Superyacht Stray-Current Corrosion, Shore Power & Galvanic Isolation: Earth Paths, Isolators, Transformers & Fault Diagnosis

Natural galvanic corrosion results from electrochemical potential differences between connected metals immersed in an electrolyte. MGDUFF separately describes electrolytic corrosion as the result of stray-current leakage from an external electrical source such as an onboard battery system or shore supply fault. The distinction matters because a sacrificial anode or shore-power galvanic isolator that is appropriate for one current path may not correct a separate electrical leakage fault driving damaging current through underwater metal.

Victron explains that when shore power is connected, the shore protective-earth conductor can electrically connect the yacht's bonding and underwater-metal system to shore-side grounding. Natural potential differences between immersed metals can then drive low-voltage DC galvanic current through this completed circuit. Other vessels connected to common marina grounding can form part of the wider electrical environment. Shore connection can therefore change an otherwise isolated yacht's corrosion-current paths without any onboard DC fault.

Victron explicitly warns against removing the shore protective-earth connection to stop galvanic corrosion because that conductor forms part of the vessel's electrical fault-protection arrangement. Disconnecting it can prevent the intended protective devices from operating correctly if an AC live conductor faults to bonded metal. Corrosion control must therefore preserve electrical shock protection. Use an approved galvanic-isolation arrangement rather than defeating the safety-earth path to achieve apparent electrical separation.

Victron's galvanic isolator is installed in the shore protective- earth path and uses opposed semiconductor junctions to block the small DC voltages associated with normal galvanic potential differences. Under a sufficiently high electrical fault condition, the device is designed to conduct so the protective-earth path can perform its safety function. The exact isolator rating, construction, monitoring and installation requirements must match the yacht's shore-power system and applicable marine electrical standards.

Because a galvanic isolator is intended to interrupt low-voltage galvanic current in the shore-earth connection, it should not be treated as a cure for every corrosion event observed while the yacht is plugged into shore power. MGDUFF identifies stray-current leakage from faulty electrical equipment or wiring as a separate electrolytic-corrosion mechanism. If current leaves an onboard DC circuit through a submerged fitting and returns through seawater, the underlying electrical fault must be located and corrected.

Victron describes a marine isolation transformer as separating the shore supply from the yacht by feeding the vessel from an electrically isolated secondary winding. This removes direct electrical continuity between the shore grounding system and the onboard system while allowing the vessel side to use the approved grounding and protective- device arrangement appropriate to the transformer installation. Unlike a simple galvanic isolator, the transformer provides galvanic separation of the shore AC supply itself.

An isolation transformer changes the relationship between shore conductors, onboard neutral, protective earth and the vessel bonding system. Victron's installation guidance explains that the vessel-side grounding arrangement is established on the transformer's secondary so protective devices can operate correctly. This wiring is safety critical. Never improvise neutral-to-earth links, remove protective conductors or copy another yacht's transformer arrangement. Follow the installed transformer's manual, yacht drawings and applicable marine electrical requirements.

MGDUFF describes stray-current leakage as a fault condition in which current from a battery or other external electrical source leaves the intended circuit through the hull or an underwater fitting and travels through the water. Severe localised corrosion can occur where current leaves a metal surface. The damage can therefore be concentrated around one shaft, through-hull, rudder, trim device or hull location rather than appearing as the more distributed wastage expected from ordinary galvanic exposure.

MGDUFF stresses proper insulation, suitable wiring, clean corrosion- resistant terminals, correct support and protection of electrical circuits from wet bilge areas. Damaged cable insulation, flooded junction boxes, contaminated terminal strips, failed pumps and chafed DC wiring can provide unintended paths from an energised circuit toward bonded or submerged metal. A corrosion investigation should therefore include recently wet, repaired or modified electrical circuits rather than focusing exclusively on the affected underwater component.

Rapid anode depletion may indicate increased protective demand, a new galvanic connection, coating damage or an external electrical-current path. Simply installing larger or additional anodes can mask the rate at which the protection system is being consumed without identifying why the demand increased. Record which anodes are wasting, their positions, elapsed service time and whether the change coincides with shore-power, electrical or refit work. Compare this evidence with hull potentials and leakage testing before changing anode design.

If hull potential, galvanic current or corrosion behaviour changes significantly when shore power is connected or disconnected, the shore interface deserves investigation. This does not by itself prove that the marina supply is faulty, because the change may result from the yacht's own grounding arrangement, a failed or bypassed isolator, an isolation-transformer wiring fault or another connected circuit. Preserve the comparative measurements and investigate the complete shore-to-vessel electrical path.

Adding a second shore inlet, charger, inverter, generator interface, communications cable, boarding equipment or other conductive connection can alter the electrical relationship between yacht and shore. Maintenance wiring can also accidentally create a parallel protective- earth path around a galvanic isolator. An isolator may then appear healthy while galvanic current bypasses it completely. After significant AC-system or shore-interface work, verify the installed topology against approved drawings and confirm that no unintended parallel bonding path defeats the corrosion-control arrangement.

A useful corrosion investigation may combine hull-to-water potential, bonding continuity, shore-earth continuity, current measurements and isolation checks. No single generic voltage or resistance value proves every yacht healthy because hull material, reference-electrode type, cathodic-protection system and electrical architecture differ. Establish the vessel's normal commissioned baseline and use manufacturer, class or competent marine-electrical criteria. Electrical measurement should be performed without bypassing residual-current, earth-leakage or other protective devices.

Shore-power systems can expose personnel to hazardous AC voltage, while onboard battery systems can deliver substantial fault current. Before opening shore-power equipment, galvanic isolators, transformers, distribution panels or bonding conductors, isolate and prove the relevant energy sources according to the yacht's electrical safety procedure. Do not disconnect protective earth on an energised system merely to observe whether corrosion current changes. Where live diagnostic measurements are genuinely required, they should be performed by competent personnel using the approved method and suitable equipment.

Begin with the exact evidence: rapid anode loss, localised pitting, corrosion appearing primarily while connected to shore power, changed hull potential, failed galvanic-isolator indication or an electrical fault associated with wet equipment. Record the yacht's shore-power state, cathodic-protection readings and recent electrical or refit work before changing anything. Confirm the approved shore inlet, protective- earth, bonding, galvanic-isolator or isolation-transformer topology and never remove protective earth as a test shortcut. Determine whether the observed current is consistent with the normal low-voltage galvanic path or with fault-driven leakage from an onboard or shore-fed circuit. Inspect isolator continuity and bypass paths by its approved test procedure, or verify transformer isolation and vessel-side grounding against the manufacturer's documentation. Trace suspicious DC circuits, wet wiring and bonded fittings while maintaining electrical safety. Correct only the confirmed wiring, isolation, bonding or equipment fault, then repeat hull-potential, leakage and shore-connected checks and record the restored electrical configuration and corrosion measurements as the verified baseline.