isolator, well above the diode threshold, so the diode bridge does conduct and the full fault current can flow unimpeded to the shore earth, exactly as needed for the earth fault protection to operate correctly.

Why this is fundamentally different from simply omitting the earth wire

It is a known but dangerous misconception to try to solve galvanic corrosion by simply removing or cutting the earth connection of the shore power cable. That does eliminate the galvanic path, but also eliminates the protective function of that same earth conductor entirely: in the event of an earth fault on board, there is then no path at all for the fault current, and the metal enclosure of the failed appliance (and, via the hull, potentially the entire vessel) can remain at a dangerous voltage without any protection tripping. A galvanic isolator resolves this conflict by fulfilling both functions at once: it blocks the permanent, low-voltage galvanic current path, but keeps the earth connection fully functional in case of a real fault — an earth conductor without an isolator and no earth conductor at all are therefore not alternative choices, but, respectively, a solution with a drawback and an outright safety hazard.

Isolation transformer: a more thorough but heavier alternative

The guide on marinas and berths also mentions the onboard isolation transformer as an alternative. Like the [separation transformer used for electrical separation under §413.3](/guides/nen-1010/413-elektrische-scheiding-scheidingstransformator), this provides a full magnetic separation between the shore network and the onboard installation: there is then no galvanic path at all between the two earthing systems, rather than a path that is only blocked below a certain voltage threshold. This is a more thorough solution — it also resolves any other galvanic or hum-loop-related issues — but is considerably heavier, more expensive and more complex than a galvanic isolator, and therefore not the chosen solution on every vessel.

Note: a galvanic isolator is a passive component with no moving parts, but a diode can indeed fail — for a regular, non-fail-safe unit typically open (breaks earth continuity entirely, an earth fault path is then lost) or, less often, shorted/punctured (the blocking function disappears, the galvanic path is open again, but earth continuity remains intact). Because the open failure mode in particular poses a serious, unnoticed safety hazard, the current ABYC A-28 requires a galvanic isolator to either have a fail-safe design — the diode bridge is then engineered to guaranteed short on failure rather than open, so only the corrosion protection is lost while earth continuity is preserved — or, if it is not fail-safe, to have a monitoring or test function (for example a status indicator or test button) that flags a failed diode bridge.

Practical relevance

When assessing or replacing a galvanic isolator on a marina shore power connection, it is important to check that the installed unit complies with ISO 13297 or ABYC A-28 and — if it is not marked fail-safe — has a monitoring function: a unit that is neither fail-safe nor has any form of status indication can be faulty for years (in either failure direction) before this comes to light, either as renewed corrosion or as a broken earth path.

Common mistakes

Omitting or cutting the earth conductor of the shore power cable to "prevent" galvanic corrosion — this also eliminates the earth fault protection function entirely and is a serious safety hazard.
Installing or keeping a galvanic isolator that is neither fail-safe nor has a monitoring function — a failed diode (bridge) is not visible from the outside and can silently undermine both the corrosion protection and (in the case of an open diode) earth continuity.
Assuming a galvanic isolator solves all forms of shore-power-related corrosion — the isolator specifically blocks the galvanic path via the shared shore power earth conductor, not corrosion caused by poor bonding on board itself or by other stray-current sources.
Treating a galvanic isolator and an isolation transformer as fully interchangeable — the isolator only blocks below a voltage threshold, the transformer provides full galvanic separation; the choice depends on the desired level of protection and the available budget/weight on board.